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 CTA2-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 (phi = 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 CTA35C1Bz, CTA4C1Bz, and CTA34C1Bz) form highly viscoelastic aqueous solutions containing entangled wormlike micelles at mM concentrations. For CTA35C1Bz, C* is less than or equal to 2 mM (phi approximate to 0.001), the micelles have a persistence length of 50 nm, and the solutions are Maxwell fluids at C greater than or equal to 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 mu m 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 greater than or equal to 70 mM.