Some Considerations on Confined Water: The Thermal Behavior of Transport Properties in Water-Glycerol and Water-Methanol Mixtures

We discuss recent literature data on the relaxation times (the primary t(alpha)), viscosity, and self-diffusion in water-glycerol and water-methanol mixtures across a wide temperature range from the stable water phase to the deep supercooled regime (373-147K). In particular, to clarify the role of hydrophilicity interactions (the hydrogen bonds) and hydrophobic interactions we study the mixture in terms of the water molar fraction (X-W) with fixed temperatures at 5K steps across the entire composition range, and we find a marked deviation from the ideal thermodynamic behavior of the transport functions. This deviation is strongly T and X-W dependent and spans values that range from two orders of magnitude at the highest temperature to more than five in the deeply supercooled regime (more precisely, at similar or equal to 200K). We analyze these deviations in terms of how the measured values differ from ideal values and find that the hydrogen-bonding water network dominates system properties up to X-W = 0.3. We also examine an Arrhenius plot of the maximum excess value (Delta t(alpha)(T) vs. 1/T) and find two significant changes due to water: one at the dynamical crossover temperature (T-L similar or equal to 225K, i.e., the locus of the Widom line), and one at T similar or equal to 315K (the water isothermal compressibility chi(T) minimum).