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 o f monolayers of F(CF2)11COOH and F(CF2)10CH2COOH on water at several t emperatures. It is found that in both monolayers the amphiphile molecu les have a very small collective tilt angle (relative to the normal to the surface). For the temperature range studied (approximately 4-30-d egrees-C) the data for both monolayers indicate the existence of only a first-order transition between an ordered condensed phase and a diso rdered dilute phase; there is no evidence, in this temperature range, for the existence of a continuous tilt transition such as has been obs erved in monolayers of arachidic acid. The experimental data suggest t hat the critical point for the transition observed is at a lower tempe rature for a monolayer of F(CF2)11COOH than for a monolayer of F(CF2)1 1COOH than for a monolayer of F(CF2)10CH2COOH.