MOLECULAR PACKING IN WATER SUPPORTED MONOLAYERS OF F(CF2)11COOH AND F(CF2)10CH2COOH

We report the results of a grazing incidence x-ray diffraction study of monolayers of F(CF2)11COOH and F(CF2)10CH2COOH on water at several temperatures. It is found that in both monolayers the amphiphile molecules have a very small collective tilt angle (relative to the normal to the surface). For the temperature range studied (approximately 4-30-degrees-C) the data for both monolayers indicate the existence of only a first-order transition between an ordered condensed phase and a disordered dilute phase; there is no evidence, in this temperature range, for the existence of a continuous tilt transition such as has been observed in monolayers of arachidic acid. The experimental data suggest that the critical point for the transition observed is at a lower temperature for a monolayer of F(CF2)11COOH than for a monolayer of F(CF2)11COOH than for a monolayer of F(CF2)10CH2COOH.