Viscoelastic properties of aqueous solutions of cetyltrimethylammonium bromide (CTAB) containing sodium salicylate (Nasal) were examined by varying the CTAB, CD, and Nasal, CS, concentrations. The results were compared with those of flexible-chain polymer solutions. With increasing salt-to-detergent ratio, C&D-l, up to a certain critical value close to 1 for the solutions of a given CD, the rheological behavior changed from that of low molecular weight polymer solutions in which the polymer chains are not entangling to behavior similar to that of high molecular weight polymer solutions in which polymer chains are fully entangling. These results suggest that threadlike micelles composed of CTAB/NaSal complexes began to form as C8Cp-l increased, resulting in an entanglement network exhibiting pronounced viscoelasticity. However, at high CSCD-~ above the critical value, their behavior can be represented by a Maxwell model with a constant plateau compliance JNO and a single relaxation time 7,. Interestingly, JNo waa proportional to CD-2.2 but independent of CSCD-', but the relaxation time 7 , decreased with increasing CSCD-l. The complex dependence of 7 , of all the solutions at high CSCD-~ may be reduced to a single composite function of CSC~-'/~. These results suggest that relaxation of the entanglement network took place by chemical processes involving actual breakdown and re-formation of the threadlike micelles.
Electrostatic features of threadlike micelles formed in aqueous solutions of cetyltrimethylammonium bromide complexed with salicylic acid or sodium salicylate, coded respectively as CTAB:HSal/W or CTAB:NaSal/W, were examined through determination of pH and bromide ion concentration in the micellar solutions by varying concentrations CD of CTAB and CA of HSal or Cs of NaSal. In the case of the CTAB:NaSal/W system, the degree of dissociation of Br" from CTAB increased with Cs from the value at Cs = 0 up to 100% at Cs = CD. On the other hand, in the CTAB:HSal/W system the degree of dissociation of Br" reached 100% at CA = 0.5CD. The pH of the system correspondingly decreased with increasing CA, changing the slope of pH versus log (CA) around CA = 0.5CD. Equimolar CTAB:HSal micelles thus carry net positive charges on their surface. The effect of adding a simple salt, NaBr, to screen the surface charges was examined by observing phase behavior of the CTAB:HSal/W system with varying CD, CA, and the salt concentration CAS. When NaBr was added beyond a certain level to a viscoelastic solution (liquid phase 1^) containing threadlike micelles or to a two-phase system consisting of the L] and crystalline HSal (S) phases, a new liquid (L2) phase emerged that was essentially an NaBr solution of HSal. The volume fraction Vu of the L] phase decreased with CAS to an asymptotic value KL1" that was proportional to CD but independent of either CA or CAS. This result suggests that the charged micelles were condensed by addition of the salt to a thicker phase without changing their structure.
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