Studies of hydrogen bonding in the vapor phase by measurement of thermal conductivity and molecular orbital calculations: Methanol–water binary mixtures

Abstract

The thermal conductivities of serveral methanol–water binary vapor mixtures were measured in a modified thick hot-wire cell between 352 and 375 K at pressures ranging from 100 to 1400 Torr. Analysis of the data indicates that molecular association to form a 2:1 methanol–water trimeric species is a major source of enhancement of the thermal conductivity. The enthalpy and entropy of association of this trimer are −10.48 kcal mol−1 and −38.24 cal mol−1 deg−1, respectively. Theoretical calculations, based on ab initio molecular orbital calculations, of the enthalpies and entropies of association for various open chain and cyclic methanol–water trimers indicate that the 2:1 methanol–water trimeric species is more stable than the 1:2 trimeric species which is consistent with the results of the analysis of the thermal conductivity data. The theoretical calculations indicate that it may be an entropic effect which makes the 2:1 trimeric species more stable than the 1:2 species.