Preparation of mesoporous structure from the flow-induced microstructure in aqueous surfactant solutions

Abstract

In this thesis, microstructure evolutions and rheological properties in dilute and semi-dilute surfactant solutions were investigated experimentally. First, the viscoelastic properties of surfactant solutions were considered in semi-dilute regime. In this regime, surfactant molecules with aromatic additive can be aggregated into long flexible wormlike micelles forming reversibly entangled structure, which leads to the viscoelastic properties. This is similar to the semi-dilute polymer solutions in many respects. We examined the effects of sodium salicylate on the microstructure evolution and rheological responses of an aqueous cetyltrimethylammonium bromide (CTAB) solution. The CTAB concentration was fixed at either 0.02M or 0.05M, both of which were far above its critical micelle concentration with the molar ratio of sodium salicylate (NaSal) to CTAB ranging from 0.1 to 10.0. The experimental results showed that the rheological responses of the surfactant solution were influenced strongly by the CTAB concentration and molar ratio. At low molar ratios below 0.3, the surfactant solutions behaved like a Newtonian fluid. However, as the molar ratio increased, the deviation from Newtonian behavior became pronounced. Specifically, for 0.05M CTAB solutions with the molar ratio ranging from 1.0 to 5.0, the apparent yield stress developed at low shear rates and the stress plateau was displayed at intermediate shear rates. When the shear rate exceeded a certain threshold value, the shear stress increased again passing over the plateau value. In addition, the viscoelasticity and relaxation behavior was distinctively observed. The relaxation behavior after the cessation of flow was strongly dependent on the molar ratio, which was also confirmed by the rheo-optical observation. The rheological responses were clearly related to the flow-induced microstructure in the solution. Secondly, to study the structural behavior of dilute surfactant solution, the rheological and rheo...