Co-Solubilization of the Binary Mixture of 1-Naphthol and 2-Naphthol in the Water Micellar Solution of Sodium Cholate and Cetyltrimethylammonium Bromide

The micellar co-solubilization of alpha- and beta-naphthol and their binary mixture with surfactants sodium cholate (SC) and cetyltrimethylammonium bromide (CTAB) is examined. The micellar solubilization of 2-naphthol (2NpOH) in SC is entropic, and it is an energetically more favored process than the micellar solubilization of 1-naphthol (1-NpOH). The capacity of the solubilization by CTAB for solubilizing 1- and 2-naphthol is about eight-times higher at 4CMC than at 2CMC (four- and two-times the critical micelle concentration, respectively), which can be explained by the change in the micelle structure with an increase in the total concentration of the CTAB surfactant. For the same CMC values of the examined surfactants, SC has a higher solubilization capacity for naphthols in comparison to the solubilization capacity of CTAB, which is explained by the existence of hydrogen bonds between the building units and solubilizates that are formed in the SC micelles. For each molar ratio of the micellar binary mixture of 1- and 2-NpOH and in both examined surfactants (except for SC solubilization with a = 0.1 and a = 0.9 at 4CMC), the interaction parameters for 1- and 2-NpOH are larger than zero, which means there is a mutual antagonistic effect of 1- and 2-naphthol on their micellar co-solubilization that is probably the consequence of the competition for the same spatial part of the hydrophobic domain of the examined micelles. The binary mixture of 1- and 2-NpOH in the micellar pseudophase possesses a higher Gibbs free energy content and is thermodynamically less stable than the monocomponent solubilizates in SC and CTAB micelles.