Geochemistry

Nanoparticle Coastlines

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Science  22 May 2009:
Vol. 324, Issue 5930, pp. 991
DOI: 10.1126/science.324_991b

Mineral nanoparticles are naturally abundant in a wide range of environments, from the oceans to the atmosphere. Despite observations that particle size influences the reactivity of a mineral surface in a number of geochemical reactions, it is still largely unknown why nanoparticles behave differently than larger particles with the same composition. Spagnoli et al. show that in aqueous solutions, the size and shape of mineral particles influence the structure of the first few layers of water on their surfaces—an important determinant of reactivity. Using molecular dynamics simulations of the common iron-oxide mineral hematite, they found that water order and layering around the particle decreased for hematite particles of decreasing size. The residence time of water molecules near the surface was shorter for smaller, less-crystalline nanoparticles than for larger nanoparticles or a bulk hematite surface. Particles with facets or low curvature, however, tended to preferentially stabilize the water network and in some cases caused faceting within the water layer itself. The dynamic nature of the water solvation shell surrounding environmental nanoparticles probably influences the energetics of crystal growth and may help explain why some surface processes—including heterogenous catalysis, bacterial metal respiration, and ion adsorption—show trends that vary with particle size.

Geochim. Cosmochim. Acta 73, 10.1016/j.gca.2009.04.005 (2009).

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