HYDROGEN-BOND CONTROL OF STRUCTURE AND CONDUCTIVITY OF LANGMUIR FILMS

Lateral conductivity and a high proton mobility at the water-Langmuir film interface appears when.the monolayer is compressed below a critic al area. For a fatty acid monolayer, this critical area lies between 3 5 and 40 Angstrom(2), and it was thought to correspond to the formatio n of a H-bonded network between the monolayer headgroups and the water molecules. In this work, the mobility and lateral conductivity are su ccessfully explained using a simple geometric model, hydrogen bond dat a, and a unidimensional model for proton transfer (PT) in hydrogen bon ds. According to the model, hydrogen bonds and PT effectively occur wh en the distance between oxygens is R<2.8 Angstrom. It is shown that th e critical value for a fatty acid monolayer corresponds to a distance of 7 Angstrom between polar heads, which leads to R = 2.8 Angstrom. Th is represents a heoretical justification for the hypothesis of proton conduction via a hop and turn mechanism. Furthermore, the strong hydro gen bonds below the critical area are responsible for the monolayer st ructuring, which causes the surface potential to increase sharply at t his area.