(474d) Effect of Contamination on the Mobility of Charged Colloids in Confined Aqueous Spaces – Isotropic Vs. Patchy Hydrophobization

Geocolloids, carried by water through underground fissures and cracks, exacerbates the spread of hydrophobic and nuclear energy-related contaminants by providing large absorbable surface area and relatively high diffusivity. The transport phenomena of geocolloids have been investigated mostly through field research on macroscopic scales. However, the driving forces controlling colloidal mobility and how they vary according to the types and degrees of contamination are still not well understood, especially in micro- and nano-sized fissures.

In this work, the surface forces apparatus (SFA) was utilized to create confining gaps between mineral surfaces to simulate the micro- and nano-sized underground fissures with gap size controllable down to Angstrom level. Pristine and hydrophobically modified mica surfaces were selected as model surfaces for underground mineral surfaces before and after contamination. Monodisperse 50 nm silica particles were selected to model mineral geocolloids. The silica particle surface was hydrophobically modified either homogeneously or anisotropically, both in controlled manners. In homogeneous modification, the particle surface was modified with hydrophobic silane in isotopically but of gradually increasing effective grafting density. Alternatively, the silica particles were modified with Janus patches, where the hydrophobic modification was systematically varied from 0%, 25%, 50% to 100% with precise control using a masking technique known for Janus particle fabrication. Two types of contaminated particles that possess the same effective silane coverage were compared in terms of transport behavior in aqueous media through the measurement of force profile between confining surfaces, as well as the effective viscosity in SFA-simulated fissures ranging from microns to a few tens of nanometer in size. Results showed a decrease of colloidal mobility, in both cases, with increasing hydrophobic contamination on mineral and colloid surfaces, which was reflected in the increasing magnitude and range of repulsion in the measured force profiles, as well as the drastic increase in effective viscosity. The increase of salinity in aqueous media emphasizes the decrease in colloidal mobility. However, the onset surface gap distance of repulsive force and increased viscosity distinguishes between homogeneous and patchy particles, indicating the isotropy of surface contamination plays an important role in the colloidal transport in confined aqueous spaces. We interpret these finding with the balance of electrostatic, Van der Waals and hydrophobic interactions between colloid and colloid, colloid and mineral, as well as mineral and mineral surfaces.

We anticipate the findings of this research to provide insights on the dynamic and transport behaviors of mineral colloidal particles in confined aqueous environments and how they can be affected by variation in surface chemistry, which is of theoretical and practical value in environmental science, geoscience and oil industry.