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

Abstract

Dielectric relaxation measurements on methanol (MA)-water, ethanol (EA)-water, and 1-propanol (1PA)-water mixtures were carried out using TDR (Time Domain Refletometry) for the entire concentration range of 0 ≤ X (molar fraction of alcohol) ≤ 1.0 up to 25GHz at 20°C-30°C. We have calculated the excess activation free energy, enthalpy, and entropy of these mixtures, ΔG E , ΔH E , and ΔS E , and the excess partial molar quantities for alcohol, ΔG A E , ΔH A E , and ΔS A E (A = MA, EA, and 1PA) and those for water, ΔG W E , ΔH W E , and Δ S 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∼0.30, 0.18, and 0.14 for methanol-water, ethanol-water, and 1-propanol-water, respectively. In the region of X ≥ X b, Δ H A E and Δ S 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, ΔH A E and ΔS A E exhibit two maxima at X 1 and X 2, where X 1∼0.045, 0.04, and 0.03, and X 20.12, 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 ΔH A E and ΔS 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 molecules around X∼ X 1 and non-clathrate shells with large cavities with three-coordinated local arrangements around X∼ X 2.