Packing characteristics of crystalline monolayers of fatty acid salts, at the air-solution interface, studied by grazing incidence x-ray diffraction

Grazing incidence X-ray diffraction (GID) studies were performed on self-assembled monolayers of carboxylic acids C_nH_2n+1CO₂H (n = 13,19,29) spread over CdCl₂ or CaCl₂ solutions at pH values 8.85 and 10, respectively, adjusted with ammonia, and subphase temperatures in the range 5 to 9 °C. The GID results show the tendency for spontaneous two-dimensional (2-D) crystalline aggregation of the molecules which pack in a rectangular cell with the hydrocarbon chains tending to be vertically aligned with respect to the solution surface. The molecular chains form a herring bone arrangement with the two molecules in the unit cell related essentially by glide symmetry. Detailed Bragg rod fitting analysis shows however, that the two molecules are crystallographically independent so that the plane group symmetry is chiral p1. For n= 29, it was possible to determine the extent of molecular thermal motion parallel to the water surface and which proved to be in the same range as in layered 3-D crystal structures containing aliphatic chains. The GID data did not provide definite evidence for ordered ion binding although the extent of crystallinity is enhanced by the presence of the bound counterion. For example, myristic acid (n = 13) yielded the GID pattern only when spread over CdCl₂ but not on pure water. A simple packing model is provided for ordered binding of counterions with high degree of coordination to the oxygen atoms of the carboxylate head groups. The proposed model is also in keeping with the observed lower area per amphiphilic molecule than over pure water.