Direct measurement of the attractive electrosolvation force between a pair of colloidal particles

TL;DR

This study directly measures the long-range attractive forces between like-charged colloidal particles, revealing that these forces depend on particle surface chemistry and challenge traditional screening models, with implications for biological systems.

Contribution

It provides the first direct measurement of the spatial profile of electrosolvation attraction between charged colloids, highlighting the influence of particle surface chemistry on interaction range.

Findings

Long-range attraction observed between charged particles with specific surface chemistries.

Decay length of the attractive force depends on particle properties, not just electrolyte.

Significant implications for biological organization and structure formation.

Abstract

In solution, electrically like-charged particles can experience a strong and long-ranged attraction that leads to the formation of stable, slowly reorganizing clusters. The attractive force underpinning this spontaneous organization process has been shown to depend on both the sign of charge of the particle and the nature of the solvent medium. The origin of the attraction has been ascribed to the preferential orientation of solvent molecules at the object-electrolyte interface. Here, we use optical imaging to directly measure the spatial profile of the potential of mean force between isolated pairs of charged microspheres. Working with particles carrying a variety of surface chemistries we find that the range of the electrosolvation attraction is substantially longer than previously held. In particular we show that particles carrying strongly anionic surface coatings composed of DNA or phospholipid bilayers display long-range attraction. We further find that the length scale governing the decay of the attractive force can depend on the properties of the interacting particles. This contrasts with the canonical expectation that the screening length governing the interaction of charged particles in solution depends exclusively on the properties of the intervening electrolyte medium. The observations point to significant departures from current thinking, and the likely need for a model of interactions that accounts for the molecular nature of the solvent, its interfacial behaviour, and spatial correlations. Finally, a strong and long-ranged attraction mediated by anionic matter constituting lipid membranes and chromatin could carry far-reaching implications for biological organization and structure formation.