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Water at the silica surface : effect of ions and temperature Lovering, Kaitlin 2018

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WATER AT THE SILICA SURFACE: EFFECTS OF IONS AND TEMPERATURE by  Kaitlin Lovering  B.A., New College of Florida, 2011  A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF  DOCTOR OF PHILOSOPHY in THE FACULTY OF GRADUATE AND POSTDOCTORAL STUDIES (Chemistry)  THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver)  August 2018  © Kaitlin Lovering, 2018   ii  The following individuals certify that they have read, and recommend to the Faculty of Graduate and Postdoctoral Studies for acceptance, the dissertation entitled:  Water at the Silica Surface: Effects of Ions and Temperature  submitted by Kaitlin Lovering in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Chemistry  Examining Committee: Allan K  Bertram Co-supervisor Keng C Chou Co-supervisor  Edward R Grant Supervisory Committee Member Xiaonan Lu University Examiner Yan Alexander Wang University Examiner  Additional Supervisory Committee Members: Pierre Kennepohl Supervisory Committee Member Daniel Bizzotto Supervisory Committee Member iii  Abstract  This dissertation studies the silica/water interface using sum frequency generation spectroscopy. The effects of alkali chloride ions and temperature on the hydrogen bonding network at the interface are examined.  We observed that the structure of water in the Stern layer depends on the identity of the cation. The ability of a cation to displace the hydration water on silica surface is in the order of Mg2+ > Ca2+ > Li+ > Na+, consistent with the trend of the acid dissociation constant of the salt. We conclude that ions with a high pKa, such as Mg2+ and Ca2+, have a local electrostatic field strong enough to polarize water molecules in the hydration shells of the ions. These partially hydrolyzed water molecules form linkages with the negative charges on the silica, forming solvent shared ion pairs.  During freezing of pure water, we observed a transient phase of ice at water/mineral interfaces, which had enhanced IR-visible sum frequency generation intensity for several minutes. Most forms of ice are centrosymmetric but a possible explanation of for the transient phase is the formation of stacking-disordered ice during the freezing process. Stacking-disordered ice, which has only been observed in the bulk ice at temperatures lower than -20 °C, is a random mixture of layers of hexagonal ice and cubic ice. The transient phase at the ice/mineral interface was observed at temperatures as high as -1 °C. This observation suggests that the mineral surface may play a role in promoting the formation of the stacking-disordered ice at the interface.  The effect of ions during freezing at the silica/water interface was investigated.  Ice is the first phase to form. NaCl·2H2O forms below the eutectic temperature, indicating that the formation and growth of ice does not push the ions out of the interfacial region. We compared iv  the surface freezing diagram with the bulk equilibrium phase diagram of aqueous sodium chloride solutions. Although the concentration of ions is higher at a charged surface, we observe that freezing point depression at the surface is analogous to freezing point depression for homogeneous freezing and bulk equilibrium phase diagram.   v  Lay Summary  Minerals and water frequently come into contact in the environment; their interactions are important in many complex processes. By studying how these interactions change as a function of temperature and ion concentration, researchers can better understand mineral weathering, ice formation, glacial creep and other environmental processes. For example, the water in clouds can exist as liquid or as ice. The phase of the cloud is an important determinant of its ability to trap heat or reflect sunlight. Consequently, the phase of the cloud impacts the earth’s radiative budget. The transition from liquid to ice in clouds often occurs at the interface of mineral dust particles commonly found in the atmosphere. It is difficult for scientists to directly probe the interface of mineral dust particles, limiting the ability to understand ice formation. This work uses a surface-specific experimental technique to examine mineral/water interactions in environmentally relevant conditions.    vi  Preface  A version of Chapter 3 has been published as: Kaitlin Lovering, Allan Bertram, Keng Chou. “New Information on the Ion-Identity-Dependent Structure of Stern Layer Revealed by Sum Frequency Generation Vibrational Spectroscopy” Journal of Physical Chemistry C 2016, 120, 18099-18104. The project was initiated by Dr. Bertram and Dr. Chou. My major contributions are literature survey, project design, data acquisition, and manuscript composition. A version of Chapter 4 has been published as Kaitlin Lovering, Allan Bertram, Keng Chou. “Transient Phase of Ice Observed by Sum Frequency Generation at the Water/Mineral Interface During Freezing.” J. Phys. Chem. Lett 2017, 8, 871-875. The project was initiated by Dr. Bertram and Dr. Chou. Experimental setup was done by Dr. Chou and Lovering. My major contributions are literature survey, project design, data acquisition, and manuscript composition.   Chapter 5 is based on work conducted in the laboratories of Dr. Chou for a project initiated by Drs. Chou and Bertram. I performed the literature survey, project design, and data acquisition. vii  Table of Contents  Abstract ......................................................................................................................................... iii Lay Summary .................................................................................................................................v Preface ........................................................................................................................................... vi Table of Contents ........................................................................................................................ vii List of Tables ..................................................................................................................................x List of Figures ............................................................................................................................... xi List of Symbols ........................................................................................................................... xvi List of Abbreviations ................................................................................................................ xvii Acknowledgements .................................................................................................................. xviii Dedication .....................................................................................................................................xx Chapter 1: Introduction ................................................................................................................1 1.1 Ice Nucleation in the Atmosphere................................................................................... 2 1.1.1 Ice Nucleating Particles .............................................................................................. 2 1.1.2 Ice Nucleation Pathways ............................................................................................. 4 1.1.3 Effect of Ions............................................................................................................... 5 1.2 Probing Buried Interfaces ............................................................................................... 6 1.2.1 Silica/Aqueous SFG Spectra ....................................................................................... 7 1.3 Overview of Thesis ......................................................................................................... 8 Chapter 2: Experimental setup...................................................................................................10 2.1 Sum frequency generation spectroscopy ...................................................................... 10 2.1.1 Surface Sensitivity .................................................................................................... 11 viii  2.2 Second Order Susceptibility Tensor ............................................................................. 12 2.2.1 Probing Different Tensor Elements .......................................................................... 13 2.3 Experimental Design ..................................................................................................... 14 2.3.1 Beam Alignment ....................................................................................................... 15 2.4 Surface Preparation ....................................................................................................... 15 2.5 Surface Cooling ............................................................................................................ 15 Chapter 3: Ions at the Silica Surface at Room Temperature ..................................................17 3.1 Background ................................................................................................................... 17 3.2 Experimental ................................................................................................................. 19 3.3 Results and Discussion ................................................................................................. 19 3.3.1 Comparison of Ionic Strength ................................................................................... 21 3.3.2 Confirmation of Localized Hydrolysis ..................................................................... 25 3.4 Conclusions ................................................................................................................... 28 Chapter 4: Transient Ice structure during freezing at the silica surface ...............................29 4.1 Background ................................................................................................................... 29 4.2 Freezing Experiments ................................................................................................... 30 4.3 Results and Discussion ................................................................................................. 31 4.3.1 Temperature of phase transition ................................................................................ 35 4.3.2 Alternative explanations for the transient phase ....................................................... 36 4.4 Conclusions ................................................................................................................... 38 Chapter 5: Freezing of NaCl Solutions at Silica Surfaces ........................................................39 5.1 Introduction ................................................................................................................... 39 5.2 Experimental ................................................................................................................. 41 ix  5.3 Results and Discussion ................................................................................................. 42 5.4 Conclusions ................................................................................................................... 49 Chapter 6: Conclusion .................................................................................................................51 6.1 Room Temperature Interactions: Ions .......................................................................... 51 6.2 Interactions during Freezing: Pure Water ..................................................................... 51 6.3 Interactions during Freezing: NaCl Solutions............................................................... 52 6.4 Outlook ......................................................................................................................... 52 Bibliography .................................................................................................................................55  x  List of Tables  Table 2.1. The polarization combinations used in SFG to probe active tensor elements. ............ 14 Table 3.1. The metal cations facilitate the hydrolysis of water. The ability of the metal cation to perturb surface water structure follows the trend in decreasing pKa for the hydrolysis reaction. 26  xi  List of Figures  Figure 1.1. Heterogeneous freezing, in which a solid phase catalyzes ice nucleation, can occur via three pathways........................................................................................................................... 4 Figure 1.2. Spectrum of liquid water (red) and hexagonal ice (blue) taken at the interface with silica. The water spectrum was taken at 22 °C, and the ice spectrum was taken at -6 °C roughly 30 min after freezing. ...................................................................................................................... 