Composition-dependent dynamical structures of monohydric alcohol-water mixtures studied by microwave dielectric analysis

Dielectric relaxation measurements on methanol (MA)-water, ethanol (EA)-water, and 1-propanol (1PA)-water mixtures were carried out using TDR (Time Domain Reflctometry) for the entire concentration range of 0less than or equal toX(molar fraction of alcohol)less than or equal to1.0 up to 25GHz at 20degreesC-30degreesC. We have calculated the excess activation free energy, enthalpy, and entropy of these mixtures, DeltaG(E), DeltaH(E),and DeltaS(E), and the excess partial molar quantities for alcohol, DeltaG(A)(E), DeltaH(A)(E), and DeltaS(A)(E) (A=MA, EA, and 1PA) and those for water, DeltaG(W)(E), DeltaH(W)(E), and DeltaS(W)(E) from the temperature-dependent relaxation times. There exists,boundary above and below which behaviors of the partial molar quantities are quite different. If we call the concentration corresponding to the boundary X-b, X(b)similar to0.30, 0.18, and 0.14 for methanol-water, ethanol-water, and 1-propanol-water, respectively. In the region of Xgreater than or equal toX(b), DeltaH(A)(E) and DeltaS(A)(E) are nearly zero. This means that, in terms of the activation enthalpy and entropy, alcohol molecules in the mixtures find themselves in not a very different environment from that in pure liquid. In water-rich region, DeltaH(A)(E) and DeltaS(A)(E) exhibit two maxima at X-1 and X-2 where X(1)similar to0.045, 0.04, and 0.03, and X(2)similar to0.12, 0.08, and 0.06, for methanol-water, ethanol-water, and 1-propanol-water, respectively. This is clearly attributed to hydrophobic hydration. Detailed analysis of the behavior of Delta(A)(E) and DeltaS(A)(E) (A=MA, EA, and 1PA) suggests the formation of two kinds of saturated hydration structures and their transition; the clathrate hydration shells with tetrahededral local arrangements of water moleculres around Xsimilar toX(1), and non-clathrate shells with large cavities with three-coordinated local arrangements around Xsimilar toX(2).