Effects of solute concentration and cosolvents on the aqueous activity coefficient of halogenated hydrocarbons

Henry's law constant of chloroform, carbon tetrachloride and hexachloroethane was measured by the multiple equilibration technique with direct aqueous phase analysis in a closed system in the absence and presence of varying concentrations of miscible cosolvents, methanol and 2-propanol. Henry's law constant was directly proportional to the solute's aqueous activity coefficient at infinite dilution. A constant value of the coefficient was a valid approximation for compounds with aqueous mole fraction solubility less than 0.001. Cosolvents reduced the solute's coefficient only at cosolvent mole fractions greater than 0.005. The effect was stronger the more hydrophobic the solute and/or the cosolvent. The validity of a semiempirical thermodynamic model, UNIFAC, was assessed in a dilute concentration range. Cosolvent effects were predicted conservatively by UNIFAC, whilst prediction of solution concentration effects agreed with the experimental results. There are 45 references.