Diffusion and structure in water-alcohol mixtures:: Water plus tert-butyl alcohol (2-methyl-2-propanol)

Data are reported for the intradiffusion coefficient of water (D-w) in aqueous solutions of 2-methyl-2-propanol (tert-butyl alcohol, TEA) as a function of pressure, temperature, and composition. In previous work (Harris and Newitt, J; Phys. Chem. B, 1998, 102, 8874), we have shown that for this system at mole fraction x(TBA) similar to 0.025 and low temperatures, D-w, shows a maximum with increasing pressure to a greater relative extent than does the self-diffusion coefficient in pure water under the same conditions. It was concluded that this is consistent with the concept that the water is more structured in the TEA solution at this composition. Our present results confirm this at other concentrations below x(TBA) similar to 0.06 This is also consistent with a previous, but unpublished, study of TEA intradiffusion by Woznyj, and the NMR relaxation and chemical shift results of Yoshida et al. (J. Chem. Phys., 1998, 108,1360). At higher concentrations, the normal behavior of a diminution of the diffusion coefficient with increasing pressure occurs. This is not consistent with ordering of hydration water suggested by certain models based on the presence of clathrate-like hydrates in solution.