Novel viscoelastic systems from cationic surfactants and hydrophobic counter-ions: influence of surfactant chain length.

In continuation of our previous investigations on the aqueous phase behavior of cetyltrimethylammonium 2-hydroxy-1-carboxy-naphthoate (CTA-2,1-HCN) (see J. Colloid Interface Sci. 288 (2005) 570), we have studied the phase behavior and the properties of the phases of the two shorter homologues, C(14)TA-2,1-HCN and C(12)TA-2,1-HCN. The phases were prepared from the alkyltrimethylammonium hydroxides RTAOH and the naphtholcarboxylic acid. By preparing the systems in this way the surfactant solutions contain no excess salt. With increasing counter-ion-surfactant ratio r we observed the same sequence of phases as for the previously studied C(16)-system, namely a L(1)-phase and a L(alpha)-phase with multilamellar vesicles (MLV). The single phases are separated by a two-phase L(1)/L(alpha) region. The phases were characterized with FF-TEM, rheological and SANS measurements. For both systems the viscosity of the L(1)-phases passes with increasing counter-ion/surfactant ratio over a maximum. The height of the maximum decreases strongly with decreasing chain length while the complex viscosity of the MLV-phase depends little on the chain length. For 100 mM solution both MLV phases behave like a weak gel and have a yield stress value. It is thus shown that it is possible to prepare viscoelastic surfactant solutions with a yield stress value from single chain surfactants with a dodecyl chain.