8 Figure 2.1. Laboratory coordinate system of the SFG experiment. The xy-plane contains the interface and the xz-plane contains the beams. The IR beam is indicated in red, the visible beam is green and the resultant SFG beam is shown in purple. ............................................................. 13 Figure 2.2. Experimental apparatus. A copper cooling block was used to cool the silica prism. The prism is attached to a Teflon flow cell, and water is introduced to the flow cell by opening the inlet valve. The laser beams are aligned to overlap at the base of the prism. The temperature of the prism is recorded by a thermocouple inserted into a hole near the base of the prism. Originally published in Lovering et al. (2017). ............................................................................ 16 Figure 3.1. Schematic representation of the Stern-Gouy-Chapman model. The Stern layer consists of ions specifically interacting with the surface. Beyond the Stern layer the ions are distributed in a diffuse Gouy-Chapman layer. Originally published in Lovering et al. (2016). ... 18 Figure 3.2. Various concentrations of alkali and alkali earth chloride salts. In (a) NaCl solution concentrations increase from 1E-4 M to 6 M. Increasing NaCl concentration causes a progressive decrease in SFG intensity until around 1 M. Above 1 M, the ability of NaCl to effect the SFG signal seems to saturate. In panel (b) CaCl2 solutions increase from 1E-4 M to 4 M. The Ca2+ ions also reduce SFG signal progressively with increasing concentration, however, the effect xii  does not saturate until > 3 M. Above this concentration, no water signal is observed. Similarly, in panel (c), MgCl2 solutions of > 1M remove almost all SFG signal. At concentrations between 1E-4 M and 1E-1 M the Mg2+ ions have minimal effect. ............................................................. 20 Figure 3.3. Spectra of pure water and the salt solutions at ionic strength of six. The concentration of the salt solutions of the divalent cations (MgCl2 and CaCl2) is 2 M and the concentration of the salt solutions of the monovalent cation (NaCl and LiCl) is 6 M. Originally published in Lovering et al. (2016). .................................................................................................................. 21 Figure 3.4. Three possible structures of the Stern layer for the case of Mg cations.  (a) Both hydration shells of the silica and ions are lost, and Si-O− - M+ is formed. (b) The hydration shells of both the ion and the silica surface are intact. (c) The hydrating water molecules are shared between Si-O− and M+ and highly polarized O-H bonds are formed. Originally published in Lovering et al. (2016). .................................................................................................................. 24 Figure 3.5. The spectra of water with CaCl2 and LiCl at an ionic strength of 12, concentration of 4 M and 12 M, respectively. A spectrum of pure water and a spectrum of CaCl2 at ionic strength (I) of 6 (M=2) are included for reference. At the ionic strength of 12 the spectrum of CaCl2 is seen to be similar to the spectrum of the MgCl2 solution at an ionic strength of 6 (Figure 2). The LiCl solution still shows some ordered water structure, despite the high concentration of LiCl. Originally published in Lovering et al. (2016). ............................................................................ 27 Figure 4.1. SFG spectra taken at the water/silica interface during the freezing process. The average temperatures are listed in the legend. The temperature at the beginning and end of each spectral collection are (a) 16.4 to 13.7 , (b) 5.2 to 3.7, (c) -0.8 to -1.2, (d) 0.3 to -1.2, (e) -2.0 to -5.2, (f) -5.0 to -5.5, (g) -5.7 to -8.4 °C. Originally published in Lovering et al. (2017). ........ 32 xiii  Figure 4.2. Typical data sets collected during cooling experiments. Results are shown for silica taken with the IR laser parked at 3100 cm-1. Originally published in Lovering et al. (2017). ..... 33 Figure 4.3. The sharp increase and slower decrease of SFG signal at the silica surface during freezing when the IR beam is fixed at 3200 cm-1, 3100 cm-1, 3000 cm-1 and 2900 cm-1.  Temperature data is excluded for clarity. ..................................................................................... 33 Figure 4.4. The cooling experiment was additionally performed at PSP and SPP polarization combinations. ................................................................................................................................ 35 Figure 4.5. The onset freezing temperatures of the water are plotted in bins of one degree. The median freezing temperature is -3.3 °C while the average is -4.5 °C. Originally published in Lovering et al. (2017). .................................................................................................................. 36 Figure 5.1. Panel (a) shows how the SFG intensity at 3200 cm-1 of a 0.5 wt% solution of NaCl changes over time (lower x-axis) as the temperature decreases (upper x-axis). As the temperature of the 0.5 wt% solution is decreased, the SFG intensity remains constant until ice is formed. A red arrow indicates the onset of ice formation. In panel (b) spectra of a 0.5 wt% solution NaCl solution are taken across the OH vibrational range at different temperatures. Each spectrum was collected stepwise over five minutes and the temperature is changing throughout. Shown in the upper right hand corner of Panel b is the average temperature during spectra collection. Panels (a) and (b) are from different cooling experiments; the difference in temperature of ice formation is reasonable due to the stochastic nature of heterogeneous freezing. .......................................... 43 Figure 5.2. Panel (a) shows how the SFG intensity at 3200 cm-1 of a 1 wt% solution of NaCl changes over time (lower x-axis) as the temperature decreases (upper x-axis). As the temperature of the 1 wt% solution is decreased, the SFG intensity remains constant until ice is formed. A red arrow indicates the onset of ice formation.  A transient signal is also observed.  The transient xiv  peak is indicated with a black star. Below the eutectic temperature, NaCl·2H2O forms, resulting in a decrease in SFG intensity at 3200 cm-1, indicated by a blue arrow in panel (a). In panel (b) spectra of a 1 wt% solution of NaCl solution are taken across the OH vibrational range at different temperatures. Each spectrum was collected stepwise over five minutes and the temperature is changing throughout. Shown in the upper right hand corner of Panel b is the average temperature during spectra collection. The red spectrum indicates ice has formed while the blue spectrum clearly shows the formation of dihydrate. Panels (a) and (b) are from different cooling experiments; the difference in temperature of ice formation is reasonable due to the stochastic nature of heterogeneous freezing. ................................................................................ 45 Figure 5.3. Panel (a) shows how the SFG intensity at 3200 cm-1 of a 9 wt% solution of NaCl changes over time (lower x-axis) as the temperature decreases (upper x-axis). The onset of ice formation is indicated with a red arrow and below the eutectic temperature NaCl·2H2O forms, indicated by a blue arrow. In contrast to the 1 wt% solution, the SFG intensity at 3200 cm-1 decreases when ice forms. As a consequence of the lower ice intensity at this frequency, the SFG intensity at 3200 cm-1 increase when the dihydrate forms. In panel (b) spectra of a 9 wt% NaCl solution are taken across the OH vibrational range at different temperatures. Each spectrum was collected stepwise over five minutes and the temperature is changing throughout. Shown in the upper right hand corner of Panel b is the average temperature during spectra collection. The ice spectrum shown in blue has a lower intensity compared to the ice spectra in figures 2 and 3; however, the NaCl·2H2O spectrum is similar. Panels (a) and (b) are from different cooling experiments; the difference in temperature of ice formation is reasonable due to the stochastic nature of heterogeneous freezing. ................................................................................................. 47 xv  Figure 5.4. Phase diagram of NaCl solutions. The black dots and lines indicate the temperature of the phase change at equilibrium. Red squares are homogeneous freezing temperatures of ~5 to 25µm diameter droplets from reference 25. Blue circles indicate the formation of ice and green indicate the formation of NaCl·2H2O based on the SFG spectra. Error bars indicate the standard error of the mean of the observed phase change. Data points for 0, 1, 9, and 13 wt% are composed of ≥ 9 data points. Data points for 0.5, 2, and 5 wt % are composed of 3-5 data points........................................................................................................................................................ 49  xvi  List of Symbols  ?⃑? …………………..Induced polarization ?⃑? …………………..Electric field 𝜒(𝑖)…………………ith order electric susceptibility  𝛽…………………..hyperpolarizibility 𝜔𝑛…………………frequency xvii  List of Abbreviations  GC………………………….Gouy-Chapman Hz………………………….Hertz INP…………………………Ice Nucleating Particle IR…………………………..infrared mJ…………………………..miliJoule ps…………………………...picosecond SFG………………….…….Sum Frequency Generation SGC………………….……Stern-Gouy-Chapman SHG……………………….Second Harmonic Generation  xviii  Acknowledgements  Firstly, I want to thank my supervisors, Allan Bertram and Keng Chou. I am certain that without their kind guidance, I would have left graduate school before the end of my second year. They have been unflaggingly understanding and helpful. Thank you for always taking the time. I will also thank my supervisory committee: Professors Pierre Kennepohl, Ed Grant, and Dan Bizzotto. Thank you for your time and interest in my studies and success. A special thanks to Prof Kennepohl for reading my thesis. Of course, nothing could have been accomplished without the hard work of the mechanical shop, IT, the electronics shop, and the administration staff. A special thanks to Ken Bach for expertly crafting my flow cell. Also, Sherri Harbour lives up to her name and calmly provides shelter from the paperwork storm with her clear and executable instructions.  I would like to thank the members of the Bertram lab for their patience with my sudden appearances asking for the location of tools and/or computer cords. And of course for the cake. I am very lucky to have begun at the same time as Vickie. From the Chou group, I would like to express my gratitude to Dan for helping me transition into grad school and to my more recent group members, I thank you for your encouragement and help with physics.  I have had the pleasure of meeting many wonderful people in grad school. Rodrigo and Ryan, I am glad we had each other’s help while we adjusted. Through Rodrigo I also met Emmanuel and Montse, who provided me a second home until they went on to make new homes. Anytime you need a place to stay, you are welcome. Moon sisters are family. I am also happy to have met Amber, with such a keen sense of humor and adventure. Damon, best coffee buddy! I want to thank you for being so honest with me. Alison, there is no way to express how glad I am xix  that we became friends. I do not know a single person with such firm principles that are so suffused with generosity.   The university is also a prime setting to meet all sorts of scholars. I have truly enjoyed my interactions with the Geography and the STS departments. In particular, thank you to Prof Adam Frank, for taking me seriously. Thank you to Cat and Sage for the solidarity. And mostly thanks to Sophie and STEM Culture[s], for saving and inspiring me.   In the larger Vancouver community, I would like to thank Battered Women Support Services for existing and the staff for all their hard work. To my fellow volunteer trainees, thank you for sharing so much during our 12 weeks of learning. I am so grateful to Sina, Flora, and especially Jheanel for making the shifts lighter. I would like to thank every woman who called me while I was working for their resistance.  I also had the great pleasure of being part of the UBC Sailing Club. I have learned more than just how to sail and met so many amazing people. Megan, Lisa, Eammonn, Robyn, Nathan, Pierre, Tony. I am so glad that I had all of you to help me relax and enjoy the beauty in Vancouver.   So many people have given me support who do not live in Vancouver. There is inexplicable happiness in me that I can count Kate, Katie, Lindsey, Kathleen, Han, Jenny, Allison, Nikki, Yael, Carson, and Mike as forever friends, having already proven ourselves over space and time.   