Adsorption of Organic Substances on Broken Clay Surfaces: A Quantum Chemical Study ADE ´ LIA J.A. AQUINO, 1,2, DANIEL TUNEGA, 1,2 GEORG HABERHAUER, 2 MARTIN H. GERZABEK, 2,3 HANS LISCHKA 1 1 Institute for Theoretical Chemistry and Structural Biology, University of Vienna, Wa ¨hringerstrasse 17, A-1090 Vienna, Austria 2 Austrian Research Centers Seibersdorf, A-2444 Seibersdorf, Austria 3 Institute of Soil Research, University of Agricultural Sciences, Gregor-Mendel Strasse 33, A-1180 Vienna, Austria Received 19 March 2003; Accepted 11 June 2003 Abstract: Hydrogen-bonded interactions between local defect structures on broken clay surfaces modeled as molec- ular clusters and the organic molecules acetic acid, acetate, and N-methylacetamide (NMA) have been investigated. Density functional theory and polarized basis sets have been used for the computation of optimized interaction complexes and formation energies. The activity of the defect structures has been characterized as physical or chemical in terms of the strength of the hydrogen bonds formed. Chemical defects lead to significantly enhanced interactions with stronger hydrogen bonds and larger elongation of OH bonds in comparison to the physical defects. The type of interaction with the defect structure significantly influences the planarity of the model peptide bond in NMA. Both cases, enhancement of the planarity by increase of the CN double bond character and strong deviations from planarity, are observed. © 2003 Wiley Periodicals, Inc. J Comput Chem 24: 1853–1863, 2003 Key words: clay surfaces; modeling of defect structures; adsorption of organic substances; density functional theory; solvation effects Introduction Adsorption of organic compounds on mineral surfaces is an im- portant process in environmental chemistry because the adsorption processes influence the transport and activity of contaminants in soils, sediments, and water, 1–3 and therefore affect the bioavail- ability, biodegradability, and destiny of harmful organic waste. To understand and influence these global processes it is desirable to get a detailed picture of individual interactions at a molecular level. However, it is difficult to obtain the required molecular information from the experiment due to the complexity of the interactions and reactions occurring at the surface or in the inter- layer space of clay minerals and related material. Computer simulations were used successfully for the investi- gation of interactions and properties of clay mineral surfaces, either regular or broken. 4 –17 Regular surfaces parallel to the (001) plane are modeled relatively easily because they are formed either from a plane of (basal) oxygen atoms or (surface) hydroxyl groups. Models of broken surfaces are not so easy to construct, and have to be deduced rather indirectly from experimental informa- tion. 18 –22 Broken surfaces are of great interest because they are expected to have significantly higher chemical activity than regular ones. Reactive defect structures arise when polar covalent bonds between oxygen atoms and central cations of the octahedral/ tetrahedral sheets are cut. Such broken surface sections are thought to consist predominantly of oxygen anions. Partially coordinated oxygen anions can strongly bind protons resulting in neutralization of the negative surface charges and formation of surface hydroxyl groups. Therefore, the properties of such surfaces in contact with solution will be strongly dependent on proton activities (for a review, see e.g., ref. 19). At high pH these surfaces will carry a negative charge, and many surface oxygen anions will lack bound protons. At low pH, singly-protonated oxygen anions can bind additional protons. In general, surface oxygen atoms on broken surfaces are amphoteric, being able to act as proton donor or as proton acceptor. In the present work, we use a cluster model for the construction of local defect structures of a broken surface in combination with Correspondence to: A.J.A. Aquino; e-mail: adelia.aquino@univie.ac.at Contract/grant sponsor: the Austrian Science Fund; contract/grant number: P15051-CHE. © 2003 Wiley Periodicals, Inc.