The water-hexafluorobenzene interaction. Second virial cross coefficients for water-hexafluorobenzene derived from gas phase excess enthalpy measurements

A flow mixing calorimeter has been used to measure the excess molar enthalp y H-m(E) of gaseous (water+hexafluorobenzene) at the composition y=0.5, at standard atmospheric pressure, and over the temperature range 383.2 to 453. 2 K. The measurements were compared with values calculated from the Kihara potential for hexafluorobenzene and from the Stockmayer potential for water in its interaction with a nonpolar fluid. To fit the measurements it was n ecessary to adjust the value of the interaction parameter xi in the combini ng rule epsilon (12)=xi>(*) over bar *(epsilon (11)epsilon (22))(1/2) to 1. 20. Adjusting the value of xi to fit the H-m(E) measurements yielded second virial cross coefficients B-12 which are tabulated. To fit similar H-m(E) measurements on water-benzene a value of xi =1.43 was needed, and this indi cates that the specific water-benzene interaction is about 2.5 times strong er than for water-hexafluorobenzene. From the minima in the water, benzene and hexafluorobenzene pair potentials, orientationally averaged binding ene rgies for water-benzene and water-hexafluorobenzene interactions were both calculated to be -(5.3 +/-0.5) kJ mol(-1). Binding energies for the van der Waals complexes in the minimum energy configuration obtained from ab initi o calculations are larger. On the basis of their ab initio calculations Dan ten et al., (Y. Danten, T. Tassaing and M. Besnard, J. Phys. Chem. A, 1999, 103, 3530), conclude that the binding energy of the water-hexafluorobenzen e complex is slightly greater than that of the water-benzene complex, and t his is in accord with our experiments.