Ion-pair extraction of pharmaceutical amines IV: Influence of anion on enthalpic, entropic, and free energy changes resulting from phase transfer
✍ Scribed by Arthur F. Michaelis; Takeru Higuchi
- Publisher
- John Wiley and Sons
- Year
- 1969
- Tongue
- English
- Weight
- 447 KB
- Volume
- 58
- Category
- Article
- ISSN
- 0022-3549
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✦ Synopsis
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 t o +I0 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.
Keyphrases 0 Ion-pair extraction-amines 0 Dextromethorphantest amine 0 Anion effect-phase transfer thermodynamics 0 Solvent effect-phase transfer thermodynamics 0 Ionic, nonionic solutes-extraction mechanism As a part of studies on ion-pair extraction of pharmaceutical amines (1-3), the enthalpy, free energy, and entropy changes associated with transfer from aqueous to lipoidal phases of cationic form of dextromethorphan, a typical pharmaceutical amine, paired with various organic and inorganic anions, have been compared with those of neutral solutes. Thermodynamic data on the ion-pair extractive process of this system, it was felt, would shed additional light on the nature of ion-pair formation in the organic phase in general, and permit more rational development of separatory systems for both analytical and isolation purposes. Although these studies have been limited to systems based on the drug cation, it is apparent that the information is applicable to other ion-pair combinptions.
Past work of this nature has been relatively sparse. Very little of the literature on the extraction of organic and inorganic compounds has treated the thermodynamics of these systems. Siddall has determined the thermodynamics of the extraction of uranyl nitrate and nitric acid by trialkyl phosphates and dialkylphosphonates (4). Krasnov el al. studied the influence of structural factors on the thermodynamic characteristics of the extraction of salts of basic dyes in a series of three papers (5-7). Beyond these few papers little is to be found in the literature concerning these thermodynamic parameters.
These experimental studies are presented in three sections. In the first part, results of an investigation into the effect of the solvating agent concentration in the organic phase on the extractive equilibria as reflected in the enthalpic, entropic, and free energy changes are presented for several ion pairs and organic phases con-