(481c) Interfacial Behavior of Anisotropic Geocolloidal Dispersions

Geocolloids are naturally occurring mineral colloids with at least one dimension between 1 nm and 1 µm. They exist in surface waters, group waters, and in soils, playing an important role on the transport of adsorbates such as human-made/produced wastes and contaminants (e.g., pharmaceutical metabolites, pesticides, lubricant oils, hydraulic fracturing byproducts, nuclear waste, diesel exhaust, and flame retardants) when contaminated. There are, however, various knowledge gaps about how the transport behaviors and diffusion dynamics of geocolloids can change upon contamination with nonpolar pollutants, especially under confined geometries and in porous media.

In this work, we aimed to investigate the interfacial interactions arising between mica surfaces (model mineral surfaces) across 50 nm silica (model geocolloids) suspensions using the Surface Forces Apparatus (SFA). Chlorotrimethylsilane (CTMS) modified silica Janus nanoparticles (model contaminated geocolloids) were systematically and precisely prepared with hydrophilic-lipophilic balance ratios of 25:75, 50:50 and 75:25 using a novel flow sweeping apparatus. In the absence of salts, the measured force profiles were found to be insensitive to the concentration of silica nanoparticles due to the exclusion and squeezing-out of nanoparticles from the confined zone via the electrostatic double-layer repulsions. However, once the electrolytes were introduced into the system (thereby increasing the salinity), the onset of long-range repulsion began at much smaller separations. Corresponding hard-wall (non-adhesive contact) distances were observed to increase as increasing the concentrations of salts and silica nanoparticles in system. These trends were ascribed to the reduction in the Debye screening length and sequential aggregation and coagulation processes, which were further promoted under nanoconfinement. The viscosity of geocolloidal suspension were also studied at varying length scales (i.e., different degrees of confinement) to determine if there is any breakdown of continuum behaviors in the relaxation and diffusion dynamics. It was found that below a critical separation distance, typically in the order of a few nanoparticle diameters, a clear transition from the Newtonian behavior to the non-Newtonian behavior was observed. Below the critical separation distance, increasing the electrolyte concentration of the KCl solution aggregated geocolloids and increased the viscosity of the suspension. Introduction of nonpolar CTMS on the surface of silica nanoparticles drastically modified overall interparticle forces by altering a relative importance of electrostatic and hydrophobic interactions, leading to substantial changes on the rheological behaviors of Janus colloidal suspensions.

We anticipate that our findings and knowledge gained through this study is important in the context of better understanding the transport phenomena of geocolloids with adsorbed contaminants, in particular, energy-related ones and their roles on the fate of such contaminants in the geosystem and environment.