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CPP: Fachverband Chemische Physik und Polymerphysik

CPP 29: Interfaces and Thin Films

CPP 29.4: Talk

Wednesday, March 29, 2023, 10:30–10:45, MER 02

Anomalous underscreening in concentrated aqueous electrolytes: myth or reality? — Saravana Kumar¹, Peter Cats², Igor Siretanu¹, René van Roij², and •Frieder Mugele¹ — ¹Physics of Complex Fluids, Univ. Twente, Enschede, The Netherlands — ²Institute for Theoretical Physics, Utrecht Univ., Utrecht, The Netherlands

Recent surface forces apparatus experiments that measured the forces between two mica surfaces 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. 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 25degC and 45 degC. 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 mica surfaces. Instead, forces are universally dominated by attractive van der Waals interactions at tip-sample separations beyond approximately 2nm 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.