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

•Mixed layer behavior of ion pair amphiphile and double-chained cationic surfactant.•Alkyl chain length effects on mixed layer behavior.•Probing molecular packing characteristics in mixed layers.•Investigating molecular desorption from mixed layers by infrared spectroscopy. 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.