Viscoelastic Fluids Originated from Enhanced Solubility of Sodium Laurate in Cetyl Trimethyl Ammonium Bromide Micelles through Cooperative Self-Assembly

Abstract

Sodium laurate (SL), which is otherwise sparingly soluble in water at room temperature, forms viscoelastic fluids when mixed with cationic cetyl trimethylammonium bromide (CTAB) micelles. This originates from a cooperative assembly of CTAB and SL through electrostatic interaction and the ability of the tetralkyl ammonium headgroup in decreasing the Kraft point of SL. Addition of SL to CTAB induces the formation of rod-like micelles, the length of which can be controlled by the composition of the mixture. The effects of electrostatic repulsion of ionic moieties and steric repulsion of hydrocarbon chains in controlling the packing of surfactant molecules within the aggregate have been identified. The variation in the optimum interfacial area occupied by the surfactant as a function of electrolyte concentration is consistent with the model predicted by Nagarajan. Small angle neutron scattering studies indicate that the micelles undergo composition dependent uniaxial growth without any significant change in the cross section radius, in the absence of electrolytes. However, in the presence of NaCl, both the length and cross section radius of the elongated micelles are sensitive to composition. At small axial ratio, these micelles behave like Newtonian fluids, while long polymer-like micelles show shear thinning behavior. The dynamic rheological responses of long micelles are consistent with a Maxwell type viscoelastic behavior.