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.

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The influence of water structuring effect of methanol and ethanol on ion pairing of two guanidinum containing pharmaceutical compounds, namely streptomycin and arginine, was studied. Congo red was used as counterion. For this purpose, the molecular modelling, physico-chemical constants (dielectric constant, conductivity) and absorption and resonance light scattering spectrometry were used. Determinations were performed using water/alcohol binary solvent mixtures containing molar fractions of 0.05 methanol, respectively 0.03 ethanol. The results showed that, in low concentrations, methanol and ethanol favours ion pairing. For streptomycin, analytical applications were studied. Rezumat A fost studiată influența efectului structurant al apei determinat de metanol și etanol asupra echilibrului de asociere ionică a unor compuși farmaceutici care conțin în structură grupări guanidinice. Roșul de Congo a fost folosit drept contraion. Analiza a fost efectuată folosind modelarea moleculară, constante fizico-chimice (constantă dielectrică, conductivitate) și spectrometria de absorbție moleculară și de difuzie de rezonanță. Determinările au fost efectuate folosind amestecuri apă/alcool conținând o fracție molară de 0,05 metanol, respectiv 0,03 etanol. Rezultatele obținute au arătat că, în concentrații mici, metanolul și etanolul favorizează formarea perechilor de ioni. Pentru streptomicină au fost studiate aplicațiile analitice.

The paper presents a new non-extractive spectrophotometric method for the assay of albendazole (ABZ) as ion pair with Congo red (CR). A soluble and stable ion pair (ABZ-CR) with an absorption maximum at 294 nm was obtained using a 2•10 -8 M CR concentration and Britton Robinson (BR) buffer with pH = 2.5. The method is linear in the range 2.5 -4.7 µg/mL, with limits of detection and quantification of 0.09 µg/mL and 0.28 µg/mL, respectively. The proposed method was applied to the assay of ABZ in bulk pharmaceutical substance. Because CR is a dye with pharmacological applications used as model in biochemical studies, the research also focused on the mechanism of interaction with ABZ. Spectral changes and physicochemical constants (conductivity, dielectric constant) indicated that ABZ-CR ion pairing takes place at the surface of CR aggregates by electrostatic and charge transfer interactions. RezumatLucrarea prezintă o nouă metodă spectrofotometrică non-extractivă de dozare a albendazolului (ABZ) ca pereche de ioni cu roșu de Congo (CR). Perechea de ioni (ABZ-CR), solubilă și stabilă, cu maxim de absorbție la 294 nm s-a obținut folosind o concentrație de CR de 2•10 -8 M și soluție tampon Britton-Robinson (BR) cu pH = 2,5. Metoda este liniară pe domeniul de concentrații 2,5 -4,7 µg/mL, cu limita de detecție de 0,09 µg/mL și limita de cuantificare de 0,28 µg/mL. Metoda a fost aplicată la dozarea substanței farmaceutice ABZ. Deoarece CR este un indicator cu proprietăți farmacologice și folosit ca model în studiile de biochimie, studiul a avut în vedere și stabilirea mecanismului de interacțiune cu ABZ. Modificările spectrale și constantele fizico-chimice (conductivitate, constantă dielectrică) au aratat că perechea de ioni ABZ-CR se formează la suprafața agregatelor de CR prin interacțiuni electrostatice și transfer de sarcină.

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