Dynamical aspects of mixing schemes in ethanol-water mixtures in terms of the excess partial molar activation free energy, enthalpy, and entropy of the dielectric relaxation process

Dielectric relaxation measurements on the ethanol-water mixture for the entire concentration range in very small increments were carried out using TDR in the frequency range from 300 MHz to 25 GHz at 20 degrees C, 22.5 degrees C, and 25 degrees C. The activation enthalpy Delta H and entropy Delta S for the mixtures were separated from the activation free energy Delta G, and hence the excess partial molar activation free energy, enthalpy, and entropy for ethanol, Delta G(EA)(E), Delta H-EA(E), and Delta S-EA(E), and those for water, Delta G(W)(E), Delta H-W(E), and Delta S-W(E) were calculated. The concentration dependence of these partial molar quantities shows the existence of two regions bound at X (molar fraction of ethanol) similar to 0.18. In the water-rich region of X<0.1, Delta H-EA(E) and Delta S-EA(E) take large positive values, exhibiting two sharp maxima at X=0.04 and X=0.08, which is clearly attributed to structural enhancement of the hydrogen bond network of water by ethanol, the so-called hydrophobic hydration. From a standpoint of dynamics, mixing schemes of ethanol and water around the two points X=0.04 and X=0.08 seem to be qualitatively different. On the other hand, in the region of X>0.18, the values of Delta H-EA(E) and Delta S-EA(E) take nearly zero. This means that ethanol molecules in the mixtures are in almost the same environment as those are in pure ethanol, forming chainlike clusters surrounded or exothermically attached to by water molecules. (C) 1999 American Institute of Physics. [S0021-9606(99)50205-5].