Solvent release upon ion association from entropy data

The translational entropy loss on the association of two ions to form an ion pair or 1:1 complex is overcompensated by a rotational entropy gain, an electrostatic entropy gain, and an entropy gain due to solvent release from translational immobilization. The first three effects can be calculated, leaving the fourth as a difference from the experimental entropy change on association. The ratio of the solvent release entropy gain to the entropy change on melting of the solvent indicates the number of solvent molecules released on the association. A similar value is obtained from data for the volume change on association.