Аннотация:Lateral conductivity and a high proton mobility at the water-Langmuir film interface appears when the monolayer is compressed below a critical area. For a fatty acid monolayer, this critical area lies between 35 and 40 \AA{}${}^{2}$, and it was thought to correspond to the formation of a H-bonded network between the monolayer headgroups and the water molecules. In this work, the mobility and lateral conductivity are successfully explained using a simple geometric model, hydrogen bond data, and a unidimensional model for proton transfer (PT) in hydrogen bonds. According to the model, hydrogen bonds and PT effectively occur when the distance between oxygens is $R<2.8$ \AA{}. It is shown that the critical value for a fatty acid monolayer corresponds to a distance of 7 \AA{} between polar heads, which leads to $R=2.8$ \AA{}. This represents a theoretical justification for the hypothesis of proton conduction via a hop and turn mechanism. Furthermore, the strong hydrogen bonds below the critical area are responsible for the monolayer structuring, which causes the surface potential to increase sharply at this area.
