Tuning of Micellar Structure and Dynamics in Aqueous Salt-Free Solutions of Cetyltrimethylammonium Mono- and Dichlorobenzoates

Micelle formation and growth in salt-free aqueous solutions have been investigated for five cetyltrimethylammonium (CTA) mono- and dichlorobenzoates using small-angle neutron scattering (SANS) and rheological measurements. Manipulation of the chlorine substitution pattern provides an excellent means of tuning micellar morphology and solution properties. Two of the surfactants, CTA 2,6-dichlorobenzoate and CTA 2-chlorobenzoate, form Newtonian aqueous solutions containing roughly spherical micelles below ca. 70 mM. There follows a region with increasing concentration of significant micellar growth to prolate ellipsoids (or rigid, short cylindrical micelles), micellar overlap, and apparent maximum aggregation numbers, 〈 n 〉's, at ca. 200 and 167 mM (φ = 0.095 and 0.073), respectively. Above these latter concentrations, the SANS Q max values scale as C 1/2, characteristic of an entangled polymer-like micellar network. The three surfactants having chlorine substituents para and/or meta to the counterion's COO - group (namely CTA35ClBz, CTA4ClBz, and CTA34ClBz) form highly viscoelastic aqueous solutions containing entangled wormlike micelles at mM concentrations. For CTA35ClBz, C * is ≤2 mM (φ ≈ 0.001), the micelles have a persistence length of 50 nm, and the solutions are Maxwell fluids at C ≥ 20 mM. The mean micellar contour length, assessed from dynamic rheological measurements analyzed using the formalism of Cates and Granek, reaches a maximum of 10 μm at ca. 15−20 mM. The C -dependences of the network correlation lengths, micellar entanglement lengths, and mean contour lengths, are analyzed to explain the recovery of Newtonian fluid behavior at C ≥ 70 mM.