Composition-dependent dynamical structures of 1-propanol–water mixtures determined by dynamical dielectric properties

Using time domain reflectometry, we carried out dielectric relaxation measurements on 1-propanol–water mixtures for the entire concentration range in the frequency range 100 MHz–25 GHz at 20, 25, and 30 °C. We have calculated the excess partial molar activation free energy, enthalpy, and entropy for 1-propanol, ΔG1PAE, ΔH1PAE, and ΔS1PAE, and those for water, ΔGWE, ΔHWE, and ΔSWE from the relaxation times. In the region of X (molar fraction of 1-propanol) ⩾0.14, ΔH1PAE and ΔS1PAE take nearly zero. This means that 1-propanol molecules in the mixtures find themselves in not a very different environment from that in pure liquid. In the water-rich region, ΔH1PAE and ΔS1PAE exhibit two maxima at X∼0.03 and X∼0.06, corresponding roughly to 0.9 and 0.78 g/cm3 of water content, respectively. This fact, together with the results of the molecular dynamics studies of Sciortino et al. suggest the formation of two kinds of saturated hydration structures: the clathrate hydration shells with tetrahedral local arrangements of water molecules around X∼0.03 and nonclathrate shells with large cavities with three-coordinated local arrangements around X∼0.06. Hydrophobic hydration seems to partly share the same mechanism with structural enhancement in pure water by lowering local density.