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

Abstract

A flow mixing calorimeter has been used to measure the excess molar enthalpy 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 necessary to adjust the value of the interaction parameter ξ in the combining rule ε₁₂ = ξ(ε₁₁ε₂₂)^1/2 to 1.20. Adjusting the value of ξ to fit the H_m^E measurements yielded second virial cross coefficients B₁₂ which are tabulated. To fit similar H_m^E measurements on water–benzene a value of ξ = 1.43 was needed, and this indicates that the specific water–benzene interaction is about 2.5 times stronger than for water–hexafluorobenzene. From the minima in the water, benzene and hexafluorobenzene pair potentials, orientationally averaged binding energies 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 initio calculations are larger. On the basis of their ab initio calculations Danten et al., (Y. Danten, T. Tassaing and M. Besnard, J. Phys. Chem. A, 1999, 103, 3530), conclude that the binding energy of the water–hexafluorobenzene complex is slightly greater than that of the water–benzene complex, and this is in accord with our experiments.