Tuning the contour lengths and persistence lengths of cationic micelles: The pole of electrostatics and specific ion binding

Small-angle scattering has been used to obtain detailed information on mice llar contour lengths and persistence lengths, a measure of micellar flexibi lity, for mixed micelles of cetyltrimethylammonium 2,6-dichlorobenzoate and cetyltrimethylammonium chloride in water as a function of surfactant and s alt concentrations and the relative amounts of the two counterions (the cou nterion inventory) at the micellar surface. Increasing amounts at the surfa ce of the penetrating counterion of the pair, 2,6-dichlorobenzoate, lowers the 1D bending modulus of the micelles, making them more flexible. Micellar contour lengths exhibit a maximum with increasing amounts of a,2,6-dichlor obenzoate at the micellar surface. This is understood in terms of decreases in the micellar end-cap energies, E-c, at high concentrations of penetrati ng counterion. By decreasing the effective micellar surface charge density, penetrating counterions also cause decreased contributions from electrosta tic repulsion, E-e, which retard micellar growth. At low ionic strengths, t his results in more extensive micellar growth upon addition of sodium 2,6-d ichlorobenzoate rather than NaCl.