To my mom and dad: Thank you so much for being proud of me. It makes me so happy. I am so proud of both of you as well. You are both so generous without even thinking about it. I hope I can be like you. Dearest Grant, you helped me become who I am today. I look forward to seeing who we will be later. xx  Dedication  To Joan Mason1  Chapter 1: Introduction  Mineral/aqueous interactions are extremely common; rocks, water, and dissolved ions are part of terrestrial, marine, and atmospheric environments (Azam et al., 2012; Dewan et al., 2013; Jubb et al., 2012; McCrum et al., 2015; Sipila et al., 2015; Yang et al., 2011; Q. Zeng et al., 2015).  These interactions can have large impacts on environmental processes. In the atmosphere, mineral/aqueous interactions are important to our understanding of the earth’s radiative budget (Carslaw et al., 2013; Koren et al., 2014; Stevens et al., 2009).   In terrestrial environments dissolved ions affect rock stability and increase mineral weathering (Dove, 1999; Dove et al., 1997; Icenhower et al., 2000; Karlsson et al., 2001). Prolonged exposure between minerals and water in subglacial lakes generates dissolved species that enter polar oceans, linking marine and terrestrial environments (Hawkings et al., 2017; Michaud et al., 2016). For example, the dissolution of silicate minerals, catalysed by alkali metals, increases the amount of dissolved silica in polar oceans, possibly contributing to blooms of diatoms (Fairchild et al., 1999; Michaud, et al., 2016). The glacier/mineral interface is also affected by dissolved ions (Knight, 1997). These chemical impurities change the stress response of glacial ice and collect at grain boundaries, which subsequently affect glacial creep (Fritz et al., 2011; Hammonds et al., 2016). In order to understand all these environmentally relevant chemical and physical processes it is necessary to fully characterize how ions, water, and minerals mutually influence each other and how these interactions are affected by changes of temperature. Below the atmospheric relevance of these interactions is described in more detail in order to illustrate the importance and complexity of mineral/aqueous interfaces before, during, and after freezing. 2  1.1  Ice Nucleation in the Atmosphere Ice formation is a kinetic non-equilibrium process; though the freezing point of ice is 0 °C at 1 atm pressure, water often remains in the liquid phase well below this temperature. This metastable liquid phase is referred to as supercooled water (Murray et al., 2012). Supercooling is common in nucleation events because the molecules must arrive, via random fluctuations, in a solid-like configuration in order to solidify. The strong interactions between water molecules enable water to supercool significantly. In terrestrial environments, water is in contact with solid surfaces that prevent water from supercooling extensively. A solid surface lowers the barrier to ice formation; this freezing process is called heterogeneous ice nucleation. In the atmosphere, however, water droplets are suspended in air and do not necessarily contain a solid surface. Water droplets in the atmosphere that freezes homogenously, that is without coming into contact a solid surface, can remain in the liquid phase at temperatures below -30 °C (D. Rosenfeld et al., 2000). Solid, insoluble aerosol particles can, however, increase the freezing temperature of droplets within clouds. Solid particles that promote freezing are referred to as ice nucleating particles (INPs). These particles have diverse origins and play an important role in climate processes. 1.1.1 Ice Nucleating Particles INPs can originate from biological, mineralogical, and anthropogenic sources (Kanji et al., 2017; Murray, et al., 2012). While it is clear from experimental data that solid surfaces help water freeze at temperatures warmer than homogeneous freezing temperatures, (Murray, et al., 2012) the mechanism is unclear. It is logically assumed that a solid surface promotes ice formation by aligning the liquid phase into a solid-like structure; however, the properties of the solid surface and the nature of the molecular interactions that occur to promote ice nucleation are poorly 3  understood. Surface charge, geometric alignment, hydrogen bonding, and the presence of active sites are all hypothesized to be responsible for the solid surface induced formation of ice (Pruppacher, 1997). These surface properties are thought to help ice formation by increasing the likelihood that water molecules form an ice-like structure. For example, silver iodide was one of the first inorganic INPs to be identified; ice and the AgI surface have a similar geometry (Vonnegut, 1947). Other inorganic particles also act as INPs though they do not have the same crystallographic alignment. Simulations of the ice nucleation ability of kaolinite, alumino-silicate clay, indicate that hydrogen bonding interactions catalyse ice formation (Zielke et al., 2016). Due to the diversity of INPs and the complexity of ice formation, it is difficult to generalize observations of INP efficiency. It is consequently difficult to incorporate INPs into atmospheric models. INPs are statistically rare in the atmosphere, however, ice formation often leads to precipitation and INPs play a large role in the hydrological cycle (Murray, et al., 2012). Additionally, ice formation effects the lifetime and albedo of clouds, impacting the radiative budget of the earth (Cziczo et al., 2000; DeMott et al., 2010). INPs can come from biological(Despres et al., 2012; MatthiasMaser et al., 1995), marine (Alpert et al., 2011a), and terrestrial sources (Bingemer et al., 2012). Though biological particles promote ice formation at the warmest temperatures (Murray, et al., 2012), mineral dust accounts for around 50% of solid particles found within ice particles collected from the atmosphere (Pratt et al., 2009). Mineral dust from deserts can be lofted into the air during wind storms and be transported over long distances (Glaser et al., 2015; Kalenderski et al., 2016; McKendry et al., 2007). 4  1.1.2 Ice Nucleation Pathways As mentioned above, ice nucleation that is facilitated by the presence of an INP is called heterogeneous nucleation. There are three distinct pathways of heterogeneous nucleation (Figure 1.1) (Murray, et al., 2012).     Figure 1.1. Heterogeneous freezing, in which a solid phase catalyzes ice nucleation, can occur via three pathways.  Deposition freezing occurs when the presence of an INP allows water vapor to transitions immediately to ice. Contact freezing occurs when an INP makes contact with a droplet of supercooled water resulting in the immediate phase transition of that droplet. Immersion freezing occurs when a super cooled droplet of water containing an INP freezes. The conditions under which each of these nucleation modes is active is an ongoing research topic. During the long-range transport of mineral dust, increases in relative humidity will coat the dust particle in water and play an important part in ice formation in midlevel clouds (Murray, et al., 2012). Additionally, the dust particles can become coated with ions from marine 5  environments or by condensation of sulfuric acid, nitric acid, and ammonium sulfate from the gas phase (Alpert, et al., 2011a; Alpert et al., 2011b; Cziczo et al., 2009; Girard et al., 2013; Kulkarni et al., 2014; Kulkarni et al., 2015; Zabori et al., 2012). 1.1.3 Effect of Ions The effect of ions on homogeneous freezing of ice has been parameterized using just water activity. Koop et al. show that, no matter the nature of the ion, the freezing of the liquid droplet is merely offset from the thermodynamic equilibrium by a constant amount in change of water activity of the droplet (Koop et al., 2000). In the atmosphere, water activity is equal to relative humidity divided by 100 because the vapor of the droplet is in equilibrium with the surroundings. The ability to use a bulk quantity to parameterize ice freezing is interesting given that ions are known to disturb the hydrogen bonding network of water differently (Robinson et al., 1941; van der Vegt et al., 2016).  Although surfaces inherently lack bulk properties, it has also been suggested that water activity can be used to parameterize heterogeneous nucleation in the immersion mode (Alpert, et al., 2011a; Knopf et al., 2013; Knopf et al., 2011; Koop et al., 2009; Q. Zeng, et al., 2015; Zobrist et al., 2008). The fit for heterogeneous ice nucleation is, however, not as convincing as homogeneous nucleation and two recent experimental papers call into question the general applicability of this parameterization for heterogeneous freezing (Kumar et al., 2018; Whale et al., 2018). Moreover, because INPs come from many sources, they have diverse architectures and may use different types of molecular interactions to catalyse ice formation. Consequently, additional research to connect the molecular level interfacial interactions to the bulk properties is necessary.  6  1.2 Probing Buried Interfaces Although the understanding the structure of water at the mineral/aqueous interface is important for several fields, studies on this topic are very limited. Obtaining molecular level information on the unique environment of surfaces is difficult because the bulk, due to its higher volume, generates significantly more signal and consequently drowns information specifically attributable to the molecules at the surface. Consequently, to obtain surface specific information, experimental techniques that only probe the surface must be used. Many of these techniques require ultra-high vacuum, and, though recent advances allow higher pressure systems, are unsuitable to monitor mineral/aqueous interfaces (Hao et al., 2016; Jiang et al., 2010; Velasco-Velez et al., 2015; Velasco-Velez et al., 2014; Weatherup et al., 2016). Additionally, high energy electromagnetic radiation, such as the x-rays often used is surface sensitive measurements, cannot be used to observe the hydrogen bonding network of water. To overcome these issues, we have used sum frequency generation (SFG).  The theory and experimental technique will be described in detail in the next chapter. Briefly, SFG requires that two electromagnetic fields overlap in space and time. The second-order material response is thus proportional to the square of the fields and, if the material is centrosymmetric, the second order response is zero under the electric dipole approximation. As a surface or interface necessarily lacks inversion symmetric, the interfacial region has a nonlinear response while the bulk is invisible. In SFG spectroscopy, an IR and visible beam are used to probe the interface. The IR beam can resonate with the vibrational modes of water molecules and give molecular level information about the hydrogen bonding network. 7  1.2.1 Silica/Aqueous SFG Spectra SFG was developed in the 1990s and many studies of the silica/aqueous interface have been performed. In contrast to linear spectra, the SFG spectrum of water shows two peaks, one around 3400 cm-1 and one around 3200 cm-1. The two peaks are present at both air/water and silica/water interfaces. The origin of these two peaks is a subject of ongoing debate. Second-harmonic generation (SHG) has been used to show that there are two different types of silanol sites with pKa values of 4.5 and 8.5 (Ong et al., 1992). A SFG study by Ostroverkhov et al. suggested that these two deprotonation sites were related to the two water species seen as bands near 3200 and 3400 cm-1, referred to as ‘ice-like’ and ‘liquid-like’ water peaks, respectively (Ostroverkhov et al., 2005). A  recent study by Myalitsin et al. showed that the water molecules at the interface experience  a broad continuum of stretching modes and the shape is due to vibrational coupling, not the presence of different hydrogen bonding configurations (Myalitsin et al., 2016). The effect of ions on the silica/mineral interface at room temperature has also been explored, particularly at low concentrations. As the silica surface has a negative charge when the pH is above around 2, cations cause a reduction in SFG intensity, though the extent of signal reduction is specific to the cation (Azam, et al., 2012; DeWalt-Kerian et al., 2017; Flores et al., 2012; Yang, et al., 2011). The SFG spectrum of hexagonal ice at silica has a higher intensity than the liquid spectrum (Figure 1.2). The higher intensity of ice may be due to dipole-dipole coupling (Shultz et al., 2014), charge transfer to the H-bond partner (Ishiyama et al., 2012), or coordinated motion (Raghunathan et al., 2011). While many studies that focus on the air/ice interface (Groenzin et al., 2007; Nojima et al., 2017; Otsuki et al., 2017; Schaefer et al., 2017; Smit et al., 2017; Smit et al., 2018), considerably fewer have been performed on the silica/ice interface.  