Fluid-crystal coexistence for proteins and inorganic nanocolloids: dependence on ionic strength

TL;DR

This paper models the fluid-crystal coexistence of charged protein and colloid solutions, analyzing how ionic strength influences phase behavior through thermodynamic calculations and electrostatic approximations.

Contribution

It introduces a theoretical framework combining the optimized Baxter model and electrostatic calculations to predict coexistence in charged nanoparticle solutions considering ionic strength effects.

Findings

Coexistence data align with model predictions when effective charge adjustments are made.

Ionic strength significantly affects chemical potentials and phase boundaries.

The approach successfully explains experimental observations for lysozyme and silicotungstates.

Abstract

We investigate theoretically the fluid-crystal coexistence of solutions of globular charged nanoparticles like proteins and inorganic colloids. The thermodynamic properties of the fluid phase are computed via the optimized Baxter model. This is done specifically for lysozyme and silicotungstates for which the bare adhesion parameters are evaluated via the experimental second virial coefficients. The electrostatic free energy of the crystal is approximated by supposing the cavities in the interstitial phase between the particles are spherical in form. In the salt-free case a Poisson-Boltzmann equation is solved to calculate the effective charge on a particle and a Donnan approximation is used to derive the chemical potential and osmotic pressure in the presence of salt. The coexistence data of lysozyme and silicotungstates are analyzed within this scheme, especially with regard to the ionic-strength dependence of the chemical potentials. The latter agree within the two phases provided some upward adjustment of the effective charge is allowed for.