Study of the interaction phenomena of cetyl-trimethyl-ammonium bromide, cetylpyridinium chloride and benzethonium chloride with C. I. Acid Orange 52 and picric acid by two spectral methods

A study of solutions containing C. I. Acid Orange 52 (AO52) or picric acid (PA) and cationic surfactants in various molar ratios by UV–vis spectrophotometry and ion pair extraction spectrophotometry has been made. Cetyl-trimethyl-ammonium bromide (CTAB), cetylpyridinium chloride (CPC) and benzethonium chloride (BEC) have been chosen as cationic surfactants. AO52 and PA have been used as substances with coloured organic anion. The study focuses on investigation of solutions containing AO52 or PA in molar excess over the surfactants. The results show significant changes of absorption maxima in solutions of CTAB or CPC and AO52. The changes of absorption maxima in the case of AO52 excess are ascribed to the formation of associates soluble in water but insoluble in non-polar solvents. This process influences negatively the equilibrium of the ion pair formation. Consequently the amount of the ion pairs extracted during the ion pair extraction spectrophotometry is lowered and a hypochromic shift is recorded as the excess of the dye is increased. Therefore it may be concluded that the results of the ion pair extraction spectrophotometry and the direct UV–vis measurement correspond to each other as far as the systems CTAB + AO52 and CPC + AO52 are concerned. The PA excess does not cause any spectral shifts. Thus, this is in the agreement with the results of the ion pair extraction spectrophotometry, since the excess of PA does not influence the absorbance of the extracted ion pairs.