8   Figure 1.2. Spectrum of liquid water (red) and hexagonal ice (blue) taken at the interface with silica. The water spectrum was taken at 22 °C, and the ice spectrum was taken at -6 °C roughly 30 min after freezing. Wei et al. monitored the melting of ice next to a silica surface (Wei et al., 2002). As the ice melted, the SFG intensity passed through a minimum. The intensity minimum was explained by water molecules at the silica surface reorienting during the phase change. No studies have been performed on the effect of ions on the silica/ice interface; however, work on the sapphire/ice interface suggests that specific ion effects are important at mineral/ice interfaces (Emmanuel Anim-Danso et al., 2013; E. Anim-Danso et al., 2016; Zhang et al., 2014).  1.3 Overview of Thesis In Chapter 2 the theory of SFG and experimental method for studying the structure of water at the mineral/aqueous interface is presented.  In Chapter 3, the structure of water at the silica/aqueous interface is discussed at room temperature. Cation specific effects of alkali and alkali earth metals on the water structure at the interface are also discussed. In Chapter 3, as well as the other research chapters, we use silica to represent the mineral surface.  Silica is a major component of mineral dust in the atmosphere and in the earth’s crust and is an appropriate 9  starting point for studying the mineral/aqueous surface.  In chapter 4 SFG is used to monitor the change in water’s hydrogen bonding network during freezing. In chapter 5 the impacts of sodium chloride on the hydrogen bonding network of water at the silica surface before, during, and after freezing are explored. 10  Chapter 2: Experimental setup  Surfaces and interfaces are usually the minority component of a system and contributions from the surface will be negligible compared to those of the bulk using most measurement techniques. Consequently, in order to obtain information about a surface or interface, the experimental technique must specifically probe the surface or interface. Surface science has traditionally relied on vacuum conditions to examine the molecular structure of surfaces and though advances have allowed for higher pressures, measurement of buried liquid/solid interfaces remains a challenge (Jiang, et al., 2010; Velasco-Velez, et al., 2015; Velasco-Velez, et al., 2014). Nonlinear spectroscopy is one of the few techniques capable of probing these buried interfaces (Shen, 1989; Shen et al., 2006). Under the electric dipole approximation, a second-order nonlinear optical processes only occur where inversion symmetry is broken. Discontinuities at interfaces by necessity break the centrosymmetry of bulk materials. In nonlinear spectroscopy, two laser beams are overlapped in space and time at a surface and the nonlinear response is collected. 2.1 Sum frequency generation spectroscopy Sum frequency generation (SFG) spectroscopy is a second-order optical process in which one of the impinging beams is in resonance with the vibrational stretches of the material. SFG gives information about the second order susceptibility of the material.  The susceptibility of a material relates the induced polarization to the applied electric field such that     ?⃑⃑? 𝒕𝒐𝒕𝒂𝒍 ∝ 𝝌(𝟏)?⃑⃑? +  𝝌(𝟐)?⃑⃑? 𝟐 + 𝝌(𝟑)?⃑⃑? 𝟑 + ⋯                                 (2.1)  11  where ?⃑? 𝑡𝑜𝑡𝑎𝑙 is the induced polarization of the material, ?⃑?  is the applied electric field and χ(i) is the ith-order electrical susceptibility. In most spectroscopic techniques only the linear relationship between induced polarization and the electric field is considered. In SFG spectroscopy, however, the second-order susceptibility, 𝜒(2), is probed.  Probing χ(2) requires two electric fields. In SFG the electric fields have different frequencies: one in the visible region and one in the IR region.                             ?⃑⃑? (𝟐)(𝝎𝑺𝑭𝑮 = 𝝎𝑰𝑹 + 𝝎𝑽𝑰𝑺) = 𝜺𝒐𝝌(𝟐)?⃑⃑? (𝝎𝑰𝑹)?⃑⃑? (𝝎𝑽𝑰𝑺)                                     (2.2)  where ?⃑? (2)is the second-order polarization of the material, 𝜔𝑖 refers to a frequency whose range is specified by the subscript, 𝜀𝑜 is the vacuum permittivity, and ?⃑?  is an electric field oscillating at the specified frequency.  The 𝜔𝐼𝑅 beam allows transitions between vibrational states and the 𝜔𝑣𝑖𝑠 beam allows electronic transitions to produce the SFG signal. When 𝜔𝐼𝑅 is in resonance with the vibrational modes of a molecule, the SFG signal is larger and information about the molecular structure of the surface can be studied.  2.1.1 Surface Sensitivity The depth of the interfacial region is specific to each system; a surface that is able to induce long-range order will have contributions from more molecules than a surface that induces less order. SFG spectroscopy does not give information about the number of layers of molecules that comprise a surface. The minimum depth of SFG probe can be as small as several monolayers (Otsuki, et al., 2017). The maximum depth of SFG spectroscopy, in reflection geometry, is one wavelength of the visible light. 12  2.2 Second Order Susceptibility Tensor The χ (2) response is a material property and is made up of the hyperpolarizability of the composite  molecules, 𝛽                                𝝌(𝟐) = 𝑵 < 𝜷 >                                                                         (2.3) where N is the number of contributing molecules and the angular brackets denote an average orientation.  The second order susceptibility is a tensor composed of 27 terms. The 27 terms arise because the material response can occur along the x-,y-, and z-axes and the response can be written as 𝜒𝑖𝑗𝑘(2). The 27 components are the possible combinations, shown below, of axial responses of the material.                                  𝒙𝒙𝒙 𝒙𝒚𝒙 𝒙𝒛𝒙𝒙𝒙𝒚 𝒙𝒚𝒚 𝒙𝒛𝒚𝒙𝒙𝒛 𝒙𝒚𝒛 𝒙𝒛𝒛     𝒚𝒙𝒙 𝒚𝒚𝒙 𝒚𝒛𝒙𝒚𝒙𝒚 𝒚𝒚𝒚 𝒚𝒛𝒚𝒚𝒙𝒛 𝒚𝒚𝒛 𝒚𝒛𝒛    𝒛𝒙𝒙 𝒛𝒚𝒙 𝒛𝒛𝒙𝒛𝒙𝒚 𝒛𝒚𝒚 𝒛𝒛𝒚𝒛𝒙𝒛 𝒛𝒚𝒛 𝒛𝒛𝒛                                       (2.4) In a laboratory coordinate system, the interface is parallel to the xy-plane while the incident plane is the xz-plane (Figure 2.1). Many of the tensor elements are not active under inversion. For example, during inversion 𝜒𝑥𝑥𝑦 → 𝜒−𝑥−𝑥−𝑦 → −𝜒𝑥𝑥𝑦. Mathematically, the 𝜒𝑥𝑥𝑦 must go to zero during inversion. Elements of the tensor shown in blue are similarly inactive. Additionally, when the xy-plane is isotropic the elements shown in green are also inactive (Yan et al., 2014). 13   Figure 2.1. Laboratory coordinate system of the SFG experiment. The xy-plane contains the interface and the xz-plane contains the beams. The IR beam is indicated in red, the visible beam is green and the resultant SFG beam is shown in purple. Isotropy of the interface in the xy-plane also means many of the elements are equivalent. Specifically, 𝜒𝑥𝑥𝑧 = 𝜒𝑦𝑦𝑧, 𝜒𝑥𝑧𝑥 = 𝜒𝑦𝑧𝑦, and 𝜒𝑧𝑥𝑥 = 𝜒𝑧𝑦𝑦. The component 𝜒𝑧𝑧𝑧 is also active. Thus there are four unique, active elements of the 𝜒(2) tensor for an interface with an isotropic surface (xy-plane). If the xy-plane is anisotropic or the interfacial region is chiral the elements shown in green in equation 2.4 are also active. 2.2.1 Probing Different Tensor Elements The polarization of the two electromagnetic fields affects the tensor element that is probed. By convention, polarization is defined with respect to the surface normal. Parallel or p-polarized light has an electric field parallel to the plane of incidence (see Figure 2.1). Perpendicular or s-polarized light has the electric field perpendicular to the plane of incidence. A p-polarized impingent beam will generate a response along the z- and x-axes while s-polarization causes a response along in the y direction. The polarization of the visible and IR beams can be selected to probe different elements of the susceptibility; the polarization of output beam is also selected.  14  The polarization combinations are listed in the order of SFG, Vis, and IR. For example, an SSP combination indicates that the SFG beam has s-polarization, the visible beam has s-polarization, and the IR beam has p-polarization. This combination probes the 𝜒𝑥𝑥𝑧 = 𝜒𝑦𝑦𝑧 tensor element. Table 1 shows the polarization combinations that probe active elements of the second order susceptibility tensor. Table 2.1. The polarization combinations used in SFG to probe active tensor elements. Polarization Combination Active Tensor Element Probed SSP 𝜒𝑥𝑥𝑧 = 𝜒𝑦𝑦𝑧 SPS 𝜒𝑥𝑧𝑥 = 𝜒𝑦𝑧𝑦 PSS 𝜒𝑧𝑥𝑥 = 𝜒𝑧𝑦𝑦 PPP 𝜒𝑧𝑧𝑧 PSP, SPP, and PPS Chiral elements  2.3 Experimental Design An Nd:YAG (neodymium:yttrium aluminum garnet) laser with an output wavelength of 1064 nm (20 ps, 40 mJ/pulse, 10 Hz) and second harmonic beam at 532 nm generated in a KTiOPO4 (KTP) crystal. A tunable IR beam was produced by difference frequency mixing of the 1064 nm beam with the 532 nm beam in an optical parametric generator/amplifier (OPG/OPA). The SFG output beam was collected by a photomultiplier tube. This experimental setup measures the intensity of the SFG output, 𝐼𝑆𝐹𝐺 ∝ |𝜒(2)|2𝐼𝐼𝑅𝐼𝑉𝐼𝑆.  15  2.3.1 Beam Alignment The visible and IR beams were overlapped in space and time at the base of an equilateral IR grade silica prism. The IR beam has an incident angle of 60 degrees and the visible beam has an incident angle of 65 degrees. Unless otherwise specified, experiments were performed with a SSP polarization combination. 2.4 Surface Preparation An equilateral IR-grade fused silica prism was washed with Extran AP12 cleaning agents, rinsed with Millipore water (resistivity > 18.2 MΩ·cm), and left to soak in concentrated sulfuric acid containing NOCHROMIX (Godax Laboratories, Inc. USA) for several hours. The prism was subsequently rinsed with Millipore water several times, soaking in the Millipore water for at least thirty minutes between each rinse. To prevent exposure to air during the course of the experiment, the prism was attached to a flow cell that allowed the addition of each solution without removing the prism. 2.5 Surface Cooling In order to study the effects of temperature on the water/mineral surface, a copper block that fits over the prism was constructed. The block was cooled by a circulation chiller that circulates cold octamethyltrisiloxane through the block. Holes were drilled into the block to allow the ingress and egress of photons (Figure 2.2). Dried N2 gas was purged through the holes to prevent water condensation on the prism surface. The temperature was measured by a thermocouple (Omega, RTD PT100) placed within a hole drilled one millimeter from the surface of the prism. 16   Figure 2.2. Experimental apparatus. A copper cooling block was used to cool the silica prism. The prism is attached to a Teflon flow cell, and water is introduced to the flow cell by opening the inlet valve. The laser beams are aligned to overlap at the base of the prism. The temperature of the prism is recorded by a thermocouple inserted into a hole near the base of the prism. Originally published in Lovering et al. (2017).   17  Chapter 3: Ions at the Silica Surface at Room Temperature  3.1 Background Interactions between ions and the silica surface are not well understood. When ions are present in water, the columbic interaction between the negatively charged silica surface and the cations perturbs the ordered water structures (Flores, et al., 2012; Jena et al., 2009; Yang, et al., 2009). While the intensity of the SFG signal is usually reduced in the presence of salts, the extent of reduction depends on the concentration of the salts and the specific effects of each ion (Darlington, et al., 2017; Schaefer, et al., 2017).  As silica has a surface charge (Barisik et al., 2014), double layer models are often used to understand the effects of ions (Jena, et al., 2009). At concentrations less than ~ 0.1 M the Gouy-Chapman (GC) model can be used (Crow, 1994). The GC model predicts that the surface potential decreases exponentially from the surface, and the thickness of the diffuse layer is characterized by the Debye length, which is a function of the ionic strength. Ionic strength takes into account both the concentration and charge of every ion present in solution and is calculated as 𝐼 =12∑ 𝑐𝑖𝑧𝑖2𝑛𝑖=1  where c is concentration and z is charge of the ion. The GC model treats ions as point charges and fails at high ion concentrations because it over predicts the number of ions that are capable of approaching the surface. At high concentrations, the Stern-Gouy-Chapman model (SGC), a modification to the GC model, has been used to explicitly account for ion-surface interactions. In the SGC model, ions at the surface are given a finite radius and occupy the Stern Layer, while ions in the diffuse layer are accounted for with the GC model, as shown in Figure 3.1.   