Tuning structure and mobility of solvation shells surrounding tracer additives

Abstract

Molecular dynamics simulations and a stochastic Fokker-Planck equation based approach are used to illuminate how position-dependent solvent mobility near one or more tracer particle(s) is affected when tracer-solvent interactions are rationally modified to affect corresponding solvation structure. For tracers in a dense hard-sphere fluid, we compare two types of tracer-solvent interactions: (1) a hard-sphere-like interaction, and (2) a soft repulsion extending beyond the hard core designed via statistical mechanical theory to enhance tracer mobility at infinite dilution by suppressing coordination-shell structure [Carmer et al., Soft Matter 8, 4083-4089 (2012)]. For the latter case, we show that the mobility of surrounding solvent particles is also increased by addition of the soft repulsive interaction, which helps to rationalize the mechanism underlying the tracer's enhanced diffusivity. However, if multiple tracer surfaces are in closer proximity (as at higher tracer concentrations), similar interactions that disrupt local solvation structure instead suppress the position-dependent solvent dynamics.

Consequences of minimising pair correlations in fluids for dynamics, thermodynamics and structure

“…These interactions are known to be of a repulsive Yukawa form at large inter-particle separations and strongly believed, in accordance with predictions from approximate integral equation theories, to be of a simple repulsive ramp-like form near contact for the step-function HS iso-g process [21,24,25]. Interestingly, a ramp-like interparticle potential was also recently shown to remove the coordination-shell structure of a tracer solute in a HS solvent [14,40], and a ramp-like fluid-wall interaction is known to similarly 'flatten' the density profile of a confined HS fluid [41]. To clarify the above issues, we seek to address the general accuracy of integral equation theory predictions for an iso-g process, as tested by molecular dynamics (MD) simulations, and confirm the possibly generic ramp-like interparticle potential form that apparently emerges for iso-g processes that initiate from HS fluid reference states.…”

Diffusion in Homogeneous and in Inhomogeneous Media: A New Unified Approach

“…von Hansen et al applied the Mean First-Passage Time analysis, disentangling the free energy and diffusivity contributions, , to the dynamics of water confined by a dipalmitoylphosphatidylcholine bilayer. Carmer et al , proposed a new approach, called the steady-state color reaction-counterdiffusion method, to determine the diffusion coefficient based on the calculation of the steady-state flux and the gradient of the local mole fraction of the labeled particle.…”