In Situ Observations of Barium Sulfate Nucleation in Nanopores

Abstract

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 nonbulk 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 ∼12% of the pores filled and was seemingly limited by the 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 ∼1.0 × 10–7 mol/m2/s, obtained from previous studies, but is consistent with TEM and STEM observations made here. The inferred nucleation rate of barium sulfate inside nanopores is estimated to be on the order of 1.0 × 109 nuclei/m2/s, which is 2 orders of magnitude higher than previous measurements on a planar silica substrate (∼1.0 × 107 nuclei/m2/s). This implies that the ability 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.

Solid Barium Sulfate Formation from Aqueous Solution: Re-examination of Key Literature and Kinetics of This Classic System in Search of a Minimum Mechanism of Formation

“…Of interest regarding efforts to obtain nucleation and growth rates is a 2022 report from Brady et al in which they investigate the formation of (BaSO 4 ) n on nanopores via X-ray scattering and pair distribution function (PDF) analysis (as well as ex situ TEM/STEM imaging). Those authors report the fraction of pore volume filled as a function of time, but then fit with the Johnson–Mehl–Avrami–Kolmogorov (JMAK) semi-empirical equation, through which nucleation and growth rates of 10 9 nuclei m –2 s –1 and 10 –7 mol m –2 s –1 , respectively, were obtained.…”

“…Of interest regarding efforts to obtain nucleation and growth rates is a 2022 report from Brady et al in which they investigate the formation of (BaSO 4 ) n on nanopores via X-ray scattering and pair distribution function (PDF) analysis (as well as ex situ TEM/STEM imaging). Those authors report the fraction of pore volume filled as a function of time, but then fit with the Johnson–Mehl–Avrami–Kolmogorov (JMAK) semi-empirical equation, through which nucleation and growth rates of 10 9 nuclei m –2 s –1 and 10 –7 mol m –2 s –1 , respectively, were obtained. Assumptions were necessary to obtain these estimates, including that the assumption that the nucleation rate will dominate growth and the assumption that the nucleus size is equal to the pore size (2.38 nm) or that the particle size at the end is 20 nm, neither of which is supported experimentally.…”

“…Dust control merits one more mention here as well. ,,, Fortunately, once again all these issues can at least in principle be dealt with via the PEStep approach of A → B (rate constant k 1 ) that has been able to deal with the following, again at least in the case of Ir(0) n particle formation reactions: mass-transfer limitations, the effects of pH, of H 2 O, of acetone solvent impurities, of trace EtOAc as an impurity, of nanocluster stabilizer, , of precursor concentration, , of dust, , and other effects that cause “super-sensitive” kinetics. Fifth, the use of multiple (ideally all), possible complimentary physical methods will help lead to greater mechanistic insights into (BaSO 4 ) n formation. These include rapid mixing, high-precision conductivity, , SAXS/WAXS studies like those done on (CdSe) n and other systems, ,, XANES, X-ray PDF analysis, nonlinear optical spectroscopy, SEM/TEM/STEM/HAADF-STEM, neutron-scattering, infrared spectroscopy, vertical scanning interferometry, atomic force microscopy, ,, and dynamic light scattering to monitor not only nucleation earlier and with greater sensitivity, but also monitor growth processes such as in Kügler et al’s important contribution. , Sixth, better insights into, and understanding of, the role(s) of ion-pairing (triple ions and higher clusters, e.g., in more concentrated solutions near or above supersaturation) and PNCs , on especially nucleation are likely to prove of interest. A key here will be to determine if any spectroscopically observable species are on-path vs off-path to the final (BaSO 4 ) n , something that cannot be attained with kinetics under steady-state conditions but, instead, requires pre- or post-steady-state kinetics studies. , Seventh, additional state-of-the-art computations ,− would be most welcome, especially those anchored to all available experimental benchmarks, calculations that ideally include explicit treatment of specific waters of hydration as well as the bulk dielectric constant of water as the so...…”

“…Of interest regarding efforts to obtain nucleation and growth rates is a 2022 report from Brady et al 108 in which they investigate the formation of (BaSO 4 ) n on nanopores via X-ray scattering and pair distribution function (PDF) analysis (as well…”

“…(v) Fifth, the use of multiple (ideally all), 34 possible complimentary physical methods will help lead to greater mechanistic insights into (BaSO 4 ) n formation. These include rapid mixing, high-precision conductivity, 103,104 SAXS/WAXS studies like those done on (CdSe) n 156 and other systems, 30,157,158 XANES, 30 X-ray PDF analysis, 108 nonlinear optical spectroscopy, 159 SEM/TEM/STEM/ HAADF-STEM, neutron-scattering, 60 infrared spectroscopy, 160 microscopy, 58,143,144 and dynamic light scattering 162 to monitor not only nucleation earlier and with greater sensitivity, but also monitor growth processes such as in Kugler et al's important contribution. 97,163 (vi) Sixth, better insights into, and understanding of, the role(s) of ion-pairing (triple ions and higher clusters, e.g., in more concentrated solutions near or above supersaturation) and PNCs 164,165 on especially nucleation are likely to prove of interest.…”