ENERGY LANDSCAPE FOR GIBBSITE NUCLEATION AT THE MICA-WATER INTERFACE

Classical nucleation theory (CNT) posits that mineral nuclei begin as small clusters of ions or molecules, which fluctuate in size until they achieve a critical size threshold. The presence of an interface can reduce the energy needed to form a critical-cluster, and thus facilitate nucleation through a process known as heterogeneous nucleation. If CNT holds, fluctuating subcritical clusters should be detectable at an interface prior to nucleation, with populations that reflect their energies of formation. Direct detection of these clusters would provide powerful insights into the energy landscape for nucleation.

In this study, we have applied high-speed in-situ atomic force microscopy (AFM) to directly observe the nucleation of gibbsite-like aluminum hydroxide films at a mica-water interface. AFM reveals a population of subcritical clusters prior to film nucleation, which fluctuate in a manner that is broadly consistent with CNT. However, an analysis of the cluster populations suggests that the energy of cluster-formation deviates significantly from simple classical predictions. In particular, the clusters display unexpectedly low edge-tensions, and unexpectedly low sensitivity to changes in solution-saturation. Furthermore, final film growth does not appear to be controlled by a rare-event nucleation process (e.g. the formation of supercritical clusters), but by a process of cluster coalescence.

Our analysis suggests that these deviations can explained by considering the electrostatic energy involved in forming positively-charged aluminum hydroxide clusters on a negatively-charged mica substrate. By including an electrostatic energy term into kinetic Monte Carlo models of film nucleation, we are able to reproduce many of the phenomena that were observed in AFM. The electrostatic term appears to favor partial film coverages that reduce the net surface charge, and drastically alters the energy of cluster formation, leading to a barrier-free nucleation process.