REMARK CONCERNING THE THEORY OF THE TILTING TRANSITION IN EXPANDED LANGMUIR MONOLAYERS

We report calculations, based on the Cai-Rice theory [J. Chem. Phys. 9 6, 6229 (1992)], of the relative importance of chain-surface and chain -chain interactions for the occurrence of the tilting transition in ex panded Langmuir monolayers. The interaction between amphiphile molecul es, and that between an amphiphile molecule and the surface, are descr ibed by Lennard-Jones potentials. It is found that for a reasonable se t of interaction parameters the structure of the expanded monolayer su pports a collective tilt, and that the existence of a nonzero amphiphi le chain-surface interaction plays an important role in determining th e collective tilt. However, over much of the range of surface density of interest, an expanded monolayer with nonzero collective tilt is uns table with respect to one with zero collective tilt and a smaller sepa ration 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 int eractions; only a modest increase in the latter is sufficient to drive the collective tilt in the monolayer toward 90 degrees. Our results a re in accord with the observed behavior of monolayers of perfluorinate d amphiphiles and ordinary amphiphiles if the ratio of chain-surface t o chain-chain interactions is allowed to vary.