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

A protocol is described for measuring and analyzing neutron reflectivity da ta from a buried oil-water interface. A hexadecane-water interface is consi dered, and data from the bare interface and from films of polybutadiene-pol y(ethylene oxide) Linear diblock copolymers with M-w similar to 60 000 g mo l(-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 a re reported. From the analysis, it emerges that the hexadecane-water interf ace is "rougher" than capillary wave theory based expectations. This could be attributable to the influence of molecular size and shape on the intrins ic thickness of the interface. When present, the amphiphilic copolymers seg regate at the interface with the blocks occupying a concentrated region (si milar to 20% polymer) with thickness similar to 20 Angstrom and a dilute re gion (much less than 10% polymer) of thickness similar to 50 Angstrom on ei ther side of a nominal dividing surface. The amount of copolymer at the int erface, determined using a Guinier-type analysis, is in line with the sprea d amount at the interface at a moderate (Gamma = 4 mg m(-2)) interfacial co ncentration but is consistent with a more expanded configuration at Gamma = 10 mg m(-2).