Molecular Orientation in a Semi-Fluorinated Alkane Monolayer Spread at the Air-Water Interface

Abstract

We show that a stable monolayer of F(CF2)8 (CH2)2-S-(CH2)16H molecules can be obtained at the air water interface. The molecular orientation within the monolayer has been determined at the air-water interface by means of surface pressure isotherms, surface charge as a function of the molecular area measurements and fluorescence microscopy. We propose a theoretical model to explain the experimental results. For molecular areas larger than 60 Å2(π=0 mN/m), half of the molecules extend their perfluorinated chains up and their hydrocarbonated chains down. At molecular areas about 30 Å2, the monolayer exhibits a condensed-like phase for which all the molecules extend their perfluorinated chains up. The transition between these two phases is first order. Eventually, we deduce a value about 4 for the dielectric constant of the monolayer.