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

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 °C and 45 °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.

References in corpus (7)

• The Electrostatic Screening Length in Concentrated Electrolytes Increases with Concentration
• Scaling analysis of the screening length in concentrated electrolytes
• Underscreening in concentrated electrolytes
• Ion-Specific Hydration Effects: Extending the Poisson-Boltzmann Theory
• Primitive Model Electrolytes in the Near and Far Field: Decay Lengths from DFT and Simulations
• Hydration interactions: aqueous solvent effects in electric double layers
• Capacitance and Structure of Electric Double Layers: Comparing Brownian Dynamics and Classical Density Functional Theory