Phase behavior of aqueous solutions of a dimeric surfactant

The effect of salt (KBr) additions on the behavior of the dimeric surfactant 12-2-12 (ethanediyl-1,2-bis(dodecyldimethylammonium bromide)) has been investigated using small angle neutron scattering (SANS) and dynamic light scattering (DLS). The effect of temperature and additions of DTAB (dodecyltrimethylammonium bromide, ''monomer'' of 12-2-12) has also been investigated. Upon increasing salt content, the partial phase diagram of the water/salt/12-2-12 system shows the sequence wormlike micelle phase, lamellar phase, and biphasic systems with a salt-rich, surfactant-poor lower phase and a salt-poor lamellar upper phase. This study is the first one reporting a salt-induced transition between a wormlike micelle phase and a lamellar phase. Additions of DTAB at low mole fraction (<5%) produced no significant changes of the phase diagram. The scattering curves for the lamellar phase show a maximum which becomes extremely sharp as the temperature is decreased and reveals the occurrence of L-alpha to L-beta phase transition at around 30 degrees C. In addition, the SANS data reveal the presence of highly curved defects. The analysis of the SANS data suggests that above 30 degrees C the lamellar phase is constituted by perforated lamellae, with the water-filled holes (defects). The existence of this phase was predicted theoretically as an intermediate one, together with branched wormlike micelles, between lamellar and wormlike micelle phases. DLS was used to determine the viscoelastic relaxation time and amplitude of the wormlike micelle phase. The increase of amplitude at the approach of the isotropic-to-lamellar phase transition suggests increasing intermicellar interactions that may be responsible for the formation of branched micelles.