In Situ Observations of Barium Sulfate Nucleation in Nanopores

The nucleation and growth of barium sulfate in nanoporous silica was investigated using in situ small-angle X-ray scattering and X-ray pair distribution function analysis, together with ex situ transmission and scanning transmission electron microscopy (TEM and STEM) imaging. We found that crystalline barite formation in micropores is likely preceded by a nonbu l k barite phase in the nanopores, indicating a possible nonclassical nucleation pathway for barium sulfate under confinement. The nucleation of barium sulfate inside the nanopores stopped at similar to 12% of the pores filled and was seemingly limited by t h e formation of crystals near the exterior of the silica particles, which likely blocked subsequent solute transport into the interior of the nanopores. The growth rate of barium sulfate was fit using the Johnson-Mehl-Avrami-Kolmogorov equation and constrained using a growth rate of barite of similar to 1.0 x 10(-7) mol/m(2)/s, obtained from previous studies, but is consistent with TEM and STEM observations made here. The inferred nucleation rate of ba r i u m sulfate inside nanopores is estimated to be on the order of 1.0 x 10(9) nuclei/m(2)/s, which is 2 orders of magnitude higher than previous measurements on a planar silica substrate (similar to 1.0 x 10(7) nuclei/m(2)/s). This implies that the abi l i t y of silica nanopores to promote barium sulfate nucleation is sufficiently high as to create a potentially self-limiting condition, where the nucleation reaction is shut down prematurely because rapid growth blocks reactant transport.