Unusual lamellar-lamellar coexistence in a catanionic surfactant-water system
Eduardo F. Marques
Department of Chemistry, Faculty of Sciences, University of Porto, Rua do Campo Alegre, nº 687, P-4169-007 Porto, Portugal.
Thu, Apr. 19th 2007, 11:00
NIMBE Bât 125, p.157, CEA-Saclay
The coexistence of two equilibrium lamellar liquid crystalline phases in binary surfactant-water systems is an uncommon and still puzzling phenomenon. In this work, we report for the first time a lamellar-lamellar coexistence for a catanionic surfactant, hexadecyltrimethylammonium octylsulfonate (TASo), in water. Small angle-X ray scattering, polarizing light microscopy and 2H NMR unequivocally show the coexistence of a dilute (or swollen) lamellar phase, L?’, and a concentrated lamellar (or collapsed) lamellar phase, L?’’. Furthermore, linear swelling is observed for the two phases, with the miscibility gap in between (15-54 wt % surfactant). In the more dilute region the swollen lamellar phase is in equilibrium with an isotropic (micellar) region. Vesicles can be observed in this two-phase region, in what appears to be a dispersion of L?’ in the solution phase.
The lamellar-lamellar coexistence is interpreted in terms of a balance between attractive and repulsive forces, which may originate from the concentration-dependent degree of issociation of the surfactant.