Adsorption kinetics of non-ionic surfactants in micellar solutions: effects of added charge

The kinetics of adsorption of micellar solutions of non-ionic surfactants have been studied in an overflowing cylinder. The addition of small amounts (<10% of the total surfactant concentration) of ionic surfactants, CTAB and STS, to solutions of C16E8 causes a dramatic reduction in the rate of adsorption of the nonionic surfactant. The results are rationalised by a combination of monomer and micelle adsorption to the air–water interface. In the presence of trace ionic surfactants, only uncharged micelles adsorb. The adsorption kinetics are independent of the sign of the charge on the micelles, only on its magnitude. The influence of ionic surfactants on the adsorption rate is reversed by addition of millimolar concentrations of salt. Electrolyte screens the repulsions between micelles and the adsorbed monolayer and allows charged micelles to adsorb without first breaking down into monomers.