(570c) 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).