18   Figure 3.1. Schematic representation of the Stern-Gouy-Chapman model. The Stern layer consists of ions specifically interacting with the surface. Beyond the Stern layer the ions are distributed in a diffuse Gouy-Chapman layer. Originally published in Lovering et al. (2016). The inclusion of the Stern layer, however, does not provide insight into the specificity of ion interactions, the mechanism of these interactions, or the structure of water in this layer. Furthermore, although many SFG studies examine the signal of water at water/mineral interfaces, the mechanism of the ion-solid interaction in the Stern layer has not been clearly understood (Azam, et al., 2012; Dewan, et al., 2013; Ruiz-Agudo et al., 2011). A study by Eftekhari-Bafrooei et al. measured the ultrafast vibrational relaxation of water at the water/silica interface and suggested that, for NaCl concentrations greater than 10-2 M, the hydrated cations accumulated near the silica surface with one or more layers of water between the silica and cations. These sandwiched aqueous layers dominated the observed vibrational relaxation dynamics (Eftekhari-Bafrooei et al., 2011). In addition, Dewan et al. (2013) carried out molecular dynamics (MD) simulations of alkali chloride solutions to study the structure of water in the solvation shell of the silica surface. Dewan et al. found that, in addition to a diffuse region consistent with the GC model, there was a “compact layer” of water next to the silica surface that was not described by standard electrical double layer theories (Dewan et al., 2014). Interestingly, 19  the depth of the “compact layer” was found to be different for Na+ vs Cs+ while the depth of the diffuse layer was independent of the identity of the cation screening the charge. The MD simulation suggested that the structure of water in the Stern layer (or the “compact layer”) depended on the specific ion properties but direct experimental observation of these specific ion effects in the Stern layer is lacking. To address this deficiency, we carried out a SFG study at the water/silica interface using solutions with high concentrations of ions.  At the high concentrations used, ions readily partition to the silica surface, surface charge screening is maximized, and the thickness of the diffuse layer is minimized. Consequently, the SFG signal from ordered water in the diffuse layer is depressed, and the SFG signal from the Stern layer dominates. The studies were carried out with solutions of NaCl, LiCl, CaCl2 and MgCl2, and the effect of the cations was compared. The experiments allowed us to investigate how the structure of water in the Stern layer depends on the identity of the cation and to infer the nature of the mineral-cation interaction.  3.2 Experimental The solutions used for the experiment were prepared using Millipore water, and the salts were purchased from Sigma Aldrich at the highest available purity: ammonium chloride 99.5%+, sodium chloride ≥99.5 %, calcium chloride ≥ 96 %, and magnesium chloride ≥98 %. All glassware used in the preparation of the solutions was cleaned using the method described for the surface preparation. 3.3 Results and Discussion Spectra as a function of concentration were recorded for NaCl, CaCl2, and MgCl2 aqueous solutions In Figure 3.2 we see that the behavior of NaCl, CaCl2, and MgCl2 solutions at high concentrations are different. The ability of NaCl to disrupt water ordering at the silica surface 20  saturates at ~1 M. The MgCl2 solutions also show little change in ability to disrupt water ordering at the surface past 1 M; from 1 M upwards, the peak centered at 3200 and 3400 cm-1 are not observed. The small remaining peak at ~3500 cm-1 is thought to originate from silanol groups on the silica surface that are not able to participate in hydrogen bonding with the liquid. The disappearance of the water OH stretching peaks indicates that the water structure at the surface is significantly perturbed.  The CaCl2 solutions also have a continuous decrease in intensity of the 3200 and 3400 cm-1 peaks, indicative of de-structuring effects of the ions.   Figure 3.2. Various concentrations of alkali and alkali earth chloride salts. In (a) NaCl solution concentrations increase from 1E-4 M to 6 M. Increasing NaCl concentration causes a progressive decrease in SFG intensity until around 1 M. Above 1 M, the ability of NaCl to effect the SFG signal seems to saturate. In panel (b) CaCl2 solutions increase from 1E-4 M to 4 M. The Ca2+ ions also reduce SFG signal progressively with increasing concentration, however, the effect does not saturate until > 3 M. Above this concentration, no water signal is observed. Similarly, in panel (c), MgCl2 solutions of > 1M remove almost all SFG signal. At concentrations between 1E-4 M and 1E-1 M the Mg2+ ions have minimal effect. 21  3.3.1 Comparison of Ionic Strength To demonstrate that the structure of water near the silica surface is not solely determined by the ionic strength, Figure 3.3 shows the SFG spectra of aqueous/silica interfaces with NaCl, LiCl, CaCl2, and MgCl2 at an ionic strength of 6. The ionic strength corresponds to salt concentrations of 2 M for the solutions of the divalent cations (MgCl2 and CaCl2) and 6 M for the solution of the monovalent cations (NaCl and LiCl), which is the saturation concentration of NaCl at 20 ºC. A SFG spectrum of pure water at the water-silica interface is included for reference. In addition to the peaks at 3200cm-1 and 3400 cm-1 discussed previously, there is a smaller peak at ~3600 cm-1. This peak has been attributed to OH groups on the surface of the silica that weakly interact with the solution or an asymmetric OH stretch (Du et al., 1994; Jena, et al., 2009). These are likely OH groups sitting on small surface detect sites as this peak is correlated with surface roughness and is consequently less affected by the addition of ions as these water molecules are less accessible to attenuation.   Figure 3.3. Spectra of pure water and the salt solutions at ionic strength of six. The concentration of the salt solutions of the divalent cations (MgCl2 and CaCl2) is 2 M and the concentration of 22  the salt solutions of the monovalent cation (NaCl and LiCl) is 6 M. Originally published in Lovering et al. (2016). The spectra of NaCl and LiCl in Figure 3.2 support the prediction by Dewan’s MD simulations showing that the structure of water in the Stern layer near the silica surface is dependent on the identity of the monovalent cation. The spectra of MgCl2 and CaCl2 indicate that specific ion effects also occur with divalent ions. The difference between the spectrum of NaCl and MgCl2 is striking. While an ordered water layer was still observable at 6M of NaCl, 2 M of MgCl2 nearly eliminated the ordered water structure at the surface.  In the following we first propose a mechanism to explain the elimination of the ordered water structure at the surface for 2 M of MgCl2, and then we show that this mechanism is consistent with the trends observed for the different cations.   In one of the few other SFG studies that have been carried out to study water structures in solutions with high ionic strength, Yang et al. observed that a high concentration of sulfuric acid caused the disappearance of the ordered water structure on mica (Yang, et al., 2011). It was thought that the sulfate ions tie up all the free water molecules at the surface, essentially dehydrating the silica. In Yang’s study, the addition of sulfuric acid neutralized the surface charge of the mineral surface. The neutralized mineral surface binds water less tightly and the sulfate ions are able to fully out-compete the mica for solvating water molecules. In the current study, the addition of MgCl2, or any of the salts studied, does not significantly alter the pH, and the silica surface remains negatively charged. It is consequently less likely that the silica surface would completely give up its solvation water molecules.  Another possible but unlikely mechanism to explain the near elimination of the ordered water structure at the surface for 2 M of MgCl2 is the higher charge density cation Mg2+ is able 23  to penetrate the solvation shell of the charged silica surface and directly bind with Si-O−, as shown in Figure 3.4a. However, direct binding between Si-O− and Mg2+ is unlikely because Mg2+ ions have large enthalpies of hydration (Jalilehvand et al., 2001; Jiao et al., 2006). It is improbable that the associated water molecules would dissociate spontaneously from the ions. Although the interaction between cations and SiO- is attractive, experimental evidence indicates that a direct interaction between the ion and surface cannot compensate for the loss of the also favorable solvent-ion interactions. Indeed the retention of the solvation shell and outer sphere interactions, in which the solvation shells of the ion and surface interact rather than the ion and surface directly, has been reported (Dewan, et al., 2014; Nihonyanagi et al., 2014; Ricci et al., 2013). Additionally, it has been shown that the dissolution of silica is faster in the presence of Na+ ions than in the presence of Mg2+ ions (Dove, et al., 1997; Wallace et al., 2010). If Mg2+ ions were forming a chemical bond with the surface, the opposite would be true because the additional charge density of the Mg2+ ion would weaken the Si-O bond more than the Na+ ion, making that bond more vulnerable to nucleophilic attack (Wallace, et al., 2010). Indeed, Dove et al. state that ‘the ability of cations to promote negative charge development on silica surfaces is directly correlated with the solvent-structuring character of the cation’ (Dove, et al., 1997). For these reasons, it is unlikely that the disappearance of the water peaks can be attributed to direct binding of the ions to the silica surface. While direct binding is unlikely, the structure in the Stern layer in the presence of Mg2+ cannot be as simple as the solvated ions interacting with solvated silica surface, as shown in Figure 3.4b, since this case should produce oriented water molecules between silica and cations and give substantial signal in the SFG spectrum and cannot explain how the ordered water is eliminated in MgCl2 solutions.    24    Figure 3.4. Three possible structures of the Stern layer for the case of Mg cations.  (a) Both hydration shells of the silica and ions are lost, and Si-O− - M+ is formed. (b) The hydration shells of both the ion and the silica surface are intact. (c) The hydrating water molecules are shared between Si-O− and M+ and highly polarized O-H bonds are formed. Originally published in Lovering et al. (2016). A possible explanation for the lack of ordered interfacial water structure in the presence of Mg2+, despite retention of the solvation shell by the cations, is that the surface deprotonated silanol and Mg2+ cation can be considered to be an ion-pair with a water molecule as an intermediary; a water molecule between the surface and the ion undergoes ‘localized hydrolysis’ in which the hydronium ion is shared (Robinson et al., 1949). Robinson and Harned first described this water mediated ion-pair in order to explain the reversal of chemical activity trends for the alkali metals in the presence of different anions (Robinson, et al., 1941). In this model, as illustrated in Figure 3.4c, the ability of the cation to polarize water leads to the localized (or 25  partial) hydrolysis of water and concomitant association of the localized hydrogen ion with a strong proton acceptor, schematically depicted for our case as Si-O− -- H+-- OH−-- Mg2+. The dipole of water in gas phase is 1.8 D, and its value increases significantly in the hydration shells of ions. With 6 water molecules in the first hydration shell, Bucher et al. reported that the dipole of water is 2.35 D for K+ and 3.06 D for Ca2+ (Bucher et al., 2008). Near a neutral pH, 15-20% of the surface silanol groups are deprotonated (Duval et al., 2002), and this concentration is sufficient for significant sharing of the partially hydrolyzed water between the cations and SiO−. Water in the hydration shell has been shown to be vibrationally decoupled from its neighbors (Ahmed et al., 2014). The vibrational resonance of [O−-- H+-- OH−] is expected to be low, as previous studies by Diken et al., using argon predissociation spectroscopy, showed the vibrational mode of [HO--H--OH]− is located at 697 cm-1 (Diken et al., 2005), which is outside the detection range of SFG. 3.3.2 Confirmation of Localized Hydrolysis The ion-pair-induced ‘localized hydrolysis’ model proposed by Robinson and Harned predicts that the equilibrium of the localized hydrolysis should be determined by the acid dissociation constant (pKa) of the ion. Table 3.1 shows that the order of pKa for the cations is Mg2+ < Ca2+ < Li+ < Na+. The spectra in Figure 3.3 are reasonably consistent with this trend with the amount of structured water in the spectrum having a trend of Na+ > Li+  Ca2+ > Mg2+.  This order exchanges the position of Ca2+ and Mg2+ from the order commonly given for the Hofmeister series (Cacace et al., 1997; Marcus, 2009).    26   Table 3.1. The metal cations facilitate the hydrolysis of water. The ability of the metal cation to perturb surface water structure follows the trend in decreasing pKa for the hydrolysis reaction. Ion Radius(Haynes et al., 2014) (pm) Acid dissociation constant, pKaa Na+ 116 14.7 Na+(OH2)n ↔Na(OH2)n-1OH + H+ Li+ 90 14.2 Na+(OH2)n ↔Na(OH2)n-1OH + H+ Ca2+ 114 13.4 Ca2+(OH2) ↔[Ca(OH2)n-1OH]+ + H+ Mg2+ 86 12.5 Mg2+(OH2) ↔[Mg(OH2)n-1OH]+ + H+ a (Burgess, 1978) To further test if the localized hydrolysis mechanism is consistent with the SFG spectra, we carried out SFG spectra with 12 M of LiCl (I = 12) and 4 M of CaCl2 (I = 12), shown in Figure 3.5.  