Effect of Biocompatible Esters and Alcohols as Cosurfactants on Structure and Solubilization Behavior of the Zwitterionic Surfactant Tetradecyldimethylamine Oxide

In this work, we compare the effect of different monoterpenoid alcohols that differ with respect to their number of double bonds and simple aromatic esters of variable molecular architecture as cosurfactants on the phase behavior of the zwitterionic surfactant tetradecyldimethylamine oxide (TDMAO) and its solubilization behavior, with respect to decane as a model paraffin oil. The esters are shown to be potent cosurfactants but require higher concentrations to achieve similar effects, with respect to structural changes and solubilization enhancement. Compared to the alcohols, they solubilize somewhat smaller amounts of decane, do reduce the interfacial tension substantially less, and also do not form an isotropic phase of unilamellar vesicles (L₄) but directly multilamellar vesicles (L_αl). A very interesting effect is the significance of the detailed molecular architecture of the esters, as ethyl benzoate and benzyl acetate, both having the same sum formula, differ significantly, with respect to their cosurfactant properties. However, all systems allow one to incorporate relatively large amounts of the oil. For the case of the esters, this always leads to the formation of oil-in-water (O/W) microemulsion droplets while the alcohols can build in relatively large amounts of oil within their vesicular structures. These findings show that these biofriendly cosurfactants allow to formulate structurally rather versatile systems and efficiently enhance oil solubility for the given surfactant system.