The Structure and Dynamics of Ions at Aqueous Interfaces Studied via Atomic Force Microscopy

Abstract

The organisation and kinetics of charges at solid and soft interfaces play a central role in biological signalling processes and are vital for energy storage technologies as well as our understanding of heterogeneous catalysis. At the molecular-scale, such interfacial behaviour remains stubbornly difficult to characterise, due to the short-ranged interactions between ions, their aqueous solvent and surface groups. Thus, continuum-scale models quickly break down, especially close to the interface and with high charge densities.

This thesis addresses the question of ionic organisation using atomic force microscopy (AFM), which uniquely combines sub-nanometre spatial resolution and the ability to probe relatively long timescales. The use of small oscillation amplitudes allows the topography of the ionic layer to be mapped while simultaneously extracting physical properties from the sample or the interface itself, with time resolution spanning from tens of milliseconds to minutes.

The structure of ions at hydrophilic interfaces is shown to be delicately sensitive to the charges’ molecular structure (in the case of larger buffering agents) and their charge density (for simple alkali cations). Specifically, the cations’ interactions with a model lipid membrane and the waters around it lead to an attractive correlation energy which generates nanoscale networks that evolve over the course of many seconds. These ionic structures directly reduce the effective stiffness of the lipids, providing a mechanism for the spontaneous control of membranes’ mechanical properties.

These ionic networks are significant in the case of confined fluids and provide an efficient means of lubrication even under high pressures in sub-nanometre gaps. When sheared, such fluid films are revealed to be non-Newtonian, with dynamics that depend on the velocity and lengthscale of the motion. The results highlight the greatly damped kinetics of ions and water molecules at interfaces, and shed light on the mechanisms behind their transport through and along biomolecules.