Short range attraction between two similarly charged silica surfaces

TL;DR

This study demonstrates that adding cobalt hexamine ions to saline solutions can induce short-range attractive forces between similarly charged silica surfaces, challenging traditional electrostatic repulsion expectations.

Contribution

It reveals that multivalent cobalt hexamine ions can cause a reversible attraction between like-charged silica surfaces, supported by surface charge measurements and exclusion of Wigner crystal interactions.

Findings

Cobalt hexamine ions suppress electrostatic repulsion at short distances.

Higher concentrations restore repulsion, indicating a non-monotonic effect.

Surface charge measurements support ion adsorption as the attraction mechanism.

Abstract

Using an Atomic Force Microscope (AFM) we measure the interaction between two identically charged silica surfaces in the presence of saline solution. For pure NaCl the interaction is always repulsive. Upon addition of cobalt hexamine ions, [Co(NH3)6]+3, the repulsion is gradually suppressed and a pronounced attraction develops at distances much shorter than the screening length. Higher concentrations of cobalt hexamine turn the attraction back into repulsion. Measurements of surface charge renormalization by the three valent cations provide their surface density and their association constant to the negatively charged silica surface. These estimates exclude interaction between two condensed Wigner crystals as an explanation for the attraction.