Investigations into the common ion effect

The molecular origins of the common ion effect and the salting out of nonpolar molecules from aqueous solutions are investigated. Thermodynamic stability criteria for a common ion mixture in a polar solvent are derived. Kirkwood–Buff statistical thermodynamics is used to make the connection with the microscopic pair correlation functions. The observed sensitivity of the compositional stability with respect to ionic strength indicates that a demixing transition is the primary cause of the instability for the common ion effect for our model Lennard-Jones plus Coulomb Hamiltonian.