Classical nucleation theory predicts that nucleation of minerals from solution requires a population of sub-critical molecular clusters that form through dynamic statistical fluctuations. These clusters are expected to occur in any solution, and their observation would provide experimental confirmation of the classical nucleation theory. However, the cluster population is expected to drop off exponentially with size (reflecting the size-dependent free energy landscape of cluster formation), making large clusters exceedingly rare. This, combined with the transitory nature of small clusters, makes observation of the subcritical cluster distribution exceedingly difficult. However, it may be feasible to detect these molecular-scale clusters during nucleation at mineral-water interfaces, where modern modern methods for high-resolution surface imaging can be applied. In this study, we utilize high-speed in-situ AFM to directly image subcritical clusters of aluminum hydroxide at a mica-water interface. When imaging a cleaved mica surface in the presence of AlCl3 aqueous solutions, we observe a population of dynamic clusters, with sizes ranging from 0.2-1.0 nm2, lifetimes on the order of minutes, and population densities that increase with aluminum hydroxide saturation in the bulk solution. When saturations are sufficiently high, a monolayer of epitaxial aluminum hydroxide is formed, with structure related to gibbsite. These observations are consistent with the classical nucleation theory, but we also identify several mechanisms that enhance nucleation rates at the surface. First, aluminum appears to be form a dense-liquid layer near the mica surface that facilitates nucleation. Furthermore, ab-initio molecular dynamics suggest that chemical interactions with the mica surface induce a proton-depleted region near the mica surface that favors formation of aluminum-hydroxide species.

Imaging nucleation at the mica-water interface with high speed molecular-resolution atomic force microscopy

255th National Meeting (2018)

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