Viscoelastic behavior of surfactant threadlike micellar solutions: effects of additives, 4.

Viscoelastic behavior of aqueous solutions of a cationic surfactant, cetyltrimethylammonium p-toluene sulfonate (CTApTS), and an organic salt, sodium p-toluene sulfonate (NapTS), was examined at room temperature by changing concentrations of CTApTS (CD) and NapTS (CS). CTApTS forms long threadlike micelles in aqueous solution even at low CD in spite of the absence of additives. Aqueous CTApTS solutions show pronounced viscoelastic behavior, which is perfectly described by Maxwell model type viscoelasticity, keeping a single set of relaxation parameters (GN and tau). The magnitude of relaxation strength, GN, is proportional to the square of CD irrespective of the value of CS. The value of a relaxation time, tau, is independent of CD, while the value remarkably reduces with increasing concentration (CS*) of excess pTS- ions in the bulk aqueous phase. The relationship between tau and CS* for the system is not identical with that for aqueous solutions of cetyltrimethylammonium bromide (CTAB) and NapTS at CS* < 30 mmolkg(-1). However, the relationships between tau and CS* for these systems agree well with each other at CS* >/= 30 mmolkg(-1). This implies that formed threadlike micelles in both systems have similar structure and the relaxation mechanism is controlled by CS*, but not influenced by the presence of Br- ions above CS* = 30 mmolkg(-1).