Evidence of Hydrophobic Effects from Critical Phenomena

Abstract

AbstractData of the temperature dependence of partial molar quantities (heat capacity and temperature derivative of partial molar volume) of perfluoromethylcyclohexane and carbon tetrachloride in a mixture of critical composition are reported. It is found that for both components the signs of the critical amplitudes of the partial molar quantities are positive. Using these data a thermodynamic relation between the critical amplitudes of specific head capacity, partial molar heat capacity, temperature derivative of specific volume and temperature derivative of partial molar volume is tested. — Analogous experiments on the system isobutyric acid/water give results which are characteristically different: (1) the sign of the critical amplitude of the partial molar heat capacity of isobutyric acid is positive whereas that of water is negative, (2) the opposite is true for the sign of the critical amplitude of the temperature derivative of the partial molar volume of the two components. The characteristics of the system isobutyric acid/water are interpreted by assuming that in a critical mixture, approaching the critical point from the one fluid phase region, the extent of hydrogen bonding between water molecules in the vicinity of the hydrophobic part of isobutyric acid molecules increases with decreasing temperature difference from the critical temperature.