Diffusive dynamics of water in tert-butyl alcohol/water mixtures

Quasielastic neutron scattering has been used to investigate the dynamical behavior of H2O in water/tert-butyl alcohol solutions. The measurements were made at fixed temperature (293 K) as a function of tert-butyl alcohol molar fraction, x, in the range 0–0.042. The data have been compared to those of pure water in the temperature range 269–293 K. The effect of tert-butyl alcohol addition on water dynamics is equivalent to that obtained by lowering the temperature of pure water by an amount proportional to the alcohol concentration. The temperature dependence of the diffusivity parameters in pure water and their concentration dependence in tert-butyl alcohol/water solutions can be rescaled to a common curve attributing to each solution a concentration-dependent “structural temperature” lower than the actual thermodynamic one. These results can be understood in terms of Stillinger’s picture of water structuring and of other more recent theoretical pictures that emphasize the influence of the geometrical properties of hydrogen bond networks on water mobility.