PHASE-BEHAVIOR OF AQUEOUS-SOLUTIONS OF A DIMERIC SURFACTANT

The effect of salt (KBr) additions on the behavior of the dimeric surf actant 12-2-12 (ethanediyl-1,2-bis(dodecyldimethylammonium bromide)) h as been investigated using small angle neutron scattering (SANS) and d ynamic light scattering (DLS). The effect of temperature and additions of DTAB (dodecyltrimethylammonium bromide, ''monomer'' of 12-2-12) ha s also been investigated. Upon increasing salt content, the partial ph ase diagram of the water/salt/12-2-12 system shows the sequence wormli ke micelle phase, lamellar phase, and biphasic systems with a salt-ric h, surfactant-poor lower phase and a salt-poor lamellar upper phase. T his study is the first one reporting a salt-induced transition between a wormlike micelle phase and a lamellar phase. Additions of DTAB at l ow mole fraction (<5%) produced no significant changes of the phase di agram. The scattering curves for the lamellar phase show a maximum whi ch becomes extremely sharp as the temperature is decreased and reveals the occurrence of L-alpha to L-beta phase transition at around 30 deg rees C. In addition, the SANS data reveal the presence of highly curve d defects. The analysis of the SANS data suggests that above 30 degree s C the lamellar phase is constituted by perforated lamellae, with the water-filled holes (defects). The existence of this phase was predict ed theoretically as an intermediate one, together with branched wormli ke micelles, between lamellar and wormlike micelle phases. DLS was use d to determine the viscoelastic relaxation time and amplitude of the w ormlike micelle phase. The increase of amplitude at the approach of th e isotropic-to-lamellar phase transition suggests increasing intermice llar interactions that may be responsible for the formation of branche d micelles.