Session BC32 - Imaging and Spectroscopy: Optical and X-Ray.
ORAL session, Sunday afternoon, March 21
Room 163W, GWCC
Using grazing incidence synchrotron x-ray diffraction (GID) we have studied the influence of the divalent cations cadmium, lead, barium, and zinc in the subphase on the structure and phase transitions of Langmuir monolayers of heneicosanoic acid. Levieller et al.(Langmuir 10, 819 (1994)) were the first to observe a superlattice of cadmium ions under the acid monolayer and we observed a similar highly ordered structure of lead ions. A clear indication of a similar counterionic lattice is not obtained with barium and zinc even at high pH values. The formation of the counterionic superlattice survives only above a particular pH threshold value and under a particular temperature and is accompanied by the appearance of the "characteristic triplet" of peaks from the acid monolayer. The calculated area per molecule depends on the type of the coordinated ion and increases with higher Pauling electronegativity in consonance with results that Schwartz et al.(J. Am. Chem. Soc. 115, 7374 (1993)) found for LB films of these molecular complexes. [Supported by DOE grant no. DE-FG02-84ER45125.]