Hydrogen-bond control of structure and conductivity of Langmuir films

Vitor B. P. Leite, Ailton Cavalli, and Osvaldo N. Oliveira, Jr.
Phys. Rev. E 57, 6835 – Published 1 June 1998
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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 Å2, 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 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 R=2.8 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

Authors & Affiliations

Vitor B. P. Leite, Ailton Cavalli, and Osvaldo N. Oliveira, Jr.*

  • Instituto de Física de São Carlos, Universidade de São Paulo, Caixa Postal 369, 13560-970 São Carlos, São Paulo, Brazil

  • *Electronic address: chu@ifsc.sc.usp.br

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Vol. 57, Iss. 6 — June 1998

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