The effect of solute concentration and the effect of model cosolvents (methanol and 2-propanol) on the solute's aqueous activity coefficient at infinite dilution, γi∞, were investigated at 20 °C. The multiple equilibration technique with direct aqueous-phase analysis was used to measure Henry's law constant of CHCl3, CCl4, and C2Cl6, which is directly proportional to γi∞. It was found that γi∞ = constant is a valid approximation for compounds with aqueous solubility mole fractions xis, ≲ 10-3. Cosolvents reduce the solute's γi, only at cosolvent mole fractions x > 5 × 10-3. The more hydrophobic the cosolvent the stronger the effect on a given solute, and the more hydrophobic the solute the stronger the effect of a given cosolvent, in agreement with general thermodynamic principles. The validity of a semiempirical thermodynamic model, UNIFAC, is assessed in the dilute concentration range. UNIFAC predicts cosolvent effects conservatively, while prediction of solute concentration effects agrees with the findings of this work.
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Münz et al. (1986) studied this question.
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