Formation of wormlike micelle in a mixed amino-acid based anionic surfactant and cationic surfactant systems.

Formation of wormlike micelles in mixed anionic/cationic system without the addition of any salt has been studied. Amino-acid based anionic surfactant N-dodecylglutamic acid (LAD), which is practically immiscible with water at 25 degrees C upon neutralization by 2,2',2''-nitrilotriethanol (TEA) forms small micellar aggregates and the solution behaves like a Newtonian fluid. The rheological behavior of LAD/water/hexadecyltrimethylammonium bromide (CTAB) and LAD/water/dodecyltrimethylammonium bromide (DTAB) systems were investigated at different degrees of neutralization of the LAD depending on the concentration of the cationic surfactants and on temperature. Addition of CTAB to the dilute aqueous solution of the LAD-TEA-x (the neutralized product, where x represents the mole ratio of TEA) causes one dimensional micellar growth. After certain concentration the elongated micelles entangle forming a rigid network of viscoelastic wormlike micelles. Thus formed viscoelastic solutions follow Maxwellian behavior over a wide range of frequency and thus are considered to consist of transient network of wormlike micelles. By varying the degree of neutralization from 1:1 via 1:1.5 to 1:2 (molar ratio) phase and rheological behavior were modified in that the highly viscous region of viscoelastic wormlike micelles shifted to higher CTAB concentrations and no maxima in the zero-shear viscosity could be observed for the higher degree of neutralization of the LAD (1:1.5 and 1:2). However, the obtained rheological parameters showed scaling relationships that were consistent with the living polymer model. The zero-shear viscosity decays exponentially with temperature following Arrhenius behavior. The flow activation energy calculated from the Arrhenius plot is very close to the value reported for the typical wormlike micellar solution. In contrast to CTAB no formation of viscoelastic wormlike micelles could be observed with DTAB, although, the solution viscosity increases. The elongated micelles could not entangle to form a rigid network of wormlike micelles in this system. For the first time viscoelastic wormlike micelles could be obtained in salt-free mixed anionic/cationic surfactant systems.