Structure and Osmotic Pressure of Ionic Microgel Dispersions

TL;DR

This study combines simulations and theoretical models to analyze the structure and osmotic pressure of ionic microgel dispersions, highlighting the validity of linear-screening approximations and the limits of cell models.

Contribution

It introduces a comparative analysis of one-component and cell model Poisson-Boltzmann theories for ionic microgels, assessing their accuracy in predicting osmotic properties.

Findings

Linear-screening approximation is valid for moderately charged microgels.

Cell model predictions align with simulations at low salt concentrations.

Limits of cell model accuracy are identified for salty dispersions.

Abstract

We investigate structural and thermodynamic properties of aqueous dispersions of ionic microgels -- soft colloidal gel particles that exhibit unusual phase behavior. Starting from a coarse-grained model of microgel macroions as charged spheres that are permeable to microions, we perform simulations and theoretical calculations using two complementary implementations of Poisson-Boltzmann (PB) theory. Within a one-component model, based on a linear-screening approximation for effective electrostatic pair interactions, we perform molecular dynamics simulations to compute macroion-macroion radial distribution functions, static structure factors, and macroion contributions to the osmotic pressure. For the same model, using a variational approximation for the free energy, we compute both macroion and microion contributions to the osmotic pressure. Within a spherical cell model, which neglects macroion correlations, we solve the nonlinear PB equation to compute microion distributions and osmotic pressures. By comparing the one-component and cell model implementations of PB theory, we demonstrate that the linear-screening approximation is valid for moderately charged microgels. By further comparing cell model predictions with simulation data for osmotic pressure, we chart the cell model's limits in predicting osmotic pressures of salty dispersions.