Intermolecular forces in lipid monolayers. Two-dimensional virial coefficients for pentadecanoic acid from micromanometry on spread monolayers at the air/water interface

Norman R. Pallas, Brian Anthony Pethica

Research output: Contribution to journalArticle

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Abstract

The lateral intermolecular forces between surfactant or lipid molecules in monolayers at interfaces are fundamental to understanding the phenomena of surface activity and the interactions of lipids in two-dimensional structures such as smectic phases and biomembranes. The classical approach to these forces is via the two-dimensional virial coefficients, which requires precise micromanometry on monolayer isotherms in the dilute gaseous region. Low pressure isotherms out to high surface areas in the two-dimensional gas range have been measured at 15, 25 and 30 °C for insoluble monolayers of n-pentadecanoic acid spread at the interface between water-vapour saturated air and a dilute aqueous solution of HCl. The data allow estimates of virial coefficients up to the third term. The second virial coefficients are compared with those predicted from a statistical mechanical model for monolayers of n-alkylcarboxylic acids treated as side-by-side parallel chains extended at the surface with the carboxyl head groups shielded in the water phase. The two sets coincide at ∼26 °C, but the experimental estimates show a much larger dependence on temperature than the model predicts. Chain conformation effects, head group interactions and surface field polarization are discussed as possible temperature-dependent contributions to the lateral potentials of mean force.

Original languageEnglish (US)
Pages (from-to)5028-5034
Number of pages7
JournalPhysical Chemistry Chemical Physics
Volume11
Issue number25
DOIs
StatePublished - Jul 20 2009

All Science Journal Classification (ASJC) codes

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

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