(13g) Influence of Nanoconfinement on Flow Dynamics and Rheological Responses of Geocolloidal Suspension

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 in geosystem.

Here, monodisperse silica nanoparticles with the radius of 50 nm (PDI = 0.035), selected as model geocolloids, were synthesized using a modified version of the Stöber process. Trichlorosilane was deposited onto geocolloid surfaces by chemical vapor deposition with precisely controlled degrees of surface coverage to serve as model energy-related contaminants. The Surface Forces Apparatus (SFA) was utilized to systematically study rheological responses and flow characteristics of geocolloidal suspensions containing bare as well as modified silica nanoparticles under different degrees of nanoconfinement. The drastic increase in viscosity and corresponding changes in storage and loss moduli were observed below a critical separation distance (i.e. level of nanoconfinement), indicative of the formation of aggregations ascribed to the attractive intermolecular interactions due to enhanced hydrophobicity and suppressed electrostatic repulsion. When the separation distance became comparable to several diameters of silica nanoparticles, geocolloidal suspensions displayed clear shear-thinning behaviors, qualitatively as well as quantitatively modified from the bulk.

This work advances the current mechanistic understanding of how geocolloids without and with covered by energy-related contaminants interact when they are forced into confined geometries, which is commonly encountered in geological flow processes. This understanding will, in turn, allow us to better evaluate the impacts of energy production and isolation of energy system wastes on subsurface geological environments. Such knowledge is significant in the context of developing mitigation strategies to reduce and minimize the adverse of energy-related contaminants in the geosystems and waterbodies.