Probing the alkyl chain length effects on molecular packing characteristics of mixed ion pair amphiphile/double-chained cationic surfactant vesicular bilayers with the Langmuir monolayer approach

Alkyl chain length effects of added double-chained cationic surfactants, dialkyldimethylammonium bromides (DXDABs), including ditetradecyldimethylammonium bromide (DTDAB), dihexadecyldimethylammonium bromide (DHDAB), and dioctadecyldimethylammonium bromide (DODAB), on the Langmuir monolayer behavior of a pseudodouble-chained ion pair amphiphile, hexadecyltrimethylammonium-dodecylsulfate (HTMA-DS), were analyzed with the Langmuir monolayer technique, infrared reflection–absorption spectroscopy, and Brewster angle microscopy. It was found that DXDA+ of DXDAB could displace HTMA+ from HTMA-DS and the displaced HTMA+ might desorb into the aqueous subphase, with the extent depending on the alkyl chain length of DXDAB. In the mixed HTMA-DS/DODAB monolayer, DODAB possessing long alkyl chains was capable of restraining HTMA+ from desorption with enhanced intermolecular interaction reflected by the favorable molecule aggregation. As for the mixed monolayers of HTMA-DS and DTDAB with short alkyl chains, incomplete displacement of HTMA+ from HTMA-DS by DTDA+ was detected and ascribed to weak intermolecular attraction exhibited by the fractal-like domain formation. Among the three DXDABs, DHDAB with the best molecular structure match to HTMA-DS in terms of alkyl chain length could form well-packed monolayer structures with HTMA-DS. The finding has the implication on the surface characteristic control of catanionic vesicles composed of ion pair amphiphile with double-chained cationic surfactant.