POLYMER-SURFACTANT AGGREGATES AT A HYDROPHOBIC SURFACE STUDIED USING SUM-FREQUENCY VIBRATIONAL SPECTROSCOPY

The structure of polymer-surfactant aggregates adsorbed at a hydrophob ic surface has been investigated using sum-frequency spectroscopy (SFS ). The surface vibrational spectra of a cationic surfactant (tetradecy ltrimethylammonium 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 surfact ant 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 b etween the polymer and surfactant influence the detailed adsorbate str ucture. The sum-frequency spectra indicate that low concentrations of negatively charged polymers cause the adsorption of surfactant at lowe r concentrations than from a solution of pure surfactant. The effects of pH, polymer concentration, and added electrolyte on the polymer-sur factant adsorbate have been examined. The SF spectrum of the poly(acry lic acid) and deuterated cationic surfactant adsorbate provides eviden ce that this polymer also adopts a preferential orientation at the sur face under certain conditions.