Neutron reflectivity studies at liquid-liquid interfaces: Methodology and analysis

A protocol is described for measuring and analyzing neutron reflectivity data from a buried oil-water interface. A hexadecane-water interface is considered, and data from the bare interface and from films of polybutadiene-poly(ethylene oxide) Linear diblock copolymers with M-w similar to 60 000 g mol(-1) and containing similar to 50 mol % poly(ethylene oxide) spread (with interfacial concentrations of Gamma = 4 and 10 mg m(-2)) at the interface are reported. From the analysis, it emerges that the hexadecane-water interface is "rougher" than capillary wave theory based expectations. This could be attributable to the influence of molecular size and shape on the intrinsic thickness of the interface. When present, the amphiphilic copolymers segregate at the interface with the blocks occupying a concentrated region (similar to 20% polymer) with thickness similar to 20 Angstrom and a dilute region (much less than 10% polymer) of thickness similar to 50 Angstrom on either side of a nominal dividing surface. The amount of copolymer at the interface, determined using a Guinier-type analysis, is in line with the spread amount at the interface at a moderate (Gamma = 4 mg m(-2)) interfacial concentration but is consistent with a more expanded configuration at Gamma = 10 mg m(-2).