pH-Responsive Behavior of Aqueous CTAB/NaSal Solutions

pH-responsive behavior of aqueous CTAB/NaSal solutions

Chinedu Umeasiegbu¹, Ramanan Krishnamoorti^1,2, and Vemuri Balakotaiah^1,2

Department of Chemical and Biomolecular Engineering¹ and Petroleum Engineering Program², University of Houston, Houston, TX 77204

Certain self-assembly systems exhibit pH response becoming viscoelastic upon an increase in pH and this can be utilized in ensuring effective stimulation in heterogeneous carbonate reservoirs. In this study, we investigate the pH-responsive behavior of an aqueous solution of cetyltrimethylammonium bromide (CTAB) in the presence of sodium salicylate (NaSal). The fluid can be switched between gel-like and a fluid-like within a narrow pH range. Dynamic light scattering (DLS) results revealed that the pH-sensitive rheological behavior was due to a transition in microstructure between wormlike and spherical micelles and that the pH-sensitivity originated from the changing hydrophobic interactions between the salicylate anion and CTAB amphiphile as pH varies. Using the bubble recoil test, a phase diagram showing the pH at which different concentrations of aqueous CTAB (C_d) and NaSal (C_s) solutions transition from viscoelastic to nonviscoelastic was obtained. These transition pH values were also investigated using microrheology data extracted from particle tracking of polystyrene probes dispersed in these solutions. It was revealed that the transition pH of these solutions and the associated change in rheology and viscosity of the system upon transition was dependent on the NaSal to CTAB ratio (C_s/C_d) and on the concentration of CTAB (C_d).