Studies on an ester-modified cationic amphiphile in aqueous systems: Behavior of binary solutions and ternary mixtures with conventional surfactants

The aqueous behavior of an ester-modified cationic amphiphile with the molecular structure CH3CH2O(C=O)(CH2)(6)(C=O)O(CH2)sN+(CH3)(3)Br-, in the following referred to as A, has been investigated. Systems with A as the only solute, as well as different aqueous mixtures with conventional cationic surfactants, primarily dodecyltrimethylammonium bromide (DTAB), were included in the study. Isotropic solution samples were characterized using H-1 NMR, C-13 NMR, NMR diffusometry, and conductivity measurements, whereas liquid crystalline samples were investigated by optical polarization microscopy and small-angle X-ray diffraction. The results are compared to the behavior of the binary system of DTAB and water. A does not exhibit a typical surfactant behavior. When it is present as the only solute in a binary aqueous system, it forms neither conventional micelles nor liquid crystalline phases. However; there is clear evidence that it assembles with lower cooperativity into loose clusters at concentrations above 25-30 mM. When A is mixed with DTAB in solution, the two amphiphiles form mixed assemblies, the structure of which varies with the total amphiphile concentration. In concentrated mixtures with alkyltrimethylammonium surfactants, A can participate in hexagonal liquid crystalline phases even when it constitutes a significant fraction of the total amphiphile content.