Direct observation of charge inversion by multivalent ions as a universal electrostatic phenomenon

TL;DR

This study provides direct atomic force microscopy evidence that multivalent ions cause charge inversion on surfaces, primarily driven by ion valence and spatial correlations, regardless of ion chemistry or surface details.

Contribution

It offers the first direct observation of charge inversion caused by multivalent ions and confirms the theoretical mechanism involving ion correlations.

Findings

Charge inversion occurs reversibly at specific multivalent ion concentrations.

Valence of ions strongly influences the charge inversion threshold.

Surface chemistry and structure have minimal impact on the inversion process.

Abstract

We have directly observed reversal of the polarity of charged surfaces in water upon the addition of tri- and quadrivalent ions using atomic force microscopy. The bulk concentration of multivalent ions at which charge inversion reversibly occurs depends only very weakly on the chemical composition, surface structure, size and lipophilicity of the ions, but is dominated by their valence. These results support the theoretical proposal that spatial correlations between ions are the driving mechanism behind charge inversion.