Electrostatics of soft (bio)interfaces: Corrections of mean-field Poisson-Boltzmann theory for ion size, dielectric decrement and ion-ion correlations

Hypothesis: Electrostatics of soft (ion-permeable) (bio)particles (e.g. microorganisms, core/shell colloids) in aqueous electrolytes is commonly formulated by the mean-field Poisson-Boltzmann theory and inte-gration of the charge contributions from electrolyte ions and soft material. However, the effects con-nected to the size of the electrolyte ions and that of the structural charges carried by the particle, dielectric decrement and ion-ion correlations on soft interface electrostatics have been so far considered at the margin, despite the limits of the Gouy theory for condensed and/or multivalent electrolytes. Experiments: Accordingly, we modify herein the Poisson-Boltzmann theory for core/shell (bio)interfaces to include the aforementioned molecular effects considered separately or concomitantly. The formalism is applicable for poorly to highly charged particles in the thin electric double layer regime and to unsym-metrical multivalent electrolytes. Findings: Computational examples of practical interests are discussed with emphasis on how each con-sidered molecular effect or combination thereof affects the interfacial potential distribution depending on size and valence of cations and anions, size of particle charges, length scale of ionic correlations and shell-to-Debye layer thickness ratio. The origins of here-evidenced pseudo-harmonic potential pro-file and ion size-dependent screening of core/shell particle charges are detailed. In addition, the existence and magnitude of the Donnan potential when reached in the shell layer are shown to depend on excluded volumes of the electrolyte ions.(c) 2023 Elsevier Inc. All rights reserved.

Automated summary

The electrostatics of soft (ion-permeable) (bio)particles in aqueous electrolytes has often been studied using mean-field Poisson-Boltzmann theory and integration of charge contributions from electrolyte ions and soft material. However, the effects of electrolyte ion size, structural charges carried by the particle, dielectric decrement, and ion-ion correlations on soft interface electrostatics have been overlooked. This study modifies the Poisson-Boltzmann theory for core/shell (bio)interfaces to include these molecular effects. The findings show how each molecular effect affects the interfacial potential distribution depending on the size and valence of ions, size of particle charges, ionic correlations, and shell-to-Debye layer thickness ratio.