Hydrophobic hydration and molecular association in methanol-water mixtures studied by microwave dielectric analysis

摘要

Dielectric relaxation measurements on the methanol-water mixtures for the entire concentration range were carried out using time domain reflectometry in the frequency range from 500 MHz to 25 GHz at 20, 25, and 30 degrees C. The excess partial molar activation free energy, enthalpy, and entropy for methanol, Delta G(MA)(E), Delta H-MA(E), and Delta S-MA(E), and those for water, Delta G(W)(E), Delta H-W(E), and Delta S-W(E), were calculated from accurately measured concentration and temperature dependence of the dielectric relaxation time of the mixtures. The behavior of the excess partial molar quantities in the regions below and above X (molar fraction of methanol) similar to 0.3 are quite different from each other. In a water-rich region, Delta H-MA(E) and Delta S-MA(E) exhibit two maxima at X similar to 0.045 and X similar to 0.12, which is clearly attributed to structural enhancement of the hydrogen bond network of water, the so-called hydrophobic hydration. Appearance of two maxima in Delta H-MA(E) and Delta S-MA(E) implies that water molecules surround methanol molecules in qualitatively different manners around the two points. In the concentrated region of X greater than or equal to 0.3, the values of Delta H-MA(E) and Delta S-MA(E) become nearly zero, which means that methanol molecules in the mixtures find themselves in not a very different environment from that in pure methanol, associated and forming chainlike clusters. Water molecules seem to exothermically attach to the hydrophilic site of methanol. (C) 2000 American Institute of Physics. [S0021-9606(00)52105-9]. [References: 27]