The structure and phase diagram of chiral alkyl-serine monolayers on mercury

The structure of liquid-mercury-supported Langmuir films (LFs) of chiral serine-modified fatty acid molecules was studied as a function of length, n = 8-22 carbons, temperature, T = 5-25 °C, and surface coverage, A ≈ 40-200 Ų per molecule, for both homochiral and heterochiral compounds. Using surface pressure π-area A isotherms and surface-specific synchrotron X-ray diffraction methods the phase diagram was determined in detail. No lateral order was found for phases comprising surface-parallel molecules, in contrast with unmodified fatty acid LFs on mercury. For phases comprising standing-up molecules, long range lateral order was found for n ≥ 12, but no order for n = 8. The molecules in the ordered phases are extended, and tilt rigidly by ∼40° from the surface normal. The homochiral LFs pack in an oblique, single-molecule, unit cell. The heterochiral LFs pack in a body-centered rectangular unit cell, containing two molecules. Unlike unmodified fatty acid LFs, the structure of the standing-up phase does not vary with n, T or A. The interactions underlying these characteristics, and the role of chirality, are discussed.