Absence of anomalous underscreening in highly concentrated aqueous electrolytes confined between smooth silica surfaces

TL;DR

This study systematically investigates electrostatic interactions in highly concentrated aqueous electrolytes confined between silica surfaces, finding no evidence of anomalous underscreening and instead observing van der Waals dominance at high salt concentrations.

Contribution

The paper provides experimental and theoretical evidence that contradicts the universality of anomalous underscreening in highly concentrated electrolytes, showing no such effect in silica confinement.

Findings

No evidence of anomalous long-range electrostatic forces in experiments.

Electrostatic forces are dominated by van der Waals interactions at high salt concentrations.

Screening length decreases with increasing ion concentration, supported by density functional theory.

Abstract

Recent experiments and a series of subsequent theoretical studies suggest the occurrence of universal underscreening in highly concentrated electrolyte solutions. We performed a set of systematic Atomic Force Spectroscopy measurements for aqueous salt solutions in a concentration range from 1 mM to 5 M using chloride salts of various alkali metals as well as mixed concentrated salt solutions (involving both mono- and divalent cations and anions), that mimic concentrated brines typically encountered in geological formations. Experiments were carried out using flat substrates and submicrometer-sized colloidal probes made of smooth oxidized silicon immersed in salt solutions at pH values of 6 and 9 and temperatures of 25 {\deg}C and 45 {\deg}C. While strong repulsive forces were observed for the smallest tip-sample separations, none of the conditions explored displayed any indication of anomalous long range electrostatic forces as reported for macroscopic mica surfaces. Instead, forces are universally dominated by attractive van der Waals interactions at tip-sample separations of $\approx$2nm and beyond for salt concentrations of 1 M and higher. Complementary calculations based on classical density functional theory for the primitive model support these experimental observations and display a consistent decrease in screening length with increasing ion concentration.