Experiments with concentrations of NaCl beyond 6 M were not possible as the solution was saturated at this concentration. Figure 3.5 illustrates that the amount of ordered water was Li+ > Ca2+ at an ionic strength of 12, consistent with the trends in the pKa values given in Table 3.1.  At 12 M (mole fraction of LiCl ~ 0.18), there are on average only 5-6 water molecules available to solvate one pair of Li+ and Cl−.  Even at this extremely high concentration, ordered water structures could still be observed, suggesting that Li+ ions are not able to interact with the solvation water of the silica surface in the way Mg2+ ions can at a much lower concentration. On the other hand, the spectrum of CaCl2 solution with I = 12 (Figure 3.5) shows that higher concentration of Ca2+ was able to nearly destroy the solvation water on silica surface.  27    Figure 3.5. The spectra of water with CaCl2 and LiCl at an ionic strength of 12, concentration of 4 M and 12 M, respectively. A spectrum of pure water and a spectrum of CaCl2 at ionic strength (I) of 6 (M=2) are included for reference. At the ionic strength of 12 the spectrum of CaCl2 is seen to be similar to the spectrum of the MgCl2 solution at an ionic strength of 6 (Figure 2). The LiCl solution still shows some ordered water structure, despite the high concentration of LiCl. Originally published in Lovering et al. (2016). A higher concentration of CaCl2 is needed to cause a similar effect as solutions of MgCl2 because the pKa of hydrolysis of water by Ca2+ ions is higher than the pKa of hydrolysis of water by Mg2+ ion. The ability to perturb water structure follows the trend of pKa constants. The ‘localized hydrolysis’ model also successfully explains the SFG spectra of NaCl and LiCl solutions. The monovalent cations, with higher hydrolysis constants, do not polarize the oxygen-hydrogen bond of water as drastically and do not promote the formation of the water mediated ion-pairs with the silica surface. Instead both the surface and the monovalent cations maintain relatively independent solvation shells, and the vibrational resonances of these water molecules are seen in the spectra of the NaCl and LiCl solutions. While at lower concentration (0.10 mM), 28  Na+ ions were seen to affect surface water structure more than Li+ ions (Flores, et al., 2012), in our study we found that Li+ ions perturb the water structure more than Na+ ions at high concentration (Figure 3.3).  3.4 Conclusions We found that the structure of water in the Stern layer is indeed dependent on the identity of the cation. For example, ordered water was always observed with NaCl solutions, even at a saturation concentration (~6 M), but MgCl2 solutions at the same ionic strength (2M) nearly destroyed the ordered water structures. While our results confirm Dewan’s MD simulations, suggesting that the nature of the “compact layer” is dependent on ion identity, existing models used to explain the surface charge screening observed by SFG could not fully explain our experimental data. However, the ‘localized hydrolysis’ of the water molecules, proposed by Robinson and Harned in 1941 to explain trends in the chemical activities of proton acceptors with different cations, is a plausible mechanism governing the structures of water and ions near the charged mineral surface. We found that the observed residual SFG signal of water in the presence of salts is consistent with the acid dissociation constant of the salt. Our results complement Dove’s hypothesis that entropy and solvent structure largely determine ion-mineral interactions by providing molecular-level insight into how the solvent shell is interacting with the surface (Dove et al., 2005). The data suggested that divalent cations, such as Mg2+ and Ca2+, have a local electrostatic force strong enough to cause localized hydrolysis while the monovalent cations, such as Li+ and Na+, do not.   29  Chapter 4: Transient Ice structure during freezing at the silica surface  4.1 Background Ice has a complex phase diagram (Salzmann et al., 2011), and under standard atmospheric conditions ice I forms.  The most stable form of ice I under atmospheric conditions is hexagonal ice, ice Ih, which has hydrogen-bonded hexagonal ring structures in an ABAB stacking arrangement. A closely related but less stable phase of ice I is called cubic ice, ice Ic. Ice Ic also has six-membered hydrogen-bonded rings but the rings are stacked in an ABCABC arrangement (Malkin et al., 2015). Although ice Ic is metastable with respect to ice Ih, X-ray diffraction (Malkin, et al., 2015; Mayer et al., 1987; Murray et al., 2006; Murray et al., 2005), electron microscopy (Kuhs et al., 2012), and neutron diffraction (Florsheimer et al., 1999; Hansen et al., 2008; Kuhs, et al., 2012; Malkin, et al., 2015) measurements have shown that these two phases can coexist under certain conditions, depending on the freezing pathway as well as the ambient temperature and pressure. Under no conditions, however, has Ic ice been found to exist as a pure phases; Ih ice is always mixed in with the Ic ice (Carr et al., 2014). Recently, these mixtures of Ic and Ih ice have become known as stacking-disordered ice, Isd ice (Malkin, et al., 2015). It was suggested that the so-called cubic ice actually refers to an ensemble of stacking-disordered ice (Hudait et al., 2016; Malkin, et al., 2015). Isd ice has been seen in both theoretical and experimental studies of bulk phase transitions (Hansen, et al., 2008; Hudait, et al., 2016; Mayer, et al., 1987; Moore et al., 2011; Reinhardt et al., 2012). Experimental studies have also shown that colder temperatures and faster cooling rates lead to more stacking faults and lock in the faults as Isd ice (Murray, et al., 2006; Reinhardt, et al., 2012). Experimentally, bulk Isd ice has only been observed at temperatures lower than 30  approximately -20 °C since annealing to Ih ice takes place at warmer temperatures (Kuhs, et al., 2012). A recent study using large-scale molecular dynamics simulations indicates that Isd ice is formed even at warmer temperatures but the relatively high temperature allows annealing before the presence of Isd ice can be observed (Hudait, et al., 2016).  Although the formation of Isd ice has been investigated in the bulk, the formation of this phase has not been investigated at the water/solid interface. Measurements at a buried surface are difficult because the surface is the minority component of most systems. Traditional methods used to identify Isd, such as diffraction or calorimetry fail, for these buried interfaces. Raman spectroscopy has also been used recently to study Isd in the bulk (Carr, et al., 2014), but this technique does not have the sensitivity to monitoring surface structures.  One of the most obvious differences between Ih, Ic and Isd ice phases is their space groups. Both Ih and Ic ice phases belong to centrosymmetric space groups, 𝑃63/𝑚𝑚𝑐 and 𝐹𝑑3̅𝑚, respectively, while Isd ice is non-centrosymmetric (Malkin, et al., 2015). This symmetry difference can be harnessed by nonlinear optical techniques that probe the second-order susceptibility. Under the electric-dipole approximation, this tensor is non-zero only when inversion symmetry is broken (Shen, 1989). For some materials this requirement means only a few surface layers, where the symmetry is necessarily broken, are visible. Other materials, however, inherently lack centrosymmetry; these materials have larger second-order responses from the bulk (Shoji et al., 1997).  4.2 Freezing Experiments Two different types of freezing experiments were carried out. Each freezing experiment began at room temperature, around 22 °C. In the first type, the IR beam was  scanned from 2800 cm-1 to 3800 cm-1 during the freezing process with a step size of 20 cm-1 and each step was averaged for 31  1 second (10 laser shots), and consequently a complete spectrum was collected in 50 seconds. The cooling rate of this experiment was ~0.001 °C/s.   In the second type, rather than scanning the IR frequency across the range of hydroxyl stretching modes, the IR laser was parked at a fixed wavelength, and the SFG signal was collected as the temperature changed. Each data point is an average over 5 seconds (50 laser shots). The cooling rate typically started at ~0.015 °C/s and decreased to ~0.008 °C/s after freezing.  4.3 Results and Discussion Figure 4.1 shows the SFG spectra of water/silica interface recorded during a slow cooling process (~0.001 °C/s). The water structure at the silica surface is stable until the freezing process begins at ~ -4 °C. Figure 4.1e and Figure 4.1f were taken during a transient transition. Therefore, they do not reflect the actual spectral profile. However, it is clear that water undergoes a transient state that has a higher SFG intensity than those of both liquid water and hexagonal ice. This transient state diminishes in intensity at -6 °C (Figure 4.1f) until the stable hexagonal ice spectrum is observed at -7 °C (Figure 4.1g).  32   Figure 4.1. SFG spectra taken at the water/silica interface during the freezing process. The average temperatures are listed in the legend. The temperature at the beginning and end of each spectral collection are (a) 16.4 to 13.7 , (b) 5.2 to 3.7, (c) -0.8 to -1.2, (d) 0.3 to -1.2, (e) -2.0 to -5.2, (f) -5.0 to -5.5, (g) -5.7 to -8.4 °C. Originally published in Lovering et al. (2017). To better capture the dynamics of the transient state, the SFG intensity was recorded during freezing at a fixed wavenumber. The onset of freezing is apparent by a sudden increase in SFG intensity, as shown in Figure 4.2. The SFG intensity has a ~10-fold transient increase before stabilization at a lower temperature. It is noted that SFG intensity is proportional to the square of the second order susceptibility and the increase in ordering is therefore only a factor of 3-4. The transient state exists for a period of ~ 3 min when the temperature of the system (blue curve in Figure 4.2) is relatively constant between 1000 and 1200 sec, which is an indication of a phase transition. Similar SFG increases were observed when the IR laser was parked at 3200 cm-1, 3100 cm-1, 3000 cm-1 and 2900 cm-1 (Figure 4.3), reinforcing the presence of a broad and intense resonance during the transition. 33   Figure 4.2. Typical data sets collected during cooling experiments. Results are shown for silica taken with the IR laser parked at 3100 cm-1. Originally published in Lovering et al. (2017).  Figure 4.3. The sharp increase and slower decrease of SFG signal at the silica surface during freezing when the IR beam is fixed at 3200 cm-1, 3100 cm-1, 3000 cm-1 and 2900 cm-1.  Temperature data is excluded for clarity. Data presented in Figure 4.1, Figure 4.2, and Figure 4.3 suggest that a non-centrosymmetric structure of water molecules is formed in the ice-growth process at the 34  water/mineral interfaces, since both liquid water and Ih ice are centrosymmetric, in which the SFG from the bulk media is forbidden under the electric dipole approximation, and the SFG intensity is dominated by the SFG generated at the water (or Ih ice) interfaces (Wei, et al., 2002). Since both Ih ice and Ic ice are centrosymmetric, the large increase in SFG intensity can be explained by the transition from seeing only the interface of the water/silica to seeing the bulk Isd ice and then to seeing only the interface of Ih ice/silica. Additionally, a transient phase was also seen observed with the polarization combination SPP (Figure 4.4 ). This polarization indicates a chiral surface (Yan, et al., 2014). The observation of a chiral surface could mean that the ice initially grows in spiral structures, as has been observed by microscopy during deposition growth (G. Sazaki et al., 2014; Gen Sazaki et al., 2010; Thurmer et al., 2013). On the other hand, the PSP configuration also accesses a chiral element of the second order susceptibility but is seen to be zero throughout the ice formation process. The observation of chirality is therefore non-conclusive and additional experiments are required to confirm or deny the transient existence of a chiral structure during ice formation at the silica surface.  35   Figure 4.4. The cooling experiment was additionally performed at PSP and SPP polarization combinations. 4.3.1 Temperature of phase transition The temperature at which the stacking-disordered ice was observed is significantly higher than previously reported temperatures observed in bulk water. Bulk Isd ice has only been observed experimentally at temperatures lower than -20 °C since annealing to Ih ice takes place at warmer temperatures (Kuhs, et al., 2012). Our results suggest that the mineral surface may play a role in stabilizing the formation of the stacking-disordered ice.  Since ice nucleation is a stochastic process, even two identical systems may freeze at different times and temperatures. Figure 4.5 shows 35 freezing processes with the freezing point in the range of -12 and -1.2 °C; the majority froze between -3 and -1 °C. All 35 freezing process show the existence of stacking-disordered ice at the water/mineral interface is ~10-20 °C higher than those in the bulk media. 