Perfluoroalkanes in water:: Experimental Henry's law coefficients for hexafluoroethane and computer simulations for tetrafluoromethane and hexafluoroethane

The solubility of hexafluoroethane in water was determined experimentally and, together with that of tetrafluoromethane, calculated by molecular simulation. A high-precision apparatus based on an extraction method was used to measure the solubility of hexafluoroethane in water in the temperature range of 287-28 K at pressures close to atmospheric. The experimental data obtained were used to calculate Henry's law coefficients H-2,H-1(p(1)(sat),T), whose temperature dependence was represented by appropriate correlations. The imprecision of the results was characterized by average deviations of H-2,H-1 from these smoothing equations and is of +/-0.17%. From the temperature variation of the Henry's law coefficients, partial molar solution quantities such as the variation of the Gibbs energy, enthalpy, entropy, and heat capacity were derived. Monte Carlo simulations, associated with Widom's test particle insertion method and the finite-difference thermodynamic integration technique, were used to calculate the residual chemical potential of low molecular weight alkanes and perfluoroalkanes in water leading to Henry's law coefficients. Simulations were performed from 280 to 500 K along the saturation line of the pure,solvent. The simulation method was validated by the calculation of H-2,H-1(p(1)(sat),T) for methane and ethane in water for which quantitative predictions were attained even at the lowest temperatures. The calculations for tetrafluoromethane and hexafluoroethane in water were compared with experimental values, when available, to test intermolecular potential models. Solute-solvent radial distribution functions were obtained from simulation at low and high temperatures.