Diffusion and structure in dilute aqueous alcohol solutions: Evidence for the effects of large apolar solutes on water

By measuring the dependence of molecular motion on pressure, p, using diffusion measurements as a probe, one can obtain information regarding the structure of solutions containing amphiphilic solutes having apolar or hydrophobic groups. We have found that at low temperatures, the intra-diffusion coefficient (D) of water in a dilute solution of aqueous tert-butyl alcohol (mole fraction, x = 0.025) shows a maximum with increasing p, to a greater relative extent than in pure water under the same conditions. This suggests the water in these solutions is more "structured" than in pure water, though there is a clear distinction from the effects produced by large "structure breaking" ions as solutes where the absolute water diffusion coefficient may show a maximum as concentration or pressure is increased. We have made a survey of a number of other water-alcohol systems; a similar large enhancement occurs in 2-propanol solutions, In solutions of methanol, ethanol, and 2,2,2-trifluoroethanol, the enhancement is similar to that of water with no additional effect. Volumes of activation (Delta V-D) have been calculated from the diffusion data, From an analysis of the T and p dependence of D and Delta V-D, we have concluded that the mechanism for the diffusion of water in these solutions seems likely to be the same as for pure water. The molecules can move around the obstacle solute molecules vis the labile hydrogen-bonded network. Where the temperature is low and the solute alcohol molecules are large, the network appears to be distorted such that the effect of pressure on water diffusion is greater with the maximum appearing at higher pressures than in pure water.