Orientation and Electronic Structure of Ion-Exchanged Pyridinium Compounds on Mica

Monomolecular films of hexadecyl- (HDP) and methylpyridinium (MP) ions were prepared on muscovite mica from aqueous solutions via ion exchange. Electronic structure and orientation in the adsorbed films were determined with near-edge X-ray absorption fine structure spectroscopy (NEXAFS). A pronounced angular dependence of several transitions was observed for the pyridine units, indicating a preferred ring orientation with the plane tilted similar to34degrees from the surface. The hexadecyl group of HDP did not show any preferential orientation and-more remarkably-the length of the alkyl group also did not influence the orientation of the rings significantly. This is most likely the result of ionic interactions between the pyridinium group and the negatively charged mica lattice as well as van der Waals forces between pyridinium units and the mica surface. Due to the relatively low packing density dictated by the limited number of ion-exchange sites in the mica basal plane, intermolecular interactions are less important. (C) 2002 Elsevier Science (USA).