Salting-out or -in by fluctuation theory

In this paper, the Kirkwood-Buff formalism was used to examine the effect of the addition of a salt on the gas-solubility. A general expression for the derivative of the Henry constant with respect to the salt concentration was thus derived. The obtained equation was used to correlate the experimental solubilities as a function of the salt molality. The correlation involves one parameter, which has to be determined from the experimental data. In addition, it requires information about the molar volume of the salt solution and the mean activity coefficient of the salt. It has been shown that the experimental solubilities can be well correlated when an accurate expression for the mean activity coefficient of the salt is used. It was also shown that the well-known Sechenov equation constitutes a particular case of the obtained expression. The general expression allowed one to find a criterion for the prediction of the kind of salting (salting-in or salting-out) for dilute salt solutions. According to this criterion, the kind of salting depends mainly on the molar volume of the salt at infinite dilution. This explains the literature observations that the salts with large molar volumes at infinite dilution usually increase the gas solubility compared to that in pure water.