Classical nucleation theory predicts that crystal nucleation 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 direct 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), and large clusters are expected to be exceedingly rare. This, combined with the transitory nature of small clusters, makes observation of the subcritical cluster distribution exceedingly difficult. In this study, we utilize high-speed in-situ atomic force microscopy (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 are able to observe single-molecule adsorbates of aluminum. The pH dependance of adsorption, combined with complimentary streaming-potential and dynamic force spectroscopy experiments indicate that hydroxide ions promote the strong-binding of aluminum species at the mica-water interface. With increased supersaturation, we observe a dynamic population of larger molecular clusters, with sizes up to 4.0 nm2. The size-distributions and cluster-dynamics are broadly consistent with expectations of classical nucleation theory, although some deviations from the ideal classical energy landscape are detectable. Further increases in supersaturation leads to growth of a monolayer of crystalline aluminum hydroxide. We find that the surface enhances nucleation rates relative to the bulk in several ways. Aluminum appears to be concentrated near the mica surface in a dense-liquid layer, and the high density of clusters at the surface leads to a growth-pathway driven by cluster-coalescence. 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.

Observing nucleation dynamics at the mica-water interface with molecular-resolution atomic force microscopy

256th National Meeting (2018)

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