X-ray scattering from monolayers of F(CF2)(10)(CH2)(2)OH at the water-(hexane solution) and water-vapor interfaces

Synchrotron x-ray reflectivity is used to study the structure of a monolaye r of F(CF2)(10)(CH2)(2)OH self-assembled at the liquid-liquid interface fro m a solution in hexane placed in contact with water. It is demonstrated tha t this monolayer is in a high density (solid) phase below a transition temp erature. This is in contrast to the conventional expectation that soluble s urfactants form disordered monolayers at the liquid- liquid interface. Abov e the transition temperature the monolayer desorbs into the hexane solution , leaving behind an interface with a very low density of surfactants. Hyste resis in the formation of the monolayer occurs when the temperature is scan ned through the transition temperature. The success of these measurements r elied upon the development of a novel technique to flatten the liquid-liqui d interface to the extent required for x-ray reflectivity. The measurements of F(CF2)(10)(CH2)(2)OH at the liquid-liquid interface are compared to x-r ay surface diffraction measurements of monolayers of the same material spre ad at the water-vapor interface. A solid to disordered-phase phase transiti on also occurs in the spread monolayer though at a slightly higher temperat ure. This indicates that the hexane acts to disorder the solid monolayer at the water-hexane interface. A measurement of the thermal expansion coeffic ient of the monolayer at the water-vapor interface is consistent with liter ature values for bulk hydrocarbon rotator phases, in contrast with previous measurements on monolayers of perfluoro-n-eicosane supported on water. (C) 1999 American Institute of Physics. [S0021-9606(99)70615-X].