Adsorption of Transition Metal Cations onto a Lamellar Poly(3-aminopropyl)silsesquioxane: Cation−Cation Interaction and Transition of Adsorption Phase

The adsorption of Cu 2+ and Co 2+ onto poly(3-aminopropylsilsesquioxane) has been carried out to elucidate the effects of the adsorbate−adsorbate interaction on aminopropyl-functionalized silica surfaces. By fitting to the Langmuir equation, N/Cu = 2 and N/Co = 1, respectively, have been elucidated at full coverage. The ESR signals of Cu 2+ gradually change with increasing adsorption, although the structures are assigned to diamino-Cu(II) complexes on the basis of g values and hyperfine coupling constants. A resonance due to Cu 2+ −Cu 2+ spin−spin coupling is observed in the low field for the saturated adsorption, even at room temperature. This is probably the first spectroscopic observation of such interactions in organic functionalized silica surfaces and implies a highly dense Cu 2+ layer. On the other hand, the adsorption isotherm of Co 2+ is composed of two stages: the adsorption is once saturated at N/Co = 2 and then increases again until N/Co = 1. This characteristic adsorption feature implies a sudden gross change of the adsorption layer when N/Co = 2 is achieved, which generates a large number of empty sites. The pre-edge peak intensities of the Co K-edge XANES spectra are plotted in accordance with the adsorption isotherm, where a sudden increase is found at the start of the second stage of adsorption. These characteristic adsorption phenomena are discussed with the structure of poly(3-aminopropylsilsesquioxane), where the aminopropyl group is located two-dimensionally with a spacing of Si−O−Si.