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 se lf-assembled monolayers of carboxylic acids CnH2n+1CO2H (n = 13, 19, 2 9) spread over CdCl2 or CaCI2 solutions at pH values 8.85 and 10, resp ectively, adjusted with ammonia, and subphase temperatures in the rang e 5 to 9-degrees-C. The GID results show the tendency for spontaneous two-dimensional (2-D) crystalline aggregation of the molecules which p ack in a rectangular cell with the hydrocarbon chains tending to be ve rtically aligned with respect to the solution surface. The molecular c hains form a herring bone arrangement with the two molecules in the un it cell related essentially by glide symmetry. Detailed Bragg rod fitt ing analysis shows however, that the two molecules are crystallographi cally independent so that the plane group symmetry is chiral p1. For n = 29, it was possible to determine the extent of molecular thermal mo tion 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 bindi ng 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 simp le packing model is provided for ordered binding of counterions with h igh 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.