Complex Fluids with Mobile Charge-Regulated Macro-Ions

TL;DR

This paper introduces a generalized framework for charge regulation in complex fluids with mobile macro-ions, revealing how charge regulation affects electrostatic properties and the structure of ionic solutions, especially for proteins.

Contribution

It develops a new theoretical model for charge regulation in complex fluids with mobile macro-ions, highlighting effects on electrostatic interactions and screening.

Findings

Effective macro-ion charge varies with position.

Debye length exhibits non-monotonic behavior with salt concentration.

Charge regulation modifies electric double-layer structure.

Abstract

We generalize the concept of charge regulation of ionic solutions, and apply it to complex fluids with mobile macro-ions having internal non-electrostatic degrees of freedom. The suggested framework provides a convenient tool for investigating systems where mobile macro-ions can self-regulate their charge (e.g., proteins). We show that even within a simplified charge-regulation model, the charge dissociation equilibrium results in different and notable properties. Consequences of the charge regulation include a positional dependence of the effective charge of the macro-ions, a non-monotonic dependence of the effective Debye screening length on the concentration of the monovalent salt, a modification of the electric double-layer structure, and buffering by the macro-ions of the background electrolyte.