Micellization of a weakly charged surfactant in aqueous salt solution: self-consistent field theory and experiments

Self-consistent field (SCF) calculations and light scattering experiments were performed to study the pH and salt response of micelles composed of surfactants with a single weak acid group in aqueous salt solution. To this end, the common surfactant Brij 35 was oxidized to yield a polyoxyethylene alkyl ether carboxylic acid with a single terminal weakly charged carboxylic acid group in alkaline media. At low pH values, the micellar hydrodynamic radii (R_h) are independent of the salt concentration. By contrast, at pH values around the acid dissociation constant (pH ≈ pK_a ± 1), the micellar radius decreases upon increasing pH until a salt-dependent plateau value is reached. The reduction in micellar size is more pronounced for lower salt concentrations. The SCF computations are in qualitative agreement with the experimental results and further reveal a limiting value for R_h corresponding approximately to the Debye length λ_D. Self-assembly into micelles is suppressed for low salt concentrations that would yield R_h < λ_D. Instead, the surfactants remain as unimers in solution. The results are summarized in a state diagram displaying the preferred surfactant configuration in solution as a function of R_h/λ_D, pH and salt concentration.