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Ion solvation dynamics in binary solvents Day, Tyler James Frederick

Abstract

The dynamics of selective ion solvation in binary solvents is investigated using molecular dynamics simulations. Initial studies of Stockmayer solvents focus on the dependence of the usual solvation response function, S(t), on solvent composition and on the relative polarity of the solvent species. Particle solvation response functions, P(t), are also introduced which describe the compositional relaxation of the first solvation shell. It is shown that the selective solvation process can be well described by a simple phenomenological model based on the ideas of elementary chemical kinetics. This model is useful and helps in the identification of two distinct timescales present in the selective solvation process. These are associated with a rapid electrostriction step during which the total number of particles in the first shell increases to its equilibrium value, and a slower spatial redistribution process during which the composition of the first shell achieves equilibrium. The redistribution phase depends on the rate of mutual ion-solvent diffusion and also on the rate of particle exchange between the first and second shells. A detailed analysis of the exchange process indicates that exchanges occur on virtually a one-to-one basis with the insertion of a stronger dipole into the first shell being mirrored by an almost immediate ejection of a weaker one. An extension to more realistic systems consisting of both water-methanol and waterdimethylsulfoxide (DMSO) mixtures was also carried out. It is found that, despite the presence of hydrogen-bonding, the dynamical behaviour in water-methanol systems is very similar to that observed for simple Stockmayer solvents. For the water-DMSO mixtures, however, it is found that the relative sizes and geometries of the solvent species can have a substantial influence on the preferential solvation process. For positively charged solutes in water-DMSO the physical picture does not differ greatly from the water-methanol case. However, for negative solutes the DMSO component shows only a weak response to the charge, and the solvation process consists largely of water molecules moving slowly to the solute through an essentially static DMSO medium. The results also illustrate that the usual solvation response function, which depends on the total solute-solvent energy, is not a very sensitive probe of selective solvation dynamics. This insensitivity has since been noticed in recent experimental studies, and an alternative function that appears to be more sensitive to the solvent composition near the solute has been suggested. A comparison of simulation results with time dependent density functional theory is also carried out. It is shown that the nonlinear version of the theory is necessary in order to obtain a good quantitative description of selective solvation dynamics. The linear theory is only of qualitative value. Also, attention is drawn to a previously little appreciated problem which arises when one attempts to compare time dependent density functional theory with computer simulation or experimental results. The difficulty involves matching the theoretical and absolute timescales. Finally, an extension of the theory to Stockmayer systems reveals that the theory in its present form is insufficient to capture the behaviour of these systems.

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