Solvent release upon ion association from entropy data. II.
When a cation and an anion associate, the charge on the product is lower than that on the individual ions and solvent is released from their solvation shells to the bulk solvent. This release occurs when the associate is a solvent-shared or contact ion pair or an inner-type complex. The measurable molar entropy change involved is considered to be made up of four contributions: translational, rotational, electrostatic, and desolvation entropies. The former three can be calculated from the properties of the ions and solvents involved; hence, the fourth is obtained by difference. The release of solvent molecules from the crystalline frozen solvent to the liquid on melting is analogous to the solvent release from translational immobilization in the solvation shells of the ions. The molar entropy of melting of the solvent is used to estimate the amount of solvent released in the association process.
Marcus
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Department of Inorganic and Analytical Chemistry, The Hebrew University, Jerusalem 91904, Israel. ymarcus@ vms.huji.ac.il