pH-Responsive self-assembly in an aqueous mixture of surfactant and hydrophobic amino acid mimic

The pH-responsive behavior of an amino acid is used to control the electrostatic interactions on a cationic micelle surface and thereby alter the morphology of self-assembled structures formed by them. The pH-induced changes in the rheology and microstructure in an aqueous mixture of cetyltrimethyl ammonium bromide (CTAB) and a hydrophobic amino acid mimic, anthranilic acid (AA) were studied by rheological measurements, small-angle neutron scattering (SANS) and light scattering. At low pH, when the net charge on the amino acid is positive, nearly globular mixed micelles are formed. With increase in pH, the AA becomes negatively charged due to the dissociation of carboxyl group and complexes with the positively charged CTAB molecule. This electrostatic complexation decreases the surface charge of the micelles, leading to a structural evolution from small globular micelles to long worm-like assemblies. Rheological studies indicate a transition from Newtonian to viscoelastic nature of the fluid with an increase in the pH of the solution. SANS studies confirm the formation of prolate ellipsoidal micelles and the axial ratio of the micelle increases with an increase in pH of the solution. The variation in the diffusion coefficient of the micelles with pH is consistent with the SANS results. This demonstrates the use of hydrophobic amino acids in creating pH-sensitive assemblies of amphiphiles. An increase in the zero shear viscosity of the fluid of four orders of magnitude could be achieved by a slight change in the pH of the solution.