Chapter 5. Activity Coefficient and Solubility in Water

Summary

The extent to which an organic compound “likes” being surrounded by liquid water is of utmost importance to the environmental behavior and impact of the compound. This chemical property is called the aqueous activity coefficient, and it is closely related to the compound's aqueous solubility. For organic liquids, both real and hypothetical ones, the aqueous activity coefficient is equal to the inverse of the mole fraction aqueous solubility. For organic gases and solids, this inverse relationship is modified to include the energetic costs of condensing the gas or melting the solids at the temperature of interest. Since intermolecular interactions dictate the extent to which an organic solute “likes” to be dissolved in water versus being mixed in a solution of itself, multiparameter linear solvation energy relationships (LSERs) are presented to show how aqueous activity coefficients are a function of their tendencies to escape themselves (vapor pressures), to create cavities in the aqueous solvent (molecular size), and to form van der Waals and hydrogen-bonding interactions with the water. Environmental factors modify these aqueous activity coefficients, and quantitative expressions for the effects of temperature, salts, and organic co-solvents are given. Finally, one-parameter expressions (e.g., based on molecular size or octanol-water partition coefficients) are also presented as powerful tools for estimating (liquid) solubilities for new chemicals of interest given experience with sets of structurally-related compounds.