Specular and Off-Specular Neutron Reflectivity of a Low-Molecular-Weight Polystyrene Surfactant At the Air-Water-Interface

Neutron reflectivity measurements were made on films at the air-water interface of a highly characterized low molecular weight polystyrene surfactant, with a single hydroxyl end group. The II-A isotherm is consistent with collapse of a classical fluid surfactant monolayer to a trilayer structure at high compressions. Before the collapse point, reflectivity measurements indicated a decrease in the average tilt angle of the molecules to the surface normal as the layer was compressed. After the collapse point, the film structure was time dependent and was best represented by a monolayer of constant thickness at the water interface, covered by a bilayer whose thickness increased with time and a time-dependent diffuse layer above the bilayer. Multidetector reflectivity profiles showed only specular reflection for surface pressures below the collapse point, but in the collapse region, some off-specular reflection was clearly present, indicating the presence of surface texture on the micron length scale in the plane of the interface. This is believed to be the first observation of well-developed off-specular neutron reflection from a Langmuir film