Interactions of charged Langmuir monolayers with dissolved ions

Dong June Ahn, Elias I. Franses

Research output: Contribution to journalArticlepeer-review

39 Citations (Scopus)


Dissociation of protons from fatty acid Langmuir monolayers causes competitive adsorption of other counterions and partial charge neutralization. A new equilibrium model incorporating surface electrochemical and thermodynamic features is presented. The Stern-Gouy-Chapman equations are used, along with a modified Langmuir isotherm in which ion activities in the mixed surface monolayer instead of concentrations are employed. A two-dimensional lattice model with Flory-Huggins interaction parameters χij is invoked to obtain the activities of the ions in the monolayer. The competitive adsorption of ions to fatty acid monolayers is either "ideal" (χij = 0), or nonideal, depending on the ions mixing behavior with protons. Previous electrochemical models apply only for ideal adsorption, which appears to occur for Cd2+ and Ca2+. The nonideal adsorption is characterized successfully by the use of the above interaction parameters. Ba2+ ions show negative deviations from ideal adsorption, i.e., they adsorb less than expected for ideal adsorption in the presence of H+ ions. In contrast, Pb2+ ions show positive deviations from ideal adsorption. The influence of ions on surface electrochemical properties of the monolayer is also calculated from the theory.

Original languageEnglish
Pages (from-to)8486-8493
Number of pages8
JournalJournal of Chemical Physics
Issue number11
Publication statusPublished - 1991
Externally publishedYes

ASJC Scopus subject areas

  • Physics and Astronomy(all)
  • Physical and Theoretical Chemistry


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