(450b) Influence of Nano and Microconfinement on Internanoparticle Interactions in Geocolloidal Dispersions

Geocolloids are known to facilitate diffusion of chemical contaminants generated from energy-related human processes. In particular, chemical contaminants (ethylbenzene, napthalene, etc.) that enter the ground during the process of hydraulic fracturing are observed to diffuse rapidly across large distances in geosystems where they are in contact with geocolloids, otherwise they stay stationary by sticking to the geosurfaces. Many critical questions remain unanswered in this aspect, in particular, as to how diffusion, viscosity, and adsorption characteristics of geocolloids that are partially or fully covered with contaminants behave under confinement and/or at varying salinity deviated from bulk and standard conditions.

In this work, highly monodisperse silica nanoparticles of diameter ranging from 50 to 400 nm, synthesized using a modified version of the Stöber process, were selected as model geocolloids. Trichlorosilane was deposited onto geocolloid surfaces by chemical vapor deposition with controlled degrees of surface coverage to serve as adsorbed chemical contaminants. Direct force measurements with geocolloids having different degrees of surface coverages were conducted using the Surface Forces Apparatus (SFA) over a distance regime starting from 8µm all the way down to molecular contact. Repulsive forces were observed on approach starting from > 3µm, followed by an exponential increase of which magnitude appears to be larger than a decay length obtained from Derjaguin–Landau–Verwey–Overbeek (DLVO) theory in pure water. When the geocolloids were confined in salted water, the magnitude of onset of repulsion was varied as a function of salinity in solution, which can significantly alter a purely repulsive screened electrostatic (coulombic) interaction arising from, among geocolloids as well as between geocolloids and geosurfaces.

We anticipate that the knowledge gained through this study will enable the scientists and researchers to better assess transport and fate behaviors of geocolloidal dispersions that can carry energy-related contaminants under realistically emulated geosystems.