A study of the solubilization of p-methoxycinnamic acid and p-dimethylaminobenzoic acid esters in long-chain surfactant systems

The aqueous solubility of water-insoluble materials can be increased by adding a third component, a surfactant. This phenomenon of solubilization is directly related to one important property of the surfactant, i.e., the primary molecules of the surfactant aggregate spontaneously to form micelles in the system. The present work was undertaken to study the solubilization of two homologous series of ten high fatty esters of p-methoxycinnamic acid and p-dimethylamino benzoic acid, which have structures similar to the active ingredients in sunscreen formulations, in several surfactant systems. The measurement of the maximum absorbance of ultraviolet (UV) radiation by the esters in the surfactant systems has been used to determine the extent of solubilization. The effect of surfactant structures and mixed surfactant systems on the extent of solubilization of the esters has been investigated. The finding of preferential solubility of dodecyl esters of p-methoxycinnamic acid and p-dimethylamino benzoic acid in the dodecyl surfactant systems by Hochberg was further confirmed by this research. Similar preferential solubility was also observed for the tetradecyl esters of p-methoxycinnamic acid and p-dimethylaminobenzoic acid in most tetradecyl surfactant systems selected for this research. The counterions on the polar heads of the tetradecyltrimethyl ammonium salts did not change the preferential solubility of the tetradecyl esters among the esters in the two homogenous series. Similar solubilization tendency curves were obtained in the same chain length surfactant systems for the two homologous series of esters, but the solubilization results were unexpectedly changed in the mixed binary homologous dodecyl and tetradecyl sulfate systems. The results suggest that the chain length of surfactants is the major factor to affect the process of solubilization of the long chain esters of p-methoxycinnamic acid and p-dimethyl aminobenzoic acid.