The solubilization of esters of p-methoxycinnamic acid and p-dimethylaminobenzoic acid in surfactant solutions

A property of surfactant solutions which is a direct consequence of their formation of micelles in the bulk phase is the phenomenon of solubilization, in aqueous solution the ability to dissolve water-insoluble materials. Ten higher fatty esters of p-methoxycinnamic acid and p-dimethylaminobenzoic acid were selected for study in part because of their relation to compounds which have found use as the active ingredient in sunscreen formulations. The present study was undertaken to obtain qualitative and quantitative data on the ability of ten commercial surfactants to solubilize homologous series of the higher molecular weight fatty esters with even-numbered straight-chain alkyl groups consisting of ten to eighteen carbon atoms. The measurement of the absorbance of the ultraviolet absorption maximum of the esters in the surfactant solutions has been used to determine the extent of solubilization. From this research it was found that, of the ten commercial surfactants studied, the two most effective solubilizers proved to be Triton X-100 and sulfobetaine. All of the surfactants utilized in this study, except Triton X-100, bear a dodecyl, or similar alkyl chain. Under the experimental conditions of this research, i.e., 40$\sp\circ$C and solubilization conditions which did not involve extensive solution agitation, a preferential solubility of the dodecyl esters has been observed in most cases. Shifts in the wavelength of maximum absorbance were also observed for the larger esters in several surfactant solutions, accompanied by low absorbance values. The results suggest that solubilization of these compounds may begin in the hydrocarbon interior of the micelle and proceed to the palisade layer as the concentration of solubilized species in the micelle increases.