title: Dissolution of ZnO nanoparticles at circumneutral pH: a study of size effects in the presence and absence of citric acid.
abstract: Understanding size-dependent processes, including dissolution, of engineered nanoparticles is essential in addressing the potential environmental and health impacts of these materials as well as their long-term stability. In this study, experimental measurements of size-dependent dissolution of well-characterized zinc oxide (ZnO) nanoparticles with particle diameters in the range of 4 to 130 nm have been measured at circumneutral pH (pH 7.5) and compared. Dissolution was found to be enhanced with smaller ZnO nanoparticles compared to larger-sized particles, even though the nanoparticles were present in solution as aggregates with hydrodynamic diameters on the order of 1-3 μm in size. The presence of citric acid significantly enhanced the extent of ZnO dissolution for all sizes, and the greatest enhancement was observed for the 4 nm particles. Although these results are found to be in qualitative agreement with theoretical predictions, a linearized form of the Kelvin equation to calculate a surface free energy yielded quantities inconsistent with expected values from the literature. Reasons for this inconsistency are discussed and include potential deviations of solubility behavior from classical thermodynamics as a result of a lack of detailed knowledge of surface structure and surface properties, including the presence of different surface crystal facets, and the aggregation state.
authors: Department of Chemistry, University of Iowa, Iowa City, Iowa 52242, United States.
authors: Imali A
authors: IA
authors: Mudunkotuwa
authors: Thilini
authors: T
authors: Rupasinghe
authors: Chia-Ming
authors: CM
authors: Wu
authors: Vicki H
authors: VH
authors: Grassian