A pH-induced transition of surfactant-polyelectrolyte aggregates from cylindrical to string-of-pearls structure

Aqueous solutions of polyelectrolyte and oppositely charged ionic surfactant are proposed to have a string-of-pearls structure under strongly acidic conditions. This differs from the cylindrical structure presented by this same system at neutral pH conditions. Experimental data are consistent with the conclusion that there is a cylinder-to-sphere transition of the adsorbed surfactant when pH is dropped below a critical value. At intermediate concentrations, this transition is manifested as a pH-induced gelation. At dilute conditions, the aggregate structure is characterized through light scattering, potentiometry, (1)H NMR, and solubility measurements. Protonation of the carboxylate groups on the polyelectrolyte and resulting restructuring of the micellar structure around the aggregate are argued to be the primary causes of the transition. Since these are completely reversible, the addition of NaOH, a strong base, is observed to reverse the structure from string-of-pearls back to cylindrical. This reversible pH-induced transition, which is caused by a shift in intermolecular forces within the aggregate, is likely to be common among self-assembled aggregates. These aggregates represent a region of the polyelectrolyte-surfactant aggregate formation (that of hydrophobic polymer and varying electrostatic attraction of opposite charged moieties) that exhibits distinctive aggregate structural behavior.