Spectrophotometric and thermodynamic studies on the acidity constants of 4-(4-nitrophenylazo) resorcin in micellar media

The acidity constants of 4-(4-nitrophenylazo) resorcin (Scheme 1) in water, water-SDS, water-Triton X100 and water-CTAB micellar media solutions at different temperature and an ionic strength of 0.1 M have been determined multiwavelength spectrophotometrically.To evaluate the pH-absorbance data, a resolution method based on the combination of soft- and hard-modeling is applied. The acidity constants of all related equilibria are estimated using the whole spectral fitting of the collected data to an established factor analysis model. DATAN program applied for determination of acidity constants. Results show that the pKa values of 4-(4-nitrophenylazo) resorcin are influenced as the percentages of a neutral and ionic surfactant such as Triton X100, SDS and CTAB, respectively, added to the solution of this reagent. Effect of surfactant on acidity constants and pure spectrum of each component are also discussed. Aqueous micellar media are widely used in different areas of chemistry and several reviews concerning their applications have been published. One important property of micelles is their ability to solubilize a wide variety of compounds which are insoluble or slightly soluble in water. The incorporation of a solute into micellar systems can lead to important changes in its molecular properties. Another important effect of micellar systems is that they can modify reaction rates and, to some extent, the nature of the products. Micelles can inhibit or accelerate reaction rates (by up to several orders of magnitude) and also shift equilibria (acid-base).Surfactants usually affect spectral parameters: the intensity and shifts in the absorption bands can be increased and shifts in the absorption maxima of reagents are observed. Micelles can affect the apparent pKa values of the reagents due to a combination of electrostatic and microenviromental effects of the micelle. Moreover, the acid-base equilibria involved in these systems are also influenced by surfactants [1-3].