Preferential solvation dynamics in liquids: How geodesic pathways through the potential energy landscape reveal mechanistic details about solute relaxation in liquids

Abstract

It is not obvious that many-body phenomena as collective as solute energy relaxation in liquid solution should ever have identifiable molecular mechanisms, at least not in the sense of the well-defined sequence of molecular events one often attributes to chemical reactions. What can define such mechanisms, though, are the most efficient relaxation paths that solutions take through their potential energy landscapes. When liquid dynamics is dominated by slow diffusive processes, there are mathematically precise and computationally accessible routes to searching for such paths. We apply this observation to the dynamics of preferential solvation, the relaxation around a newly excited solute by a solvent composed of different components with different solvating abilities. The slow solvation seen experimentally in these mixtures stems from the dual needs to compress the solvent and to do solvent-solvent exchanges near the solute. By studying the geodesic (most efficient) paths for this combined process in a simple atomic liquid mixture, we show that the mechanism for preferential solvation features a reasonably sharp onset for slow diffusion, and that this diffusion involves a sequential, rather than concerted, series of solvent exchanges.

The inherent dynamics of a molecular liquid: Geodesic pathways through the potential energy landscape of a liquid of linear molecules

“…That is, we have been asking precisely how slowly rearranging liquids move most efficiently from place to place in their full, many-dimensional, configuration space. 6,7 The goal of the present paper is to generalize these studies by moving from atomic to molecular liquids. Part of the motivation for pursuing this direction is the existence of examples such as liquid crystals for which reorientational motion is the fundamental source of the slow dynamics.…”

“…6,7 These examples made attractive targets because aptly chosen mixtures can display much of the slow dynamics characteristic of supercooled liquids 6 and of preferential solvation systems. 7 The principal rationales for choosing atomic models, though, were technical: it was possible to devise practical algorithms for finding the necessary geodesic-like paths; there was a direct connection between the lengths of those paths and the diffusion constant (enabling us to test the ability of geodesic theory to predict dynamics); 6 and it was straightforward to analyze the microscopic mechanism underlying the dynamics by computing the average number of participating translational degrees of freedom along the geodesic path. 7 The question this paper pursues is whether we can extend this geodesic formalism to cover instances of molecular liquids.…”

“…[5][6][7][8] When a many-body system undergoes slow diffusion, its motion will automatically be dominated by the most efficient pathways it can take. In analogy with the development of inherent-structure theory, what we have begun studying is how those pathways are defined by the shape of the potential energy landscape.…”

“…7 The principal rationales for choosing atomic models, though, were technical: it was possible to devise practical algorithms for finding the necessary geodesic-like paths; there was a direct connection between the lengths of those paths and the diffusion constant (enabling us to test the ability of geodesic theory to predict dynamics); 6 and it was straightforward to analyze the microscopic mechanism underlying the dynamics by computing the average number of participating translational degrees of freedom along the geodesic path. 7 The question this paper pursues is whether we can extend this geodesic formalism to cover instances of molecular liquids. Even shifting our focus to rigid linear molecules, the specific subject of the present work, raises the question of what the kinematic length defined in Eq.…”

Solvent-Mediated Vibrational Energy Relaxation from Vaska’s Complex Adducts in Binary Solvent Mixtures

“…34 The mechanism of this process was elegantly described by Nguyen and Stratt as originating from the exchange of molecules from the first solvation shell to re-equilibrate around the excited state. 35 Hence, the preferential solvation of a solute by one component of a binary mixture over the other, which occurs for the VC adducts studied here (see Figures 2 and 3), could have an influence on the time-dependent reorganization of the solvation-shell molecules. Viewing the role of the solvent in VER as modulating the force applied to the solute, 36 it is conceivable that preferential solvation might also affect the CO relaxation times.…”