6th International Conference on Gas Hydrates


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  ZETA POTENTIAL OF THF HYDRATES IN SDS AQUEOUS SOLUTIONS   C. Lo, J. Zhang, A. Couzis, and J. W. Lee    Department of Chemical Engineering  The City College of New York  New York, NY 10031, USA  P. Somasundaran Department of Earth and Environmental Engineering Columbia University New York, NY 10027, USA  ABSTRACT In this study, Tetrahydrofuran (THF) hydrates were formed in-situ in the Zetasizer Nano ZS90. With various concentrations of SDS, we attempted to characterize the SDS adsorption on the surface of the hydrate particles. In doing so, we tried to correlate the adsorption of SDS to THF hydrate induction times with respect to SDS concentration (0 – 3.47 mM), to determine whether the fast nucleation of THF hydrates is due to the adsorption of SDS. The measured ζ-potential for pure THF hydrates was -100 ± 10 mV, indicating anion adsorption. An adsorption curve was observed where there is saturation leveling. Correlating this data to the hydrate induction times, we see that when the saturation level is reached, a significant reduction in induction time can be seen.  Keywords: Gas hydrates, SDS adsorption, Induction time    Corresponding author: Phone: +1 212 650 6688 Fax +1 212 650 6660 E-mail: lee@che.ccny.cuny.edu NOMENCLATURE E potential gradient [mV] mM [mmol/L] ve mobility of particles [µmcm/Vs] ε dielectric constant μ viscosity [cP] ζ zeta potential [mV]  INTRODUCTION Gas hydrates are inclusion compounds, where water molecules form a caged structure surrounding suitable guest molecules under high pressure and low temperature [1]. These guest molecules usually are small gas molecules (e.g. CH4, C2H6, CO2, etc.) or are organic compounds (e.g. THF [2]). Snow like in appearance, gas hydrates can store up to 170 volumes of gas at STP conditions. Gas hydrates have been studied as a promising potential for safe solid storage. However there are certain obstacles that hinder its industrial usage, like the long induction time and slow hydrate growth rate. Recently, studies have shown that the uses of sodium dodecyl sulfate (SDS) can reduce the induction time and can accelerate hydrate growth [3-6].  Though studies have been done on the role of SDS, its role at a molecular level has not been investigated. It was suggested that SDS micelles act as nucleation sites where hydrate growth can begin [3]. However, recent studies have found that below the normal Krafft point (271-289 K) for SDS no micelles are able to form [5-11]. That leads us to believe that SDS monomers act on hydrate nuclei to reduce induction time and increase hydrate growth [6]. Therefore a molecular level analysis is needed to clarify the role of SDS on hydrate nuclei. The goal of this study is to clarify the relationship of SDS monomers on Proceedings of the 6th International Conference on Gas Hydrates (ICGH 2008), Vancouver, British Columbia, CANADA, July 6-10, 2008. hydrate surfaces and its relation to shorter induction times.  THF hydrates were used as the model system, because unlike many other gas hydrates (e.g. methane), THF hydrates are stable at atmospheric conditions. THF can form sII hydrates with an equilibrium melting point of 277.9 K at atmospheric pressure [12,13]. This study will investigate the adsorption of SDS monomers on THF hydrate/liquid interface by -potential measurements. -potential measurements will qualitatively show the SDS adsorption isotherm. Upon which we will present a possible mechanism of SDS adsorption at the hydrate/liquid interface.  MATERIALS AND METHODS Materials Tetrahydrofuran (THF) and sodium dodecyl sulfate (SDS) with a purity of + 99% were purchased from Sigma-Aldrich. Deionized water was produced in our lab with a resistivity of 17 mΩ cm-1.  Samples THF solutions were gravimetrically weighed. THF was weighed then deionized water was added so that the final concentration was 10wt% THF solution. Adding varying concentration of sodium dodecyl sulfate (SDS): 0, 0.17, 0.35, 0.87, 1.73, 2.60, and 3.47 mM; each of the seven samples were placed in a chiller at 269 K until visual confirmation of hydrate formation then at 276.2 K overnight. These mixtures tended to produce slurries. Prior to testing these samples for - potential, samples were dissociated in an ultrasonicator to reduce bubble formation. The samples of THF hydrate solution for -potential were formed in-situ in the -potential cuvette.  -Potential THF hydrates were formed in-situ in the Zetasizer Nano ZS90 (Malvern Instruments). The cuvette cells were filled with 1 cm 3 of sample, after dissociating the sample. The ZS90 was set at 276.2 K, just below the equilibrium temperature, and allowed 30 minutes of equilibration time. This allowed enough time for the hydrates to reform inside the cuvette cell. Once reformed, a particle size measurement was done. Measuring the particle size ensures that hydrates were inside the cuvette cell. Ten -potential measurements were performed. The Zetasizer Nano ZS90 calculates the -potential by inducing a potential across the cell, then measures the velocity of the particles by doppler effect. Using the Helmholtz- Smoluchowski formula:   4  E v e        (1) where ve is the mobility of particles, E is the potential gradient, ε is the dielectric constant, ζ is the zeta potential, and μ is the viscosity.  Induction time Induction time is defined as the time from the start of the set temperature to the time of hydrate formation, where there is a temperature spike. A thermocouple was placed in middle of the vessel and one on the reactor wall. Usually formation of THF hydrates occurs faster along the wall. Figure 1 is a schematic diagram of the experimental set- up. 40 grams of a 20 wt% THF solution, with the same varying concentrations of SDS, was placed in an atmospheric vessel and allowed to equilibrate for 30 minutes at 278K; then the temperature was set at 268 K for the remaining time. A 20wt% THF solution ensured that there was no ice formation based on the phase diagram of THF hydrates.  The rm ostat TC TC TC TC The rm ocouple A /D TC  Figure 1. Schematic diagram of experimental set- up.  RESULTS AND DISCUSSIONS -Potential of THF Hydrates at 276.2 K The melting point temperature of 10 wt% THF solutions is around 276.8 K under atmospheric pressure [12, 13]. At 276.2K the dissociation mass concentration is 9.6 wt% [12]. Therefore the mass fraction is about 4.0%, with an initial concentration of 10 wt% THF, based on the phase diagram of THF hydrates [12]. The low mass fraction is used so that the hydrate particles are under detectable limits for -potential measurements.  The measured particle size was greater than 10 μm under experimental conditions. Assuming that hydrate particles are spherical, the estimated surface area is less than 0.3 m 2 g -1 . Under measurement conditions the maximum total surface area of hydrates is 12.6 m 2 L -1 . If SDS headgroups form a closed pack monolayer, the cross sectional area is 0.42 nm 2 . Therefore the maximum decrease in SDS concentration is 5.0x10 -2  mmol/L [15]. 5.0x10 -2 mM is well below the initial concentration of SDS. In this paper, it is assumed that the concentration of SDS (CSDS) is the same as the initial concentration, due to the small decrease in SDS adsorption.  Figure 2 shows a charge of-100 ± 10 mV at the shear plane of the THF hydrate/liquid interface. In the absence of SDS, the negative charge is due to the adsorption of anions at the hydrate/liquid interface. The samples are unavoidably exposed to the atmosphere during preparation and measurements, and so carbon dioxide from the air dissolves into water, which shift the pH below 6. The measure pH of the solution was 5.88. It was reported that there is no preferential adsorption of OH -  over H + at the ice/water interface [16]. Therefore ice particles below pH 7 are positive, unless other anions (e.g. bicarbonate) other than OH -  adsorbs at ice/water interface. Ice and hydrates surfaces are similar because the molecular arrangement of the hydrogen bonds are not too different from each other [17]. Thus, the negative charge of THF hydrates is due to anion adsorption, specifically bicarbonate, at the hydrate/liquid interface. A possible explanation for anion adsorption at ice/water is that pendant hydrogens on the crystal surface form hydrogen bonds between the anions and this could be the same for anion adsorption at the hydrate/liquid interface.  