36   Figure 4.5. The onset freezing temperatures of the water are plotted in bins of one degree. The median freezing temperature is -3.3 °C while the average is -4.5 °C. Originally published in Lovering et al. (2017). 4.3.2 Alternative explanations for the transient phase In addition to the positive evidence presented supporting the formation of Isd ice, other possible explanations for the large SFG intensity increase are unsatisfactory. For example, although there is evidence of a liquid-liquid phase transition prior to freezing, and that the pre-freeze liquid has a higher order parameter, the liquid-liquid transition occurs at super cooled conditions that are not reached in this study (Palmer et al., 2014).  Is it possible that another phase of ice, not Isd ice, is forming before Ih ice? Russo et al. defined a new phase of ice called Ice 0, which also lacks inversion symmetry (Russo et al., 2014). This phase, however, exists on the femtosecond time scale and would be too short-lived to account for the relatively long lifetime of the transient phase seen in this study (Russo, et al., 2014). Moreover, their simulations are done for homogenous nucleation when no other structure-inducing substrate is present.  37  There are also phases of ice in which the protons are locked in place and therefore are more ordered (Salzmann, et al., 2011); some of these phases also lack inversion symmetry. However, these proton-ordered phases occur only at high pressure and are not expected to form under the present experimental conditions (Salzmann, et al., 2011). The formation of ferroelectric ice has recently been observed by SFG (Sugimoto et al., 2016). This ice was formed by epitaxial growth on Pt(111) by vapor deposition. The authors note the observation of a high crucial temperature for ferroelectric ice of -98 °C. Ferroelectric ice has also been made in one dimension via growth within nano-channels (Gorshunov et al., 2016; Zhao et al., 2011). Although water has a large molecular electric dipole moment, formation of hydrogen bonds suppresses the ferroelectric ordering of ice. Therefore, ferroelectric ice has been observed only when water is confined and the temperature is significantly below 0 °C. It has been suggested that a hydroxylated silica surface could provide a template for growth of ferroelectric ice, similarly to the Pt(111) surface (Parkkinen et al., 2014). However, it requires depositional growth at much lower temperatures (Iedema et al., 1998; Sugimoto, et al., 2016). Once formed, ferroelectric ice is unstable due to the depolarization field inherent to ferroelectric materials (Parkkinen, et al., 2014). The field needs to be compensated by ions, free surface charge, or surface restructuring (Parkkinen, et al., 2014; Sugimoto, et al., 2016). In the current study, there is no mechanism to stabilize the depolarization field as there are no ions present, silica is a semiconductor and will not inject a hole to stabilize the ice, as is seen with Pt (Sugimoto, et al., 2016), and surface restructuring of the silica mineral is improbable in the given time scales and is not suggested by the data. Additionally, proton ordered ice, including ferroelectric ice, is not expected to from at the warm temperatures and high dimensional system of the current study. 38   Of the other studies where the freezing process of water is monitored by SFG spectroscopy one, which is done on a sapphire surface, also sees a transient increase in SFG signal after freezing but only at pH 9.8. The transient increase is attributed to the presence of Na+ ions in the solution that disrupts the charge transfer and the stitching bilayer (Emmanuel Anim-Danso, et al., 2013). This explanation is insufficient for our study because no ions are present in the current system.  4.4 Conclusions We have observed the formation and destruction of a transient non-centrosymmetric phase of ice at water/mineral interfaces during freezing. While the formation of stacking-disordered ice is a plausible explanation for the observed data, the temperature observed at the interface is ~20 °C higher than those observed in the bulk ice. Our results suggest that the mineral surface may play a role in promoting and stabilizing the formation of the stacking-disordered ice at the interface.    39  Chapter 5: Freezing of NaCl Solutions at Silica Surfaces  5.1 Introduction The liquid to solid phase change at solid surfaces has attracted significant interest because it is important in geological (Hawkings, et al., 2017; Michaud, et al., 2016; Q. Zeng, et al., 2015), biological (Christner et al., 2008; Lindow et al., 1982; Wilson et al., 2003), and atmospheric contexts (Carslaw, et al., 2013; Koren, et al., 2014; Daniel Rosenfeld et al., 2014; Stevens, et al., 2009). As an example, the liquid to solid phase change at solid surfaces is important for the formation of ice in atmospheric clouds(Carslaw, et al., 2013; Koren, et al., 2014; Stevens, et al., 2009). Without the presence of a solid surface, water in cloud droplets can remain in the liquid phase well below the equilibrium liquid to solid phase transition temperature (Lawson et al., 2017; D. Rosenfeld, et al., 2000). However, the presence of a solid surface in cloud droplets raises the temperature of ice formation (Cziczo, et al., 2009; Eastwood et al., 2008; Koop, et al., 2009; Kulkarni et al., 2010; Murray, et al., 2012; Zobrist, et al., 2008). The ability of a solid surface to promote ice formation is attributed to a property of the surface, such as crystal structure, presence of active sites, or ability to hydrogen bond (Pruppacher, 1997), however, the ability of a solid surface to promote ice formation (referred to as heterogeneous freezing) is not understood at a molecular level. Monitoring water structure at a solid surface before, during, and after freezing, will help us better understand heterogeneous freezing. Additionally, the structure of water at a solid surface after freezing may be relevant for understanding creep of glaciers (Knight, 1997). Nonlinear spectroscopy is inherently surface selective under the electric dipole approximation and can be used to monitor liquid/air as well as buried liquid/solid interfaces. 40  Previous nonlinear spectroscopic studies indicate that mineral surfaces can affect the hydrogen bonding network of pure water before and during freezing. In a second harmonic generation (SHG) study, mica induced ordering of water molecules before freezing but sapphire, which is a poor ice nucleating surface, did not (Abdelmonem et al., 2015). The silica surface was observed to affect the hydrogen bonding network during freezing; a large, transient signal, attributed to the formation of a metastable stacking structure, was observed by sum frequency generation (SFG) (Lovering et al., 2017).   The studies discussed above were performed with pure water, but most systems in the environment contain ions. Koop et al. show that, no matter the nature of the ion, the freezing temperature of a liquid droplet that does not contain a solid phase can be predicted just with the water activity of the droplet (Koop, et al., 2000).  Although surfaces have different properties than the bulk, it has also been suggested that water activity can also be used to predict heterogeneous freezing temperature (Alpert, et al., 2011a; Knopf, et al., 2013; Knopf, et al., 2011; Koop, et al., 2009; Q. Zeng, et al., 2015; Zobrist, et al., 2008). On the other hand, recent experimental papers call into question this suggestion (Kumar, et al., 2018; Whale, et al., 2018). The need to directly monitor the solid surface during the freezing of a salt solution was recognized by Anim-Danso et al. (2013) who monitored the phase change of NaCl solutions in contact with sapphire using SFG spectroscopy (Emmanuel Anim-Danso, et al., 2013). They found that 0.1 M (~0.6 wt%) solutions of NaCl first formed a layer of brine, with ice formation occurring around -10 °C with subsequent formation of NaCl·2H2O below the eutectic temperature. When NaCl·2H2O forms, no ice is observed at the surface. Another study by the same group shows that CaCl2 and MgCl2 hydrate crystals are mixed in with the ice crystals after freezing, indicating that there are specific ion effects on the structure observed at the interface 41  after freezing (Zhang, et al., 2014). In these studies, however, ice formation was not initiated at the mineral surface and questions about the impact of the surface on the expulsion of ions from the ice structure remain. The present study is carried out on silica and designed to ensure ice formation is initiated at the surface of the silica. We find that as the solution cools the SFG intensity does not change significantly prior to ice formation. This suggests that the ion concentration at the interface is not changing with temperature. We also find that sodium chloride dihydrate forms below the eutectic temperature and after ice formation within the probing depth of the measurements (~50 nm) indicating that after ice formation the ions are not pushed far from the surface (Wei et al., 2000). In our experiments, both ice and dihydrate are visible in the SFG spectrum. Finally,  the trend in freezing temperatures of NaCl solutions as a function concentration at the silica  surface suggest that that freezing point depression at the surface is analogous to freezing point depression for homogeneous freezing and the equilibrium liquid-to-solid phase transition. 5.2 Experimental All solutions were prepared with sodium chloride ≥99.5 % from Sigma Aldrich. Similarly to experiments discussed in chapter 4, rather than scanning the IR frequency across the range of hydroxyl stretching modes, the IR laser was parked at a fixed wavelength of 3200 cm-1, and the SFG signal was collected as the temperature changed. Each data point is an average over 5 seconds (50 laser shots).The cooling rate typically started at ~0.015 °C/s and decreased to ~0.008 °C/s after freezing.  In separate experiments, spectra were taken across the OH stretching region during cooling. These spectra were collected over ~5 minutes with the temperature decreasing during the collection. 42  5.3 Results and Discussion In Figure 5.1a we show the behaviour of the SFG signal of a 0.5 wt% solution at 3200cm-1 during cooling. The SFG intensity is stable until an abrupt increase in intensity a temperature of approximately -6 °C. The signal is then stable until below -21 °C. Spectra taken across the OH vibrational range, shown in Figure 5.1b, confirm that the increase in SFG signal is due to the formation of ice in the interfacial region. The ice that forms from this low wt% solution has a higher intensity than the ice formed from pure water measured with the in the same sample cell and reported previously (Lovering, et al., 2017). Ions are thought to decrease the SFG intensity of ice because they interfere with the hydrogen bonding network (Emmanuel Anim-Danso, et al., 2013). Nevertheless, previous studies have shown that presence of NH3 can enhanced the ice signal at a sapphire surface.(E. Anim-Danso, et al., 2016) The enhancement was attributed to hydrogen bonding between NH3 and water.(E. Anim-Danso, et al., 2016) While Na+ will not directly hydrogen bond, it is possible that, at these low concentrations, the cations promote structuring in the ice.    43   Figure 5.1. Panel (a) shows how the SFG intensity at 3200 cm-1 of a 0.5 wt% solution of NaCl changes over time (lower x-axis) as the temperature decreases (upper x-axis). As the temperature of the 0.5 wt% solution is decreased, the SFG intensity remains constant until ice is formed. A red arrow indicates the onset of ice formation. In panel (b) spectra of a 0.5 wt% solution NaCl solution are taken across the OH vibrational range at different temperatures. Each spectrum was collected stepwise over five minutes and the temperature is changing throughout. Shown in the upper right hand corner of Panel b is the average temperature during spectra collection. Panels (a) and (b) are from different cooling experiments; the difference in temperature of ice formation is reasonable due to the stochastic nature of heterogeneous freezing. Figure 5.2a shows how the SFG intensity at 3200 cm-1 of a 1 wt% NaCl solution changes with temperature. Similarly to the 0.5 wt% solution, the SFG intensity is stable until the onset of freezing. At the onset of freezing  in Figure 5.2a we observe a transient SFG signal, which has been observed during the freezing of pure water (Lovering, et al., 2017). This transient state is seen in 0.5 and 1 wt% NaCl solutions in ~ 40% of freezing experiments but is not seen for solutions with higher concentrations of NaCl.  