Remark concerning the theory of the tilting transition in expanded Langmuir monolayers

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Abstract
We report calculations, based on the Cai–Rice theory [J. Chem. Phys. 96, 6229 (1992)], of the relative importance of chain–surface and chain–chain interactions for the occurrence of the tilting transition in expanded Langmuirmonolayers. The interaction between amphiphile molecules, and that between an amphiphile molecule and the surface, are described by Lennard‐Jones potentials. It is found that for a reasonable set of interaction parameters the structure of the expanded monolayer supports a collective tilt, and that the existence of a nonzero amphiphile chain–surface interaction plays an important role in determining the collective tilt. However, over much of the range of surface density of interest, an expanded monolayer with nonzero collective tilt is unstable with respect to one with zero collective tilt and a smaller separation of the molecules. The existence or nonexistence of a collective tilt in the monolayer is very sensitive to the relative magnitude of the amphiphile chain–amphiphile chain and amphiphile chain–surface interactions; only a modest increase in the latter is sufficient to drive the collective tilt in the monolayer toward 90°. Our results are in accord with the observed behavior of monolayers of perfluorinated amphiphiles and ordinary amphiphiles if the ratio of chain–surface to chain–chain interactions is allowed to vary.