Benzene-polar fluid association. An analysis of measurements of the vapour phase excess molar enthalpy of mixtures of (cyclohexane, or benzene plus polar and non-polar fluids)

A differential flow mixing calorimeter has been used to measure the excess molar enthalpy H-m(E) of gaseous mixtures containing either cyclohexane or benzene mixed with 14 other gases. These are nitrogen, methane, carbon dioxide, dimethyl ketone, diethyl ketone, ether, dioxane, chloromethane, dichloromethane, chloroform, tetrachloromethane, water, methanol, and ethanol. For the mixtures with cyclohexane vapour, the excess enthalpy of the mixtures with nitrogen, methane, and carbon dioxide is in accord with values calculated by using the usual combining rules to obtain the interaction parameter (1 - k(12)) in the combining rule epsilon (12) = (1 - k(12))(epsilon (11)epsilon (22))(1/2). For (cyclohexane + polar fluids) the interaction parameter is slightly larger than expected. The experimental values of (1 - k(12)) obtained from the H-m(E) measurements on the cyclohexane mixtures were used to calculate H-m(E) for mixtures in which benzene rather than cyclohexane was used. However, with the exception of the mixtures of benzene with nitrogen and methane, the excess enthalpies of an the other 12 gases are smaller than predicted, and this suggests that there is a specific interaction between benzene and the polar component. The difference between the calculated and experimental excess enthalpies was fitted by an association model which describes the unlike molecular interaction in terms of an equilibrium constant K-12, and an enthalpy DeltaH(12) of dimer formation. No obvious relationship between the electrical properties of the polar molecule and the enthalpy of association was found, but when plotted against Fitter's acentric factor omega the values of K-12 and DeltaH(12) were discovered to be in three groups: chlorocarbons, ketones and ethers, and alcohols. In each group the values of K-12 and DeltaH(12) are a linear function of omega. The H-m(E) measurements on (carbon dioxide + benzene) and (dioxane + benzene) make it clear that contributions to the pair potential due to quadrupolar forces must be included if benzene-polar fluid interactions are to be understood. (C) 2000 Academic Press.