Behavior of cationic surfactants in polyistyrene sulfonate brushes

The adsorption behavior of a cationic surfactant (cetyl trimethyl ammonium bromide, CTAB) in a sodium poly(styrene sulfonate) (NaPSS) brush in the presence of 1 mM NaNO₃ was investigated by ellipsometry. The interactions between surfactants and brush chains determined from these results are compared with data from other types of experiments on similar systems. Four adsorption regimes were found, including regimes dominated by electrostatic interactions and by hydrophobic interactions. As the concentration of the surfactant increased from zero in the bulk solution surrounding the PSS brush, (1) surfactant monomers replaced Na⁺ ions reversibly, in a manner analogous to ion exchange, until β (the ratio of the number of bound surfactants to the total number of negatively charged monomers of PSS) reached 0.2-0.3 (1 × 10^-6 M CTAB), (2) surfactants in the brushes interacted with each other hydrophobically, making adsorption irreversible and producing contraction of the brush (0.3 < β < 0.4), (3) the rate of adsorption increase was suppressed, by the squeezing out of available volume in the shrinking brush (β = 0.4), (4) additional uptake of surfactants took place above 1 × 10^-4 M CTAB in bulk solution (still 1 order of magnitude lower than the critical micelle concentration (cmc) of CTAB), because of the hydrophobic aggregation of surfactants on surfactant-polyelectrolyte complexes in the brushes. The overall conclusion and insight gained from this work is that surfactants are taken up, even at micromolar concentrations, extensively and in a nonlinear, cooperative fashion, in polyelectrolyte brushes. These data imply that the properties expected from highly hydrated polyelectrolyte brushes (steric protection, lubricity) are strongly modified by the presence of oppositely charged surfactants.