Mesophases and crystalline phases in fatty acid monolayers

Abstract

The polymorphism in docosanoic (behenic) acid monolayers at the air/water interface is investigated by means of X-raydiffraction. Analysis of the diffraction spot profiles parallel and perpendicular to the surface provides detailed data on theunit cells of five different phases (L2,Li,S,CS, and LS) with uniform aliphatic tail alignment and regular two-dimensionallattice structure. The first four of these display centered rectangular symmetry, while the LS phase is hexagonal. In theLS, S,and CS phases the tails are oriented normal to the surface, whereas in Lzand Li they are tilted toward the nearestand next-nearest neighbors, respectively. The unit cells of the S and CS phases have the same symmetry. However, thelatter is the only phase to display the packing density (C0.19 nmz aliphatic chain cross section), the very low compressibility(<3 X 10-4 m/mN), and the resolution-limited peak width (C0.03nm-l, corresponding to a characteristiclength for exponentialdecay of positional correlations equal to more than 160 lattice spacings) expected from a close-packed crystalline solid. Incontrast, the peak width in the Lz,L2/,LS, and S phases is more than 0.09 nm-’, corresponding to a correlation length ofless than 54 lattice spacings. Their similarities to mesophases of liquid crystals are pointed out.