Measurement of Nucleation Rates during Nonclassical Nucleation of Cerium Oxalate: Comparison of Incubation-Quenching and In Situ X-ray Scattering

There is a growing consensus that numerous crystallization processes occur via a "nonclassical" mechanism, that is, involve transient noncrystalline structures (molecular clusters, solid nanoparticles, liquid nanodroplets, etc.). Because these transient noncrystalline structures have been so far overlooked in chemical engineering approaches, their impact on reactor-scale models of precipitation processes still needs assessment. Here we show that standard incubation-counting methods underestimate nucleation rates of cerium oxalate by 1 to more than 3 orders of magnitude at concentrations between 1 and ca. 100 mmol L–1. Because the transient nanoparticles and nanodroplets have lifetimes in the 100 ms to 100 s range, it becomes impossible to choose a priori an incubation time that allows complete crystallization without aggregation or ripening. This discrepancy calls for a revision of databases of nucleation rates, using instead in situ, time-resolved techniques with resolutions at the nanometer scale able to count and discriminate between crystals and noncrystalline objects, such as X-ray scattering used in this report.