Figure 2 shows the effect of SDS on THF hydrates by -potential measurements. Three distinct regions in the plot of -potential vs. SDS Concentration (CSDS) are observed. In region I (CSDS < 0.17 mM), the -potential maintains constant since the change in the -potential is within the measurement error. This is complementary to the mechanism proposed to anion adsorption. In region I, bicarbonate anions are being replaced by the DS -  monomers. Region II (0.17 mM < CSDS < 0.35 mM) is where the - potential decreases sharply, indicating that the negative charge at the slipping plane of hydrate/interface increases significantly, and this increase to more negative direction is due to the adsorption of DS - monomers. This DS -  adsorption is past the anion exchange of bicarbonate. Therefore from this point the DS -  monomers are arranging themselves into some sort of packing order.  Region III (0.35 mM < CSDS) is where the - potential decreases linearly with increasing SDS concentrations. The slope of the plot is -336 mV mM -1  in Region II and -20 mV mM -1  in the region III. The slope change indicates that the packing density of the anion headgroup is limiting the packing. From these three regions we see an adsorption isotherm, where there is saturation beyond 0.35 mM. 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 -260 -240 -220 -200 -180 -160 -140 -120 -100 -80 IIIII Z e ta  p o te n ti a l (m V ) SDS concentration (mM) I  Figure 2. -Potential of THF hydrates as a function of SDS concentrations.  Induction Time Induction time is determined by monitoring the temperature spike of THF solutions. According to nucleation theory some hydrate nuclei exist in the THF solution before a temperature spike is detected [1, 19]. Therefore, the dramatic change in temperature occurs at the onset of hydrate crystal growth. These hydrate nuclei are small and sparse [20, 21], so the concentration of SDS remains constant before the onset of crystal growth that is associated with the temperature spike.  Figure 3 shows the effect of SDS in the induction time of THF hydrates. Table 1 shows the induction times for three trials and the statistical error measurements. The SDS concentrations above 0.17 mM are within statistical measurement error. The results indicate that the induction time is reduced in the presence of SDS concentration above 0.17 mM SDS, and that the induction time is independent of SDS at SDS concentration ranging form 0.17 to 3.47 mM.  0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 0 1 2 3 4 5 In d u c ti o n  t im e  ( h r) SDS concentration (mM) Figure 3. Relationship between the induction time for THF hydrates at 268 K and SDS  ADSORPTION MECHANISM A possible mechanism, which was led by the data that was collected, could be that SDS is adsorbed via an anionic adsorption of the headgroup of SDS. Once the replacement of bicarbonate to SDS monomers is fulfilled, aggregated growth happens along side the SDS pendants until a saturated adsorption level, as interpreted by region II in Figure 2.  One possible effect of SDS reducing the induction time is that the interfacial tension of hydrate/liquid interface decreases after DS -  adsorption. It was reported that the induction time for gas hydrates decreases with decreasing hydrate-liquid interfacial energy [21]. At present, no report on the effect of SDS on the interfacial tension of hydrate- liquid is available to clarify this point. Another possible effect of SDS is that hydrate formers such as methane and THF are solubilized between adsorbed DS - , which increases the surface concentration of hydrate formers at hydrate/liquid interface and thereby enhances crystal growth.  Table 1. THF hydrate induction times at 268 K. *95% confidence, sd: standard deviation, se: standard error.  CONCLUSION From this work, we have qualitatively analyzed the adsorption of SDS on the THF hydrate/liquid interface. In the absence of SDS, the associated negative charge is due to anionic adsorption of bicarbonate. -potential of THF hydrates show a constant negative charge, indicating SDS competes with bicarbonate for adsorption sites. The further decreasing negative charge indicates aggregated growth of SDS until the saturated point.  The reduction in induction time is due to SDS monomers adsorption at the THF hydrate/liquid interface. Above a SDS concentration of 0.17 mM, there is a reduction in induction time, and from 0.17 mM to 3.47 mM SDS concentration, the short induction time is statistically constant.  REFERENCE [1] Sloan, E. D. Clathrate Hydrates of Natural Gas, 3 rd  ed.; CRC press: Boca Raton, 2008. [2] Sloan, E. D. Fundamental principles and applications of natural gas hydrates. Nature 2003, 426, 353-359. [3] Zhong, Y.; Roger, R. E. 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