Liquid structure and dynamics of aqueous isopropanol over γ-alumina

The liquid structures of thin films of aqueous solutions of 0, 7, 19, 50, and 100 mol % isopropanol above O/Al-terminated γ-alumina 〈001〉 surfaces have been investigated by means of classical molecular dynamics simulations. The structuring effect of the oxide on the liquid mixtures is strong and heavily dependent on the local structure of the oxide. Two distinct regions are found on the oxide surface characterized by the degree of coordination of Al atoms. Above octahedral Al atoms, water and isopropanol molecules adsorb via the oxygen atoms to maximize the electrostatic interaction, whereas above tetrahedral Al sites the solvent molecules adsorb via hydrogen atoms with the oxygen atoms away from the surface. More mobility is found in the second layer compared with the first; however, its structure is still influenced significantly by the orientation of molecules in the first adsorbed layer. Qualitatively, the displacement of water from the surface by the adsorption of isopropanol occurs with 2.6 water molecules lost for every alcohol molecule present based on the effective surface areas of the two species calculated from the pure simulations. Diffusion in the liquid has been investigated by both molecular dynamics and PFG-NMR studies. Both show that the first adsorbed layer is slower moving than the bulk by several orders of magnitude, as expected, and thereafter the simulations show a gradual increase in diffusivity with increasing distance from the interface, tending toward the bulk value. Experimental diffusion coefficients of isopropanol inside the γ-alumina pore are found to be approximately one-quarter of that found in the bulk liquid. When compared with the simulated values, this suggests that the surface properties probed by the NMR technique encompass the first two layers interacting with the surface. In addition, because of the time scale of the measurements, the largest diffusion coefficient obtained includes the pore tortuosity and, therefore, a reduced experimental value is found compared with that obtained from the simulations.