ATR FTIR spectroscopy of protein and surfactant adsorption to the oil-water interface

An oil-infused polyelectrolyte multilayer (PEM) immersed in aqueous solution is a valid mimic for an oil-water interface, that can be used to probe adsorption of surfactants (including co-/competitive adsorption) using attenuated total reflection Fourier transform infrared spectroscopy (ATR FTIR). A PEM has been formed on an ATR internal reflection element, dried and then re-solvated with oil, then exposed to a flowing solution of protein, and subsequently a solution of surfactant. The PEM had undergone prior characterization with atomic force microscopy (AFM). The experiment was replicated using profile analysis tensiometry (PAT). The PEM displays a high degree of solvation with oil; it retains most of this oil in the presence of flowing aqueous solution. FTIR spectra confirmed adsorption of a dairy protein (beta-lactoglobulin) to the medium chain triglyceride (MCT)-water interface, and the response of the adsorbed protein layer to a common food formulation surfactant (Tween 60) in the aqueous phase above the interface. The protein is observed to adsorb strongly and irreversibly to the MCT oil-water interface, and the Tween 60 molecules are seen to co-adsorb rather than displace the protein. This observation is supported by measurements of sequential co-adsorption undertaken with PAT.