Excess enthalpies and cross-term second virial coefficients for mixtures containing water vapour

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Measurements of the excess molar enthalpy for {x H2O +(1 –x) CH3Cl}(g) and {x H2O–(1 +x) C2H5Cl}(g) mixtures at pressures around 60 kPa and over the temperature range 363.2–423.2 K are reported. Cross-term second virial coefficients, B12, have been derived from the measurements. A method of calculating third virial coefficients of mixtures containing water vapour is suggested. The method allows a more detailed analysis of HEm measurements than was previously possible when third virial terms were ignored and a small systematic error was introduced into derived B12 values. Previously reported HEm values for 11 mixtures containing water have been reanalysed and better B12 values have been obtained: these agree well with values obtained from measurements of the solubility of water in compressed gases. HEm values for water–chloroalkane mixtures, together with previously reported measurements on 11 mixtures of water with non-polar fluids have been used to obtain parameters of the Stockmayer potential for water in its interaction with any substance with which it does not associate. It was found that the interaction parameter ξ= 2(σ311σ322)½(σ–312)(I1I2)½(1+I2)–1 when used in the combining rule Iµ12=ξ(Iµ11Iµ22)1/2 works as well for water–n-alkane mixtures as it does for alkane-alkane mixtures. Using this combining rule together with the Stockmayer parameters Iµ/kB= 233 K, σ= 0.312 nm and t*= 1.238 for water, B12 values can be calculated for mixtures of water with any polar or non-polar substance. For mixtures of water with carbon dioxide, ethene, propene or benzene the experimental B12 values are significantly more negative than those calculated. This is attributed to specific interactions between the water lone-pair electrons and unfilled orbitals in the π-electron system of the other component of the mixture.