Atomic force microscopy determination of the molecular orientation and array structure of a pyridine adlayer on heulandite(010) adsorbed from its aqueous solution

For the first time the array structure of liquid-phase-adsorbed aromatics on a zeolite surface was determined, and the molecular orientation within the adsorbed phase was estimated. Pyridine molecules adsorbed on a cleaved heulandite(010) surface from an aqueous solution were imaged with a contact-mode atomic force microscope. The adsorbed pyridine molecules formed a well-ordered two-dimensional (2D) hexagonal array on the surface, with the rhombic unit cell dimension being 0.55 nm. The molecular plane is found to be tilted, with the estimated tilt angle being ca. 43 degrees to the surface. The ring plane also makes an estimated angle of ca. 20 degrees with respect to one of the 2D lattice vectors of the adsorbed pyridine unit cell.