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-l-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, C14TA-2,1-HCN and C12TA-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 counterion-surfactant ratio r we observed the same sequence of phases as for the previously studied C16-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/(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. (c) 2006 Elsevier Inc. All rights reserved.