Triangular phase diagram of the catanionic system dodecyltrimethylammonium bromide–disodium dodecanephosphonate–water

The catanionic system dodecyltrimethylammonium bromide (DTAB)–disodium dodecanephosphonate (DSDP)–water phase behaviour differs from that shown by other catanionic systems (e.g. low solubility, highly non-ideal interactions in micelles, formation of lamellar mesophases and vesicles and/or liposomes, strong reduction of the critical micelle concentration (CMC) and the domains of existence of micelles). The system is highly soluble at all DTAB:DSDP ratios. The CMC has an almost ideal dependence on the DTAB:DSDP ratio. There is a wide domain of existence of spherical micelles, followed by a much largest domain of existence of rodlike micelles. At high concentration, only hexagonal liquid crystal exist. No vesicles nor lamellar mesophase were found. An explanation of this behaviour is proposed, based on steric effects due to the size of the headgroups.