Polymer-Surfactant Aggregates at a Hydrophobic Surface Studied Using Sum-Frequency Vibrational Spectroscopy

Abstract

The structure of polymer-surfactant aggregates adsorbed at a hydrophobic surface has been investigated using sum-frequency spectroscopy (SFS). The surface vibrational spectra of a cationic surfactant (tetradecyltrimethylammonium bromide) show that the surfactant molecules form an oriented monolayer at the surface, with the polymer electrostatically binding to the charged head groups of the surfactant at the interface with the aqueous phase. The polymer-surfactant interaction, however, does not appear to increase the maximum packing density of the surfactant layer above that of a monolayer of pure surfactant at its cmc. The general structural features of the surfactant layer remain unaffected by the presence of polymer, but steric and hydrophobic interactions between the polymer and surfactant influence the detailed adsorbate structure. The sum-frequency spectra indicate that low concentrations of negatively charged polymers cause the adsorption of surfactant at lower concentrations than from a solution of pure surfactant. The effects of pH, polymer concentration, and added electrolyte on the polymer-surfactant adsorbate have been examined. The SF spectrum of the poly(acrylic acid) and deuterated cationic surfactant adsorbate provides evidence that this polymer also adopts a preferential orientation at the surface under certain conditions.