As was observed in Figure 5.1a, the SFG intensity 0 6 12 18 24 3016 4 -6 -13 -19 -21012345SFG Intensity (arb.u.)Time (min)Temperature (°C)2800 3000 3200 3400 3600 3800012345678  SFG (arb.u.)Wavenumber (cm-1) -23 °C -17 °C -9 °C 22 °C(a) (b) 44  remains high once ice forms. Abrupt decreases in SFG are due to ice formation cover the outlet of the SFG beam. The slight downward trajectory is due to sample drift as the optical stands cool due to their proximity to the copper cooling block. Below the eutectic temperature, however, the SFG intensity decreases sharply. Spectra taken across the OH region indicate that this is due to the formation of the NaCl·2H2O (Baumgartner et al., 2010). The NaCl·2H2O structure has been observed previously with SFG at the sapphire surface, however, in that study the peaks associated with OH stretching modes of ice are absent (Emmanuel Anim-Danso, et al., 2013). Here we observe both ice and dihydrate in the interfacial region; the difference is likely due to how the solutions were frozen. Though ice formation is initiated at the silica surface, the ice does not push Na+ or Cl- ions outside the detection range of SFG.      45   Figure 5.2. Panel (a) shows how the SFG intensity at 3200 cm-1 of a 1 wt% solution of NaCl changes over time (lower x-axis) as the temperature decreases (upper x-axis). As the temperature of the 1 wt% solution is decreased, the SFG intensity remains constant until ice is formed. A red arrow indicates the onset of ice formation.  A transient signal is also observed.  The transient peak is indicated with a black star. Below the eutectic temperature, NaCl·2H2O forms, resulting in a decrease in SFG intensity at 3200 cm-1, indicated by a blue arrow in panel (a). In panel (b) spectra of a 1 wt% solution of NaCl solution are taken across the OH vibrational range at different temperatures. Each spectrum was collected stepwise over five minutes and the temperature is changing throughout. Shown in the upper right hand corner of Panel b is the average temperature during spectra collection. The red spectrum indicates ice has formed while the blue spectrum clearly shows the formation of dihydrate. Panels (a) and (b) are from different cooling experiments; the difference in temperature of ice formation is reasonable due to the stochastic nature of heterogeneous freezing.  0 6 12 18 24 30 36 4215 5 -5 -13 -20 -25 -29 -3101234Temperature (oC)SFG (arb. u.)Time (min)2800 3000 3200 3400 3600 380002468  SFG (arb.u.)Wavenumber (cm-1) -28 °C -14 °C 21 °C(a) (b) * 46   The freezing behaviour of a 9 wt% NaCl solution is different from the lower concentration solutions (≤ 5 wt %). For 9 wt%, when ice forms, the signal decreases (Figure 5.3a). The decrease is likely due to perturbation of the hydrogen bonding network by the ions as Figure 5.3b shows that spectrum of this frozen structure does not contain the blue-shifted water-like peaks (3400 cm-1 peak). Since the ice spectrum has a low signal in this case, when the dihydrate forms the SFG intensity increases (Figure 5.3a). Comparison of Figure 5.3 to Figure 5.2 shows that the structure of NaCl·2H2O is consistent with the structure of NaCl·2H2O formed from the less concentrated solution.         47   Figure 5.3. Panel (a) shows how the SFG intensity at 3200 cm-1 of a 9 wt% solution of NaCl changes over time (lower x-axis) as the temperature decreases (upper x-axis). The onset of ice formation is indicated with a red arrow and below the eutectic temperature NaCl·2H2O forms, indicated by a blue arrow. In contrast to the 1 wt% solution, the SFG intensity at 3200 cm-1 decreases when ice forms. As a consequence of the lower ice intensity at this frequency, the SFG intensity at 3200 cm-1 increase when the dihydrate forms. In panel (b) spectra of a 9 wt% NaCl solution are taken across the OH vibrational range at different temperatures. Each spectrum was collected stepwise over five minutes and the temperature is changing throughout. Shown in the upper right hand corner of Panel b is the average temperature during spectra collection. The ice spectrum shown in blue has a lower intensity compared to the ice spectra in figures 2 and 3; however, the NaCl·2H2O spectrum is similar. Panels (a) and (b) are from different cooling experiments; the difference in temperature of ice formation is reasonable due to the stochastic nature of heterogeneous freezing.  2800 3000 3200 3400 3600 38000123456  SFG  (arb.u.)Wavenumber (cm-1) -24 °C -18 °C 21 °C0 6 12 18 24 30 36 4216 6 -4 -12 -19 -24 -28 -310.00.20.40.60.81.0SFG (arb.u.)Time (min)Temperature (°C)(a) (b) 48   In Figure 5.4, we plot the temperature of the onset of the phase change from both ice and NaCl·2H2O determined by changes in SFG intensity at 3200 cm-1 (e.g. the arrows in Figure 5.1a, Figure 5.2a, and Figure 5.3a). Solutions of 2, 5 and 13 wt% were examined in addition to the 0.5, 1, and 9 wt% solutions shown. The 2 and 5 wt% solutions behaved similarly to the 0.5 wt% while the freezing process of the 13 wt% solution is similar to the 9 wt% solution. Figure 5.4 also includes the temperatures of the equilibrium liquid to solid phase change for bulk solutions of NaCl as well as the homogeneous freezing temperatures of droplets (5-25 μm diameters in size) of NaCl solutions not in contact with a solid phase. The freezing temperatures determined from changes in the SFG intensity are lower than the equilibrium liquid to solid phase transition since there is a kinetic barrier to freezing.  A comparison between the homogeneous freezing temperatures and the freezing temperature determined from the SFG intensities illustrate that the silica surface decreases the kinetic barrier to freezing.  Latent heat releases recorded by the thermocouple in the silica prism within seconds of the change in SFG intensity indicate that freezing at the surface rapidly propagates through the entirety of the solution. Figure 5.4 and many other studies show that surfaces elevate freezing temperatures compared to homogeneous freezing (Christner, et al., 2008; Davies, 2014; Eastwood, et al., 2008; Kulkarni, et al., 2010; Murray, et al., 2012; Wagner et al., 2011; Qiang Zeng et al., 2014). Comparison of the shape of the freezing and melting curves suggests that NaCl has a similar colligative effect on the freezing temperature at the silica surface as the temperature for the equilibrium solid to liquid phase transition and for the freezing of solution droplets not in contact with a solid surface (i.e. homogeneous freezing temperatures).This is surprising as silica has a negative surface charge and cations will consequently be more concentrated at the surface. The surface should be 49  saturated with cations before the bulk and after saturation, as concentration of the solution changes, the concentration at the surface will not change.   Figure 5.4. Phase diagram of NaCl solutions. The black dots and lines indicate the temperature of the phase change at equilibrium. Red squares are homogeneous freezing temperatures of ~5 to 25µm diameter droplets from reference 25. Blue circles indicate the formation of ice and green indicate the formation of NaCl·2H2O based on the SFG spectra. Error bars indicate the standard error of the mean of the observed phase change. Data points for 0, 1, 9, and 13 wt% are composed of ≥ 9 data points. Data points for 0.5, 2, and 5 wt % are composed of 3-5 data points.  5.4 Conclusions We have examined the freezing behaviour of NaCl solutions at the silica surface. We find that for all solutions studied, ice is the first phase to form and we do not see significant changes in the SFG intensity prior to the formation of ice. We additionally observe the formation of NaCl·2H2O below the eutectic temperature and after the formation of ice.  This indicates that the formation 0 2 4 6 8 10 12 14-60-50-40-30-20-100  Temperature (°C)NaCl (wt%)50  and growth of ice does not push the ions out of the interfacial region probed by SFG. Furthermore, we observe that freezing point depression at the surface is analogous to freezing point depression for homogeneous freezing and the equilibrium liquid-to-solid phase transition.       51  Chapter 6: Conclusion  6.1 Room Temperature Interactions: Ions Chapter 3 shows that at room temperature, ions in solution have different effects on the hydrogen bonds of water at the silica/water interface. While it is well established that water ordering at a charged surface is decreased by positively charged ions due to charge shielding, my work showed that some ions are able to remove nearly all ordered water detected with SFG at high concentrations. Mg2+ ions cause the most disruption to the ordered water, followed by Ca2+, Li+, and Na+ ions. Previous work has shown that ions with high charge density are able to polarize the bond between oxygen and hydrogen in a water molecule. We propose that this localized hydrolysis leads to the formation of solvent-shared ion pairs between the surface hydroxyl group and high charge density ion. Molecular level understanding of the interactions of ions with silica in aqueous environments will help explain the reactions that occur during prolonged exposure between minerals and water, such as ion exchange reactions.  6.2 Interactions during Freezing: Pure Water Chapter 4 discusses how the hydrogen bonding network of pure water changes as the surface temperature of the silica decreases. We see a transient increase in SFG intensity during freezing. Due to the increase in SFG intensity, we suggest that the transient structure is a metastable stacking structure of ice. This ice, called stacking disordered ice, is a mixture of thermodynamically stable hexagonal ice and the closely related cubic ice. Stacking disordered ice is non-centrosymmetric and when it forms the SFG signal originates from the crystal structure rather than the silica/ice interface. The presence of a stacking disordered transient phase 52  during ice formation highlights the dynamisms of the hydrogen bonding network during freezing.  6.3 Interactions during Freezing: NaCl Solutions In Chapter 5, the effect of NaCl on the formation and structure of ice at the silica surface is examined. A transient SFG signal after freezing is observed only for low concentrations of NaCl. The ice formed from low concentration solutions has higher SFG intensity than the ice structure formed from pure water shown in Chapter 4. Meanwhile, the ice structure formed from higher concentrations is less intense than the pure water ice. Though ice forms first at the surface, for all concentrations ≥ 1 wt%, both ice and dihydrate are observed in the interfacial region below the eutectic temperature. The mixture of ice and dihydrate crystals will affect adhesion and may have ramifications in the movement of glaciers because incorporation of ions into ice networks affects glacial creep (Hammonds, et al., 2016; Knight, 1997). We also observe that freezing point depression at the surface follows the expected freezing point depression trends for both equilibrium bulk and homogeneous freezing. This suggests that in some cases trends measured from the bulk accurately reflect trends at the surface. 6.4 Outlook Chapters 3 and 4 show that the presence of a solid surface changes how water molecules interact with each other and with ions. Chapter 5 suggests that for silica/NaCl solutions the unique environment provided by the surface, though it promotes freezing at relatively high temperatures, does not significantly change the expected colligative effects of NaCl during freezing.   Preliminary data from 20 wt% solutions suggest, however, that the effect of ions at the surface may saturate. At a certain concentration of NaCl, it is possible that no additional 53  lowering of freezing will be observed and the surface freezing behavior deviates from established colligative trends. Additional studies should be done with 20 wt% NaCl solutions.  Experiments to explore the generalizability of the coincidence of the bulk and surface trends during freezing also need to be performed. If even some Na+ ions, which have the relatively weak interactions with silica, remain in the interfacial region during ice formation, then the other ions studied in Chapter 3 should as well. Chapter 4 indicates that the hydrogen bonding network changes during freezing.   Ions at the surface with more strongly coordinated solvation shells may impact the freezing temperature more than ions with weakly interacting coordination spheres (Lanaro et al., 2018).  The work in this dissertation reflects both a growing interesting in understanding, at a molecular level, the unique interactions that occur at surfaces in environmentally relevant systems and builds on previous spectroscopic work that can directly probed buried mineral/water interfaces. The silica/water interface is a good model system for the environment and has been previously studied with SFG. By adding temperature, as well as high concentrations of ions, as variables to the silica/water system we expand on the fundamental knowledge of environmentally relevant systems. These studies will also serve as a touchstone for work on more complex buried interfaces.  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