Rheology of Cetyltrimethylammonium p-Toluenesulfonate−Water System. 3. Nonlinear Viscoelasticity

Abstract

The nonlinear viscoelastic behavior of the cetyltrimethylammonium p-toluenesulfonate (CTAT)−water system is investigated in steady and unsteady shear flow as a function of surfactant concentration and temperature. A rheo-optical study which includes measurements of dichroism, birefringence, and turbidity under flow at various shear rates is also discussed. The shear viscosity data in steady shear agree with the complex viscosity in the limit of low deformation rates. For moderate deformation rates, in the shear thinning region, the Cox-Merz rule is not followed. In all cases, a limiting stress or plateau stress was observed at shear rates that exceed one-half of the reciprocal of the main relaxation time [(2τd)-1]. At the stress plateau, the micellar solution most likely undergoes an isotropic-to-nematic phase transition induced by shear. However, our results do not conclusively exclude the possibility of a constitutive instability with respect to shear banding, in which simultaneous shear rates coexist under controlled stress experiments. In unsteady shear flow, CTAT−water micellar solutions exhibit a slow transient behavior in which the system achieves steady state in starting up experiments after tens to hundreds of Maxwell relaxation times. This is consistent with the existence of shear banding. Metastable branches are also observed in thixotropic loops produced under exponential shear. The time scale of this branch coincides with that of the inception of shear flow just before the overshoot peak. Moreover, the system exhibits a quasilinear rheological behavior at long times characterized by an exponential relaxation with a single time constant. A simple model consisting of the co-deformational Maxwell constitutive equation and a kinetic equation for construction and destruction of structure is proposed to predict distinct features of the complex rheological behavior of the elongated micellar solutions.

Phase and rheological behavior of the polymerizable surfactant CTAVB and water

“…In time-dependent experiments, long transients and oscillations typically accompany shear-banding flow [58][59][60][61]. These transients and oscillations in the shear stress have been asso- ciated with the kinetics of nucleation and growth of a second phase, similar to phenomena reported in equilibrium first-order phase transitions [54].…”

Rheological behavior of surfactant-based precursors of silica mesoporous materials

“…Notice that the magnitude of the overshoots diminishes and the overshoot maximum shifts to longer times with increasing TEOS content. Moreover, oscillations are detected at longer times that last tens of relaxation times in the TEOS-free sample, which is typical of wormlike micelles undergoing shear-banding flow [3,29,30]. With increasing TEOS, the oscillations damp down quite rapidly and at high T/C ratios, no oscillations are observed and the steady state is achieved quite rapidly.…”

“…1 to 25 wt% at room temperature [1]. These polymer-like solutions exhibit near Maxwell behavior in linear oscillatory flow and shear thinning as well as shear banding in steady shear flow [2][3][4]. This complex behavior is related to the relaxation mechanisms in this kind of systems, mainly, i.e., kinetic-controlled and diffusion-controlled relaxation mechanisms [5][6][7][8].…”

On the shear thickening behavior of micellar aqueous solutions of cetyltrimethylammonium fluorobenzoates: Effect of the fluor position

“…Hydrotropes cannot form organized self-associated structures, such as micelles, because their hydrophobicity is lower than that of surfactants; however, they decrease substantially the critical micellar concentration (cmc) and induce the transition from spherical to rod micelles (cmc 2 ) as well as wormlike micelles formation at lower surfactant concentrations. This ability typically increases when the hydrotrope molecules have aromatic structures [3][4][5][6][7][8][9][10][11][12][13][14][15]. For instance, the addition of hydrotrope salts such as 2-hydroxybenzoate to cetyltrimethylammonium bromide (CTAB) solutions induces the formation of wormlike micelles at very low concentration whereas the addition of 3-and 4-hydroxybenzoate counterions does not.…”