Ions in mixed dielectric solvents: density profiles and osmotic pressure between charged interfaces

TL;DR

This paper develops a mean-field theoretical model incorporating solvent composition effects to better understand osmotic pressure between charged interfaces in mixed solvents, explaining experimental observations on DNA solutions.

Contribution

It introduces a generalized expression for osmotic pressure considering solvent inhomogeneity and preferential solvation, advancing the understanding of ionic interactions in mixed solvents.

Findings

Osmotic pressure is reduced at small separations due to solvent effects.

Preferential solvation causes solvent depletion near charged interfaces.

The model explains experimental data on DNA osmotic pressure.

Abstract

The forces between charged macromolecules, usually given in terms of osmotic pressure, are highly affected by the intervening ionic solution. While in most theoretical studies the solution is treated as a homogeneous structureless dielectric medium, recent experimental studies concluded that, for a bathing solution composed of two solvents (binary mixture), the osmotic pressure between charged macromolecules is affected by the binary solvent composition. By adding local solvent composition terms to the free energy, we obtain a general expression for the osmotic pressure, in planar geometry and within the mean-field framework. The added effect is due to the permeability inhomogeneity and nonelectrostatic short-range interactions between the ions and solvents (preferential solvation). This effect is mostly pronounced at small distances and leads to a reduction in the osmotic pressure for macromolecular separations of the order 1--2 nm. Furthermore, it leads to a depletion of one of the two solvents from the charged macromolecules (modeled as planar interfaces). Lastly, by comparing the theoretical results with experimental ones, an explanation based on preferential solvation is offered for recent experiments on the osmotic pressure of DNA solutions.