In this work, the linear viscoelastic properties of the cetyltrimethylammonium tosilate (CTAT)−water system are examined in detail. This system forms elongated micelles at low and intermediate concentrations, and it yields a hexagonal phase above 27 wt % CTAT at 25 °C. Rheological behavior at low frequencies in a small-amplitude oscillatory shear experiments or at long times in stress relaxation measurements is governed by a single dominant relaxation time, although deviations from the limiting slope of the elastic modulus in the terminal region are observed at high CTAT concentrations. For higher frequencies, however, there is an additional mechanism whose dependence on frequency is analyzed with several rheological models. Analysis of data in terms of the theory of Cates demonstrates that the system consists of flexible micelles in the slow-breaking limit and it exhibits a constant entanglement density along the whole micellar region, even though the average micellar length decreases monotonically with concentration. Under these conditions, reptation speed up by the kinetics process of breaking and re-forming is the controlling relaxation mechanism.

The partial phase behavior of CTAT/water is investigated here as a function of temperature by WAXS, DSC, polarizing microscopy, conductometry, 1H-NMR, and FTIR spectroscopy. Oscillatory strain and temperature sweeps are also reported. The Krafft temperature (7k) of CTAT/water is 23 °C. Below this value, triclinic crystals of CTAT coexist with an isotropic solution. Above 7k and at low concentrations, spherical micellar solutions are Newtonian and exhibit low viscosities. At higher concentrations (ct), cylindrical micelles form and viscosity increases dramatically with CTAT concentration, but no elastic effects are noticed. When micelles are long enough to entangle (0.9-27 wt % at 25 °C), clear viscoelastic solutions form. At higher concentrations and up to 47 wt %, an hexagonal phase appears. This phase exhibits yield stress and viscoelasticity. At higher concentrations, a nonelastic, viscous solid paste forms. Micellar solutions and hexagonal phase depicts three regimes of viscoelasticity with temperature. These regimes are bounded by T\ and by the temperature (TV) at which the system exhibits its main relaxation time. 77 moves to lower temperatures as CTAT concentration increases indicating that the main relaxation time decreases upon increasing concentration.

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The polymerization of styrene in oil-in-water microemulsions made with the cationic surfactants dodecyltrimethylammonium bromide or chloride is studied as a function of inorganic electrolyte (KBr, KCl, or K2SO4) concentration. The resulting microlatex is stable, but as the electrolyte concentration increases, both the average radius and the polymer molecular weight decrease. The presence of electrolyte slows the polymerization rate and diminishes final conversion as followed by gravimetry, dilatometry, and calorimetry. Both particle radius, determined by quasielastic light scattering, and molecular weight show only limited growth as styrene conversion increases, suggesting continuous nucleation of latex particles and termination by chain transfer to monomer. Small-angle neutron scattering (SANS) of undiluted parent and polymerized microemulsions shows that a unimodal population of swollen micelles evolves into a bimodal population of empty micelles coexisting with large polymer particles. Structural details of the parent and polymerized microemulsions as determined by SANS are used to assess nucleation mechanisms previously proposed for emulsion polymerization.

The flow in a pipe of wormlike micellar solutions is examined using a simple model that consists of the codeformational Maxwell constitutive equation and a kinetic equation that accounts for the breaking and reformation of micelles. The model needs six parameters, all of which are extracted from single independent rheological experiments. One of the parameters, the shear-banding intensity parameter is associated with the stress plateau in the shear-banding region. The stress plateau is set in our model by the criterion of equal extended Gibbs free energy of the bands. The model predicts a Newtonian (parabolic profile) flow at low-shear rates or low-pressure gradients, followed by shear thinning up to a critical rate where instabilities and long transients appear. At this critical shear rate, a shear-banding flow region arises near the pipe wall. The model indicates that tube lengths up to 400 diameters are required to obtain fully developed flow, where a pluglike profile at the center of the tube coexists with a region supporting a much higher shear rate next to the wall. Shear-banding flow is present up to a second critical shear rate. At shear rates larger than the second critical rate, the parabolic velocity profile is recovered, except near the center of the tube where a small shear-banding flow region remains because the stress at that radial position is equal to the plateau stress. This is a consequence of the linear dependence of the shear stress with the pipe radius. The predictions of the model are compared with experimental results from the literature.

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.

ConclusionsExamination of six dyes of increasing amphiphilicity in water-rich microemulsions incorporating oils of different polarity and surfactants with different charge reveals important factors for optimizing dye solubility. In almost all cases, the solubility of dye is much enhanced in microemulsions over that found in pure solvents. It appears that the primary variable is dye amphiphilicity: the more surfactant-like the dye becomes, the greater the solubility in water-rich microemulsions. From the data, it is inferred that the primary site for solubilization of polar dyes is the surfactant-rich interfacial region separating oil and water domains.Acknowledgment. This work was supported by IBM, and we are grateful for the useful comments of Dr. A. H. Sporer.