A simple method has been developed to produce a stable, thin (~90 Å) oil film on the surface of pure water, suitable for direct measurements of the oil-water interface using ellipsometry, X-ray or neutron reflectometry, or other experimental methods. The layer is simply formed from the co-spreading of an aliphatic non-volatile hydrocarbon oil (chain lengths between 8-30 methylene repeats) and a polymeric polyisobutylene- succinic amide (PIBSA) of mean molecular mass of 1200 g mol–1 from a solution in toluene. The toluene solution is directly spread on an aqueous surface to produce the homogenous oil layer. This mixed oil-water surface layer has a well-defined surface pressure-surface area isotherm (measured on a Langmuir trough) and reflects X-rays strongly - indicating a smoothness of a few Ångstroms at most.
The figure shows the X-ray reflectivity (R) multiplied by the fourth power of the vertical momentum transfer (Q4) as a function of Q. The fringe pattern contains information on the density and thickness of both the oil over-layer and the surfactant at the oil-water interface. The scattering length density profile of these layers from entry into the oil layer from the air and into the underlying water is shown in the inset.