NER: Can Aluminum Nanocluster Complexes Yield Rates of Ligand Exchange at Aluminous Mineral Surfaces?

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CaseyNER-0207709This proposal was received in response to the Nanoscale Science and Engineering initiative NSF 01-157, category NER, and is co-funded by the GEO Directorate. The reactions that concern geochemists are often ligand exchange reactions, where one atom in the inner-coordination sphere of a metal is replaced with another. Even complicated processes like adsorption and mineral dissolution are really ligand-exchange reactions because the coordination number of the metal is unchanged by the process. The rates of these reactions vary enormously for different metals yet very little is known about the reaction rates at mineral surfaces, which are the key sites that affect natural water chemistry. Stated differently, the functional groups on clays and metal-hydroxide minerals are ligands too. Geochemists are coming to rely heavily on computational methods of predicting reaction properties because so many of the key reactions are difficult to probe experimentally. Unfortunately, the computational cost of predicting rates of ligand exchanges at the aqueous-mineral interface are enormous because so many atoms must be included in a realistic simulation. We pursue an alternative strategy where we attempt to establish a correlation between measured rate coefficients of water exchange ( ) in aluminum complexes and nanoparticles and structural parameters in the complexes that were calculated using ab initio methods. This correlation is novel because it is based on the easily calculated properties of the complexes instead of on the costly transition-state structures and provides a simple and computationally inexpensive way to predict rates of one of the most fundamental classes of geochemical reactions. By hypothesis, this simple approach can be extended to nanoparticles, colloids, and clays and is limited only by the feasibility of computer simulations. This research is risky because there is no guarantee that the important variables will scale with molecular size. Nevertheless, if successful, it promises an inexpensive method to predict rates of the most fundamental of reactions in aqueous media across an enormous range of molecular sizes. It is a novel and far-reaching approach because it is based on properties of ground-state structures rather than transition states.