(73b) Influence of Nanoconfinement on Geocolloidal Interactions and Relaxation Dynamics

It is known that geocolloids play a major role on the transport and distribution of energy-related contaminants (e.g. naphthalene and cesium etc.) in environment. However, there are many fundamental questions to be answered pertaining to their transport processes and molecular interactions at the nano- and micro-length scales, that are closely related to corresponding relaxation dynamics of colloidal suspension.

Here, monodisperse silica nanoparticles with the radius of 50 nm were synthesized and selected as model geocolloids. The Surface Forces Apparatus (SFA) was used to measure collective surface forces arising between confining mica surfaces (i.e. model geosurfaces) across colloidal dispersions of silica nanoparticles at varying degrees of confinement as well as dispersant conditions. In the presence of silica nanoparticles in 0.1 M KCl, the measured forces were purely repulsive on approach and separation: the repulsive forces were long-ranged starting from about 0.3 – 1 µm contrasting to the case without silica nanoparticles where the onset of repulsion was less than 5 nm. In contrast, when silica nanoparticles were dispersed in pure water, the magnitude of onset of repulsion significantly diminished, giving rise to similar magnitudes of adhesive instabilities (jumps-in and -out) regardless of silica concentrations in suspension. When silica nanoparticles were confined to a gap of several colloidal diameters between mica surfaces, a dramatic increase in viscosity of > 3 orders of magnitude from the bulk was observed depending on the ionic strength in medium, which can be described by the density distribution function and potential mean force characteristics of intermolecular interactions in fluids. Our results provide a picture of the complex colloidal dynamics and interactions between geosurfaces across geocolloidal suspensions, highlighting the impact of nanoconfinement and colloidal stability manipulated by salts.