Comparison of the influence of 1-propanol and of 2-propanol on the viscoelastic solutions of cetyltrimethylammonium bromide (CTAB) and sodium salicylate (NaSal)

It is known that mixtures of CTAB and NaSal form viscoelastic solutions. For a 100 mM CTAB solution and increasing NaSal concentration, the viscosity increases strongly at a concentration of 10 mM NaSal, reaches a maximum at 60 mM NaSal, decreases then to a minimum at 100 mM, and, on further, increases passes over a second maximum at a NaSal concentration of 220 mM. The changes of the viscosity at the first maximum, at the minimum, and at the second maximum are due to changes of the mechanism that effect the structural relaxation times while the shear modulus remains constant. On addition of 1-propanol to the solutions at the maximum and at the minimum, the viscosities decrease strongly to the viscosity slightly above water while the shear modulus in the viscous solution remains constant. The data can be explained by assuming that part of the 1-propanol adsorbs at the surface of the three-dimensional network that is formed from wormlike micelles and makes the hydrophobic surface hydrophilic. On addition of 2-propanol to the solutions at the first maximum, the viscosity decreases also at first up to a concentration of 4% 2-propanol. On further increase of 2-propanol, a second relaxation effect with a different relaxation time and a different shear modulus appear. The sum of the two moduli remains about the same as the shear modulus of the solution without isopropanol. The second relaxation effect can be explained by assuming that only one methyl group of 2-propanol binds to the surface of the network while the other one sticks into water. The surface remains therefore hydrophobic and parts of the network can bind to each other what gives rise to the second relaxation effect.