Experimental and theoretical studies of the mechanisms that underlay ion-pair formation, their properties and applications in various fields have been and still are focused by researchers since the introduction of the concept in 1926, by Bjerrum. Ion pairs are distinct chemical entities, electrically neutral, formed between ions of opposite charge and held together by Coulomb forces, without formation of a covalent bond. Investigation methods used are various, from classical conductometric measurements to up-to-date methods, such as spectrophotometry, chromatography and capillary electrophoresis. In the pharmaceutical field, ion pairs were used to develop methods of separation, identification and assay for the active substances in complex matrices, to obtain pharmaceutical formulations with controlled release and to explain the mechanisms of transport and action for certain drugs. The chapter is an attempt to describe new trends in the spectrophotometry of ion pairs and their applications in the pharmaceutical field. The development of the concept and types of ion pairs are first presented; further, examples of applications using molecular absorption, fluorimetry and resonance light scattering spectrophotometry are presented. Based on the literature data and the authors’ experience in the field, challenges and perspectives in the ion-pair spectrophotometry are also considered.
Part of the book: Spectroscopic Analyses