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Structure and dynamics of worm -like surfactant micelles: Effect of salt

Ignatius Anthony Kadoma, University of Massachusetts - Amherst


We investigate the effect of excess salt and simple shear on the dynamics and structure of semi-dilute aqueous solutions of cetyltrimethylatnmonium bromide and sodium salicylate. Small-amplitude oscillatory rheological measurements suggest a structural evolution from an entangled to a multi-connected network as the salt concentration is increased. Steady-shear measurements, however, show a significant departure from the Cox-Merz rule. This departure occurs at shear rates where $\eta\sp{\*}\propto\omega\sp{-0.92\pm0.08}$ and $\eta\propto\dot\gamma\sp{-0.51\pm0.06}$ and is attributed to the formation of large shear-induced structures. The critical shear rate $\dot\gamma\sb{\rm c}$ at which the Cox-Merz rule fails approximates the inverse of the terminal relaxation time, $\tau.$ Small angle light scattering (SALS) under shear was used to probe the mesoscopic structure and revealed novel scattering patterns exhibiting two-fold symmetry along the flow direction at low salt concentration, four-fold symmetry along the vorticity direction at moderate salt concentration and two-fold symmetry along the vorticity direction at high salt concentration. The SALS patterns were in qualitative agreement with the structural evolution from entangled to connected network as the salt concentration is increased and were suggestive of a local shear induced isotropic-to-nematic transition as well as a shear-induced string phase. Three-dimensional flow birefringence was used to probe average micellar orientation and revealed that at high salt concentration and high shear rate the micelles appear to retain a considerable degree of their connected structure. ^

Subject Area

Chemistry, Polymer|Engineering, Chemical

Recommended Citation

Kadoma, Ignatius Anthony, "Structure and dynamics of worm -like surfactant micelles: Effect of salt" (1997). Doctoral Dissertations Available from Proquest. AAI9737546.