Binding of oppositely charged surfactants to poly(methacrylic acid) brushes

The interaction of poly(methacrylic acid) (PMAA) brushes with oppositely charged surfactants has been studied as a function of the polymer brush graft density, the surfactant concentration, and the surfactant alkyl chain length at neutral pH. The changes in the degree of dissociation of the polyacid a and in the amount of surfactant uptake P were determined from infrared and multiple-angle ellipsometry measurements, respectively. The brush shrinks already strongly at a surfactant concentration of around 10(-5) mol L-1, despite that the surfactant uptake in this regime is still low. A strong surfactant uptake sets in only above a critical concentration, which is slightly below the critical micelle concentration (cmc) of the respective surfactant. A comparison of dissocation alpha and surfactant uptake beta reveals that the surfactant is not exclusively bound electrostatically by an exchange of the carboxylic acid protons for surfactant ions, but also through hydrophobic interactions. The influence of surfactant concentration, alkyl chain length, and graft density on the complexation behavior is studied.