Charge affinity and solvent effects in numerical simulations of ionic microgels

TL;DR

This paper advances a numerical model for ionic microgels by improving ionic group treatment and explicitly including solvent effects, enabling better predictions of swelling behavior and structure in line with experimental observations.

Contribution

It introduces an improved coarse-grained model for charged microgels that accurately incorporates ionic groups and solvent effects, enhancing predictive capabilities.

Findings

Highly charged microgels do not fully collapse when charged groups are hydrophilic.

The solvent-monomer interaction mapping is highly accurate across charge fractions.

The model aligns well with experimental swelling and structural data.

Abstract

Ionic microgel particles are intriguing systems in which the properties of thermo-responsive polymeric colloids are enriched by the presence of charged groups. In order to rationalize their properties and predict the behaviour of microgel suspensions, it is necessary to develop a coarse-graining strategy that starts from the accurate modelling of single particles. Here, we provide a numerical advancement of a recently-introduced model for charged co-polymerized microgels by improving the treatment of ionic groups in the polymer network. We investigate the thermoresponsive properties of the particles, in particular their swelling behaviour and structure, finding that, when charged groups are considered to be hydrophilic at all temperatures, highly charged microgels do not achieve a fully collapsed state, in favorable comparison to experiments. In addition, we explicitly include the solvent in the description and put forward a mapping between the solvophobic potential in the absence of the solvent and the monomer-solvent interactions in its presence, which is found to work very accurately for any charge fraction of the microgel. Our work paves the way for comparing single-particle properties and swelling behaviour of ionic microgels to experiments and to tackle the study of these charged soft particles at a liquid-liquid interface.