Solvent-induced ion clusters generate long-ranged double-layer forces at high ionic strengths
Year: 2025
Type: article
Abstract: Recent experimental results by the surface force apparatus (SFA) have identified a dramatic deviation from previously established theories of simple electrolytes. This deviation, referred to as anomalous underscreening, suggests that the range of electrostatic interactions increase upon a further addition of salt, beyond some threshold concentration (usually about 1 M). In this theoretical work, we explore an extension of the restricted primitive model (RPM) wherein a short-ranged pair potential of mean force (sPMF) is added to the usual Coulombic interactions so as to mimic changes of the hydration as two ions approach one another. The strength of this potential is adjusted so that the modified RPM saturates at a realistic concentration level (within a range 4-7 M, typical to aqueous 1 : 1 salts). We utilise grand canonical simulations to establish surface forces predicted by the model and compare them directly with SFA data. We explore different sPMF models, which in all cases display significant clustering at concentrations above about 1 M. In these models, we find significant double-layer repulsion at separations that significantly exceed those expected from standard RPM predictions. We do not, however, observe an increase of the screening length with salt concentration, but rather that this screening length seemingly saturates at a (rather high) value. The simulated long-ranged interactions are shown to correlate with ion cluster formation, implicating the important role of accompanying cluster-cluster interactions. In particular, steric interactions between clusters (manifested in density-density correlations) are quite relevant in these systems.
Source: Soft Matter
DOI: 10.1039/d5sm00027k
Authors: David Ribar, Clifford E. Woodward, Jan Forsman
Institutions: Lund University, UNSW Sydney, Australian Defence Force Academy
Language: English
Subfield: Physical and Theoretical Chemistry
Field: Chemistry
Domain: Physical Sciences
Open Access status: hybrid
Funders: Lunds Universitet, Vetenskapsrådet
OpenAlex ID: W4411284350