NANOSCALE IMAGING OF MOLECULAR ADSORPTION

In situ atomic force microscope observations were made of the adsorpti on of anions (1- or 2-) of the organic diacid 5-benzoyl-4-hydroxy-2-me thoxybenzenesulfonic acid from aqueous solution onto the (0001) surfac e of hydrotalcite (HT), a layered clay. This adsorption process is bel ieved to mimic the ion-exchange reactions that occur within the layers of HT and other layered clays. Atomic force microscope images of the (0001) surfaces of HT, acquired in aqueous solutions, reveal an ordere d structure with respect to magnesium and aluminum atoms. In the prese nce of the anions, atomic force microscopy indicates pH-dependent adso rption onto the formally cationic HT surface. The anion coverage is go verned by electroneutrality and steric interactions between the bulky anions within the adsorbed layer, whereas the orientation of the anion s with respect to the HT surface is dictated by coulombic interactions and hydrogen bonding between the anion's sulfonate moiety and clay hy droxyl triads. These observations reveal that the reversible adsorptio n of molecular species can be examined directly by in situ atomic forc e microscopy, providing details of surface stoichiometry and adlayer s ymmetry on the local, molecular level.