Surface complexes of acetate on edge surfaces of 2:1 type phyllosilicate: Insights from density functional theory calculation Xiandong Liu, Xiancai Lu * , Rucheng Wang, Huiqun Zhou, Shijin Xu State Key Laboratory for Mineral Deposit Research, School of Earth Sciences and Engineering, Nanjing University, 22 Hankou Road, Nanjing 210093, PR China Received 3 March 2008; accepted in revised form 19 September 2008; available online 4 October 2008 Abstract To explore the complexation mechanisms of carboxylate on phyllosilicate edge surfaces, we simulate acetate complexes on the (0 1 0) type edge of pyrophyllite by using density functional theory method. We take into account the intrinsic long-range order and all the possible complex sets under common environments. This study discloses that H-bonding interactions occur widely and play important roles in both inner-sphere and outer-sphere fashions. In inner-sphere complexes, one acetate C–O bond elongates to form a covalent bond with surface Al atom; the other C–O either forms a covalent bond with Al or interacts with surface hydroxyls via H-bonds. In outer-sphere complexes, the acetate can capture a proton from the surface groups to form an acid molecule. For the groups of both substrate and ligand, the variations in geometrical parameters caused by H- bonding interactions depend on the role it plays (i.e., proton donor or acceptor). By comparing the edge structures before and after interaction, we found that the carboxylate binding can modify the surface structures. In the inner-sphere complexes, the exposed Al atom can be stabilized by a single acetate ion through either monodentate or bidentate schemes, whereas the Al atoms complexing both an acetate and a hydroxyl may significantly deviate outwards from the bulk equilibrium positions. In the outer-sphere complexes, some H-bondings are strong enough to polarize the metal–oxygen bonds and therefore distort the local coordination structure of metal in the substrate, which may make the metal susceptible to release. Ó 2008 Elsevier Ltd. All rights reserved. 1. INTRODUCTION Interactions of clay minerals with organics containing carboxylate group affect many geochemical processes in aquifers, soils and oil reservoirs (Schnitzer, 2000). The bonding of carboxylate species can significantly change the surface properties of clay minerals, e.g., the surface hydrophobicity of minerals can be enhanced by adsorption of organics, which promotes adsorption of hydrophobic or- ganic compounds and thence influences their fate in the environments (Laor et al., 1998; Wang et al., 2005). The formed complexes also remarkably influence the dissolution processes and surface reactions of minerals (Stumm, 1987; Axe and Persson, 2001; Johnson et al., 2004a,b, 2005; Yoon et al., 2004, 2005). Therefore, understandings of carboxyl- ate complexation mechanisms are very important in geo- chemistry and soil chemistry. Based on extensive experimental studies of anion com- plexes on surfaces of oxides and hydroxides (e.g., Dobson and McQuillan, 1999; Axe and Persson, 2001; Duckworth and Martin, 2001; Sherman and Randall, 2003; Yoon et al., 2004), it has been well accepted that the bonding of carboxylate species commonly occurs through two mecha- nisms: inner-sphere complexation in which the organic anion forms a direct bond with surface cations, and outer-sphere complexation, in which the anion is held by the hydrogen-bond and electrostatics. Some study has fo- cused on the types and structures of carboxylate complexes on clay surfaces (Kang and Xing, 2007), but the molecular level information is still incomplete because of the compli- cated surface structures of clays. The 2:1 type phyllosilicate, a major class of clays, is stacked by ‘‘T–O–T” layers 0016-7037/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.gca.2008.09.026 * Corresponding author. Fax: +86 25 83686016. E-mail address: xcljun@nju.edu.cn (X. Lu). www.elsevier.com/locate/gca Available online at www.sciencedirect.com Geochimica et Cosmochimica Acta 72 (2008) 5896–5907