Interaction of oppositely charged polyelectrolyte–ionic surfactant mixtures: adsorption of sodium poly(acrylic acid)–dodecyl trimethyl ammonium bromide mixtures at the air–water interface

Abstract

The adsorption of the polyelectrolyte–cationic surfactant mixture of sodium poly(acrylic acid), NaPAA, and dodecyl trimethyl ammonium bromide, C₁₂TAB, at the air–water interface has been studied using neutron reflectivity and surface tension. The results provide direct evidence of the relative roles of the electrostatic and hydrophobic interactions between the polyelectrolyte and surfactant on the adsorption behaviour. At both pH 4.2 and 9.2 the addition of NaPAA results in a strong reduction in the surface tension at low C₁₂TAB concentrations, consistent with the adsorption of polymer–surfactant complexes at the interface. This is confirmed and quantified by neutron reflectivity measurements. At pH 4.2 a mixed polymer–surfactant monolayer (∼20 Å thick) is adsorbed at the interface over the entire surfactant concentration range measured. Whereas at pH 9.2, where the polyelectrolyte is more highly charged, the adsorption pattern is similar at low surfactant concentrations to that measured at low pH. However, at higher surfactant concentrations there is a transition from monolayer adsorption to the formation of a ‘layered’ structure at the interface. This is indicative of a stronger polymer–surfactant interaction at the surface, and has some similarity with that observed in other polyelectrolyte–ionic surfactant mixtures. At high pH, the results imply that at the lower surfactant concentrations the adsorption is dominated by the hydrophobic interaction, whereas at higher surfactant concentrations it is dominated by the electrostatic interaction. At low pH the dominant interaction would appear to be hydrophobic in nature.