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Simulations of water adsorption and structure on kaolinite surfaces Croteau, Timothé

Abstract

Grand canonical Monte Carlo calculations are used to determine water adsorption and structure on kaolinite surfaces, with and without the presence of trench-like defects, as a function of relative humidity (RH), at 235 K and 298 K. Both basal planes (the Al- and Si-surfaces), as well as two edge-like, defect free surfaces are considered. The trenches simulated are rectangular in geometry, and have a fixed depth and varying width. The general force field CLAYFF is used together with the SPC/E and TIP5P-E models for water. At both 235 K and 298 K, the edges, Al-surface, and trenches adsorb water at sub-saturation, in the atmospherically relevant pressure range. The Si-surface remains dry up to saturation. Both edges and the Al-surface adsorb water up to monolayer coverage. Adsorption on the Al-surface exhibits properties of a first-order process with evidence of collective behavior, whereas adsorption on the edges is essentially continuous and appears dominated by strong water lattice interactions. Only next to the Al-surface, were hexagonal rings observed in the water layer. However, they did not match hexagonal ice Ih. The results obtained using trenches show that the granularity of the surfaces can play a major role in the adsorption of multiple layers of water over a large range of RH. Our calculations suggest that water adsorption in trenches, and possibly in other similar defects, can offer an explanation of the large water coverages reported experimentally. Related to ice, the very dense, proton ordered, ferroelectric structures found in the trenches at 235 K do not correspond to any recognizable form of bulk ice. We speculate how these structures might aid ice nucleation and growth, and suggest how this possibility could be further explored with simulations and experiments.

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Attribution-NonCommercial-NoDerivatives 4.0 International

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