Characteristics of mixed systems of phenol red and cetylpyridinium chloride

Abstract

Dye–surfactant systems known to exhibit two critical micelle concentrations (cmcs) are not many and the behavior of such systems in the mixed form has not been fully explored and reported. Phenol red (PR) and cetylpyridinium chloride (CPC) form one such dye–surfactant pair and its mixtures are studied here in the entire composition range using surface tension, conductance and UV–Visible spectroscopy methods. In this mixed system, mixed micelles are formed with one cmc in the region of 0<αCPC<0.5, while mixed micelles as well as micelles of the surfactant alone are formed with two cmcs if 0.5≤αCPC<1 (αCPC is the mole ratio of CPC to PR in solution). The mixture shows strong synergism in cmc with high negative value for interaction parameter. The bathochromic shift of the 432nm band indicated hydrogen bonding between PR− species in the region of αCPC<0.4 and this hydrogen bonding weakened more in the presence of CPC due to ion pair formation than in the presence of a non-ionic surfactant TX-100. The relative amount of CPC in the mixture and the nature of the surfactant are found to influence the maximum wavelength of the PR2− band also. The excess standard free energy of micellization has a linear correlation with the interaction parameter. Surface excess values of the individual components of the mixed system were evaluated. The compositions of the mixed micelle and the adsorbed layer reveal that some CP+ monomers also aggregate/adsorb along with the CP+–PR− ion pairs.