The heat of mixing and second virial cross coefficient of water + oxygen and water + nitrogen

A vapour phase flow mixing calorimeter has been used to measure the heat of mixing HmE of (0.5H2O + 0.5O2)(g) and (0.5H2O + 0.5N2)(g) at the pressure p = 0.101325 MPa and at temperatures from (373.15 to 493.15) K. The HmE measurements were analysed using an association model for water in which the non-specific part of the interaction energy was calculated from the Stockmayer potential with the parameters ε/k = 233 K, σ = 0.312 nm, and t∗ = 1.278. The specific part of the energy was calculated from an equilibrium constant for the association of two water molecules K298 K = 0.36 MPa−1 together with an enthalpy of association ΔH = −16.85 kJ · mol−1. The second virial coefficients of O2 and N2 were calculated from the Lennard-Jones potential. Cross coefficients B12 were calculated from a combining rule containing an adjustable parameter ξ which is known to be satisfactory for mixtures of (water + hydrogen, or nitrogen, or argon, or methane). For all these mixtures the combining rule gives ξ = 0.99, and this value gives a good fit to the new HmE measurements on (water + nitrogen). For (water + oxygen) it was necessary to reduce ξ from 0.99 to 0.83 to fit the measurements. The values of B12 for both mixtures are close to those obtained from measurements of the solubility of water in compressed oxygen or nitrogen, and confirm that B12 for water–oxygen is less negative than that for water–nitrogen. This is in accord with recent theoretical calculations of the potential surfaces for water–oxygen and water–nitrogen.