Abstract
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 Å, 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 Å. It is shown that the critical value for a fatty acid monolayer corresponds to a distance of 7 Å between polar heads, which leads to Å. 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.
- Received 9 October 1997
DOI:https://doi.org/10.1103/PhysRevE.57.6835
©1998 American Physical Society