(16f) Catanionic Surfactant Self-Assembly System Assessed from Membrane Properties

Catanionic surfactant system can be defined as a solution including self-assembly formed by cationic- and anionic- surfactants, where various self-assembly structures such as micelles, vesicles, and coacervates (aqueous two-phase system; ATPS) are spontaneously formed. Their structural diversity is known to be varied, depending onthe ratio of cationic and anionic surfactants, their composition, chemical structure of phase(self-assembly)-constitutive molecules and so on. Herein, two types of catanionic surfactant system were systematically characterized; (i) cetyl trimethyl ammonium bromide (CTAB) and sodium dodecyl sulfate (SDS), both of which are typical ionic surfactants, and (ii) dodecylmethylimidazoliumammonium bromide ([C₁₂mim]Br) and SDS, on of which consists of an ionic liquid (IL). The phase state of each system was investigated based on the total concentration of surfactants and, also, the molar ratio of cationic surfactants against its total concentration by physically observing sample appearance, turbidity, and particle size. Especially in the specific compositions that vesicle could form, the closed structure of vesicle was confirmed through the calcein release and then the membrane properties of the prepared self-assemblies were evaluated by using a fluorescence probe, Laurdan, for the polarity of the interface of the self-assembly and, also, by analyzing Raman spectra to estimate the packing parameter of molecules at the formed membrane structure. measured from the Raman spectroscopy. The obtained results clearly indicate a consistent tendency for the phase transition behaviors in the (i) CTAB-SDS system and (ii) [C₁₂mim]Br-SDS system depending on the total surfactant concentration and their composition, while the membrane properties were different between the above systems. It has been shown that the vesicle membrane formed in CTAB-SDS system was more hydrophobic and at tightly-packed membrane state than those of [C₁₂mim]Br-SDS system , owing to the difference of geometric structure of vesicle-constituting molecule: [C₁₂mim]Br has larger head group and shorter acyl chain in contrast to CTAB.