ION-PAIR EXTRACTION OF PHARMACEUTICAL AMINES .4. INFLUENCE OF ANION ON ENTHALPIC ENTROPIC AND FREE ENERGY CHANGES RESULTING FROM PHASE TRANSFER

Studies on the influence of temperature on the distribution ratio of pharmaceutical ammonium species paired with various anions and between aqueous and organic phases have permitted determination of thermodynamic parameters associated with these extractive processes. Unlike transfer of uncharged solute molecules, ion‐pair transfers involving inorganic anions appear to be largely entropically controlled. The process of movement from water to chloroform of dextromethorphanium halides, for example, involves material increase in ordering. The entropic change associated with the chloride corresponds to −29.7 e.s.u., that for bromide to −17.6 e.s.u. These compare with +10.7 e.s.u. for the free base and +7 to +10 e.s.u. for benzyl alcohol, phenol, and aniline. Less polar organic phases tend to lead to less negative entropic changes. In addition to the halides, data are provided for picrate, trichloroacetate, and several perfluorocarboxylate systems. Copyright © 1969 Wiley‐Liss, Inc., A Wiley Company