Structural studies of amphiphiles adsorbed at liquid-liquid interfaces using neutron reflectometry

We report the application and refinement of a recently developed method for structural studies at a liquid liquid interface using neutron reflectometry. The technique involves the entrapment of a thin oil layer between a silicon substrate and an aqueous subphase. The thin oil film is prepared by spin-coating an oil film on to an oleophilically treated silicon substrate. During the reflectivity measurement the sample is maintained in a horizontal position, and the angle of incidence of the neutron beam is varied using a supermirror. Attenuation of neutron reflectivity at the lowest angle of incidence is used to determine the oil-layer thickness. We report information regarding the structure at the interface between hexadecane and a 0.1% w/v aqueous solution of the triblock copolymer Pluronic L64 with EO13PO30EO13 (EO = ethylene oxide; PO = propylene oxide) and the interface between hexadecane and a 3.7 mmol dm(-3) (approximately critical micelle concentration) aqueous solution of the cationic surfactant tetradecyltrimethylammonium bromide (C14TAB). For the C14TAB system, the reflectivity data unambiguously reveal the presence of a region highly concentrated in C14TAB on the oil side of the interface. For the Pluronic L64 system, the data suggest that the polymer adsorbs at the interface occupying both oil and water sides of the interface. Model scattering length density profiles that capture these features are presented and further models that better fit the data are discussed.