Effect of Co-surfactants on Bending Stiffness and Scission Energy of Wormlike Micelles: a Rheometry and Diffusing Wave Spectroscopy (DWS) Study

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Recently, we have established a combination of mechanical high frequency rheology and optical microrheology to gain new insight into the structure and dynamics of wormlike micellar solutions. We have used this approach to show, that the micellar bending stiffness (here expressed in terms of the persistence length lp) strongly decreases with increasing binding efficiency, whereas scission energy Esciss and hence contour length are hardly affected. For the penetrating counterion sodium salicylate (NaSal) an increasing salt/surfactant ratio results in a non-monotonic variation of Esciss and a strong decrease in lp, which is attributed to the incorporation of the salicylate into the interior of the micelles. In this follow-up project we want to get deeper insight how to control length and flexibility of wormlike surfactant micelles using the following experimental approach; 1. Mixtures of the cationic surfactant hexadecyltrimethyl ammonium bromide (CTAB) with n = 16 carbon atoms in the alkyl chain with other trimethyl ammonium bromide surfactants with shorter hydrophobic tails (n = 8, 10, 12, 14) as well as the individual surfactants with n = 16 and 14 will be used to study the effect of the hydrophobic tail group on Esciss and lp systematically in comparison with simulation results from the literature. 2. Esciss and lp will be determined for the system CTAB/NaSal. Comparison of results with those for the mixed micelles as well as previous data for cetylpyridinium chloride (CPyCl)/NaSal will provide further insight how to control length and flexibility of wormlike micelles. This system will also be used to benchmark our method with literature data for lp from neutron spin echo experiments. 3. The high frequency setup will be used for an accurate determination of the plateau modulus for the system CPyCl with the non-penetrating salt NaClO3, since literature data obtained by classical rotational rheometry are not accurate and have lead to a wrong interpretation of the viscosity decrease occurring with increasing salt content.