Rheological Experimental Data on Micellar CTAB Aqueous Solutions, using HSal and NaSal

Leal, C.¹, Pereira, M.¹ and Scheven, U.²

¹ Instituto Superior de Engenharia de Lisboa – IPL, Portugal; ² Dep. Química da Faculdade de Ciências e Tecnologia da Universidade Nova de Lisboa, Portugal

The surfactant CTAB (cetyltrimethylammonium bromide) in aqueous solution forms cationic micelles above a critical concentration, which grow in length if the charge of the head group is screened. In the present study, an acid (salicylic acid - HSal) and its conjugate base (sodium salicylate - NaSal) were used as additives. The presence of the acid changes the solution pH from neutral to acidic media.

Rheological measurements were carried out using two approaches: flow curves in a steady simple shear flow and temperature sweep tests. In order to study the additive concentration dependence, the flow curves were measured for shear rates from 0.1 to 2000 s-1 at 30ºC. The CTAB concentration was fixed at 100mM and additives concentration was varied between 22.5-40mM. The obtained flow curves showed different behaviours for each system, within the accessible shear rate range. The CTAB/NaSal system is predominantly in the shear thinning regime (some samples present a Newtonian plateau at low shear rates), and the viscosity increases with NaSal concentration. For CTAB/HSal, the addition of acid has a less dramatic effect on the viscosity values, but the system shows both shear thinning and thickening behaviour; the transition between the shear thinning to shear thickening regime is dislocated for higher shear rates as acid concentration increases.

Temperature sweep tests were carried out in the temperature range 20 – 50 ºC, for shear rate values 0.1; 0.5; 1; 5; 10; 50 and 100 s-1. A similar behaviour is observed for both systems but within different temperature ranges. The temperature dependence can be described as follows: after an initial plateau, viscosity increases with temperature until a viscosity peak is reached; for higher temperatures, the systems’ viscosity is a decreasing function with T. Although after reaching the viscosity maximum all curves collapse in the same decreasing slope for the CTAB/NaSal system, while for the CTAB/HSal system this phenomena occurs gradually and is dependent of the shear rate value. The occurrence of the maximum viscosity value is dislocated to higher temperatures with the increase of the applied shear rate value.