Giannakopoulos Evangelos, Stathi Panagiota, Dimos Kostas, Gournis Dimitris, Sanakis Yiannis, Deligiannakis Yiannis
Laboratory of Physical Chemistry, Department of Environmental and Natural Resources Management, University of Ioannina, Seferi 2, 30100 Agrinio, Greece.
Langmuir. 2006 Aug 1;22(16):6863-73. doi: 10.1021/la053273m.
Humic acids have stable radicals that are indigenous to their structure. Hydroxybenzoic acid derivatives such as gallic acid (GA) and protocatechuic acid are appropriate models for the radical properties of humic acids. Here we show that the adsorption or intercalation of gallic acid in Laponite clay results in a significant thermodynamic stabilization of gallic acid radicals. Moreover, the formed organoclay shows enhanced stability against acid dissolution. The structural details of the association of gallic acid with Laponite depend on the GA/Laponite loading. At low GA/Laponite ratios (approximately 10(-6) M of gallic acid per gram of clay), gallic acid is adsorbed at the variable charge sites of Laponite. This adsorption can be adequately described by surface complexation modeling. At higher GA/Laponite ratios (approximately 10(-3) M of gallic acid per gram of clay), X-ray diffraction data show that gallic acid is intercalated at the interlamellar sites of Laponite. In the presence of Pb2+ ions, the formed GA/Pb complex is associated with Laponite in an analogous structural manner, that is, adsorption at variable charge sites or intercalation at the interlamellar sites of Laponite, depending on the loading. Laponite stabilizes the GA/Pb radicals. At prolonged exposure to ambient O2, Laponite promotes the formation of stable oligomeric GA/Pb radical species, which are intercalated into interlamellar sites.
腐殖酸具有其结构中固有的稳定自由基。诸如没食子酸（GA）和原儿茶酸等羟基苯甲酸衍生物是腐殖酸自由基性质的合适模型。在此我们表明，没食子酸在锂皂石粘土中的吸附或插层会导致没食子酸自由基的显著热力学稳定。此外，形成的有机粘土对酸溶解的稳定性增强。没食子酸与锂皂石缔合的结构细节取决于GA/锂皂石负载量。在低GA/锂皂石比率（每克粘土约10⁻⁶ M没食子酸）下，没食子酸吸附在锂皂石的可变电荷位点上。这种吸附可以通过表面络合模型充分描述。在较高GA/锂皂石比率（每克粘土约10⁻³ M没食子酸）下，X射线衍射数据表明没食子酸插层在锂皂石的层间位点。在Pb²⁺离子存在下，形成的GA/Pb络合物以类似的结构方式与锂皂石缔合，即根据负载量吸附在可变电荷位点或插层在锂皂石的层间位点。锂皂石稳定GA/Pb自由基。在长时间暴露于环境O₂时，锂皂石促进形成稳定的低聚GA/Pb自由基物种，这些物种插层到层间位点。
