The chemical and electrostatic interactions occurring at interfaces of crystalline metal-oxides and aqueous solutions are of fundamental importance in myriad geochemical, materials sciences and technological processes. Reactions of particular importance to geochemists that take place between minerals and fluids include, soil formation and weathering; uptake, transport and release of inorganic and organic contaminants; biomineralization; dissolution, aggregation and precipitation; and numerous other natural processes. A comparable broad list of interface reactions of relevance to the materials sciences and industry could also be presented. Intellectual Merit: The goal of this proposal is to investigate the effect of particle size on the chemical and electrostatic interactions between nanocrystalline metal-oxides and aqueous solutions. Nanosized particles are ubiquitous at the Earth?s surface, and may play a considerable role in biogeochemical and environmental processes. Furthermore engineered metal-oxide nanomaterials are used extensively in a variety of industrial applications and commercial products. The growing use of manufactured nanomaterials raises concern for the potential release of these particles into the environment. The role of nanoparticles (natural and manufactured) at the Earth?s surface is of importance because of the variation in the chemical and physical properties of these particles as a function of particle size. Equally, the large surface area of nanoparticles and variations in atomic structure possibly will change the surface reactivity of nanoparticles towards aqueous solutions as a function of particle size. No single experimental or theoretical technique will provide a coherent and comprehensive view of the properties of nanoparticle-solution interfacial reactions in the environment. Accordingly, the proposed research will integrate macroscopic experimental studies, molecular-scale computational studies, and surface complexation models. All three focus areas of this proposal will primarily investigate the interfacial behavior of anatase (TiO2) powders of discrete nanosize; additionally, the surface properties of MnO2 nanophases will be explored. The experiments will for the first time, provide a systematic, coherent and extensive experimental data set that will document fundamental differences in surface reactivity between nanoparticles and macroscopic particles of the same bulk material. The experiments will be performed over a range of temperatures (5 ? 75 °C) and solution compositions representative of those encountered at the Earth?s surface. The variable temperature studies will be the first to document the effect of temperature on nanoparticle surface reactivity. The experimental studies will be strongly coupled with theoretical studies. The molecular simulation studies will probe the atomic-scale properties of hydrated anatase nanoparticles, and will provide detailed information on surface bonding structures, H-bonding distributions, surface relaxation, and adsorption geometries. The ultimate goal of the proposed research is to merge the experimental and theoretical results into surface complexation models that can describe and predict interfacial properties that govern particle-size effects. Such an integrated approach is the state-of-the-art for interface science. Broader Impacts: The proposed studies will considerably advance fundamental understanding of the behavior of nanoparticles interacting with aqueous solutions at the Earth?s surface. Furthermore, the anticipated results will have wide-ranging application and relevance that extends well beyond the earth and environmental sciences; for instance, to materials sciences, chemistry, industry, and medical fields. For example, the controlled growth of nanoparticles and larger crystals from nanoparticles depends on aggregation, which in turn is influenced directly by particle-water interface chemistry. The students and post-doctoral associates involved in the project will receive training in both experimental and theoretical methods, learn how to integrate results from these approaches, and develop a more comprehensive understanding of nanoparticle interfaces. Moreover, the proposed studies are well suited to the involvement of both graduate and undergraduate students. All research results will be disseminated via publications in peer reviewed journals, and presented at national and international meetings.
