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 s urfactant, with a single hydroxyl end group. The II-A isotherm is cons istent with collapse of a classical fluid surfactant monolayer to a tr ilayer structure at high compressions. Before the collapse point, refl ectivity measurements indicated a decrease in the average tilt angle o f the molecules to the surface normal as the layer was compressed. Aft er the collapse point, the film structure was time dependent and was b est represented by a monolayer of constant thickness at the water inte rface, covered by a bilayer whose thickness increased with time and a time-dependent diffuse layer above the bilayer. Multidetector reflecti vity profiles showed only specular reflection for surface pressures be low the collapse point, but in the collapse region, some off-specular reflection was clearly present, indicating the presence of surface tex ture on the micron length scale in the plane of the interface. This is believed to be the first observation of well-developed off-specular n eutron reflection from a Langmuir film.