Mixed wormlike micelles of cationic surfactants: Effect of the cosurfactant chain length on the bending elasticity and rheological properties

We have investigated the behavior of mixed wormlike micelles (WLM) of cationic surfactants with hydrophobic moieties of the same nature but differing in chain length. Hexadecyltrimethylammonium bromide (C16TAB) with n = 16 carbon atoms in the alkyl chain mixed with other trimethylammonium bromide (CnTAB) surfactants with shorter hydrophobic tails (n = 14, 12, 10, and 8) in presence of 0.4 M KBr has been selected as model system. Mechanical high frequency rheometry experiments permit characterization of the linear viscoelasticity of the mixtures covering the frequency range up to 106 rad s−1. From these measurements we have, for the first time, determined the variation of the persistence length lp as a function of the addition of short chain surfactant using a recently established approach (Willenbacher et al. [46]). We found that lp decreases exponentially when increasing the short chain content in the mixture in agreement with theoretical predictions. The zero-shear viscosity η0 as well as the longest relaxation time TR revealed a monotonic decrease upon increasing the concentration of cosurfactant with shorter chain length. The plateau modulus G0 decreases for all mixtures except for the C16TAB–C14TAB mixture where it increases slightly. The latter results are explained by a progressive diminution of the micellar length with increasing content of short chain cosurfactant.