Salt-Induced Phase Separation of Water and Cyclohexane within a Kaolinite Nanopore: A Molecular Dynamics Study

MH Anvari and P Choi, JOURNAL OF PHYSICAL CHEMISTRY C, 122, 24215-24225 (2018).

DOI: 10.1021/acs.jpcc.8b09615

Using a molecular dynamics simulation, the behavior of water and cyclohexane confined in a model clay (kaolinite) nanopore (similar to 4-6 nm) was studied at 298.15 K and 1 atm. On a dry clay basis, the water weight concentration ranged from 6 to 30% while that of cyclohexane was kept at 14%. The formation of water bridge(s) connecting the two basal surfaces was observed at all concentrations. Upon analyzing the surface potentials using the Gouy-Chapman theory, the observation was found to be attributed to the overlapping of the positive potential (octahedral) and the negative potential (tetrahedral) of the two surfaces in the interior region of such a confined environment. The wettability of the tetrahedral sheet was found to be dependent on the water content. Larger areas of this surface would become water-wet with the increase in water concentration to minimize the contact area between the oil and water phases. Addition of sodium chloride to the aqueous phase at the concentrations of 0.1, 0.S, and 1 M substantially improved the wetting of basal surfaces. At the highest salt concentration, breakage of the water bridge was observed, and a phase-separated, three-layer structure (water-cyclohexane-water) was formed within the nanopore because of the screening effect of the adsorbed counterions.

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