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

Saravana Kumar*, Peter Cats, Mohammed B. Alotaibi, Subhash C. Ayirala, Ali A. Yousef, René van Roij, Igor Siretanu, Frieder Mugele

*Corresponding author for this work

Research output: Contribution to journalArticleAcademicpeer-review

1 Citation (Scopus)
3 Downloads (Pure)

Abstract

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 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 mica surfaces. Instead, forces are universally dominated by attractive van der Waals interactions at tip-sample separations of ≈2 nm 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.

Original languageEnglish
Pages (from-to)819-827
Number of pages9
JournalJournal of colloid and interface science
Volume622
DOIs
Publication statusPublished - 15 Sep 2022

Keywords

  • AFM
  • Colloidal interaction
  • Decay lengths
  • DFT
  • Surface forces
  • Underscreening
  • UT-Hybrid-D

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