The Influence of Supersaturation and the Presence of Biuret on the Nucleation, Growth and Morphology of Urea Crystallised from Ethanolic Solutions

Abstract

AbstractThe influence of biuret, a degradation impurity, on the crystallisability, nucleation, growth and morphology of urea, is examined as a function of supersaturation in ethanolic solutions, through a combination of polythermal turbidometry assessment and optical microscopy measurements, complemented by molecular modelling.The inclusion of a small amount of impurity is found to increase the stability of supersaturated solutions through widening the metastable zone width. Polythermal analysis of crystallisation and dissolution on‐sets as a function of cooling and heating rate, respectively, is found to be concentration‐dependent associated with both the pure and doped systems, with the nucleation mechanism changing from progressive to instantaneous with increasing solute concentration. The addition of biuret significantly increases nucleation rate when compared to the pure system, from 9.2–20.5 to 9.3–67.7 nm−3 s−1, along with a decrease in the critical nucleus size, as would be consistent with the impurity promoting nucleation i. e. resulting in more nuclei and smaller crystals.The crystal habit of urea in ethanolic solutions is found to be dominated by the prismatic {110} faces, and smaller polar {111} capping faces. The mean crystal growth rates of these faces are found to increase with respect to supersaturation, with the {111} faces exhibiting a greater level of increase than {110}, consistent with stronger solvent binding which restricts the growth of the {110} face. The addition of biuret is found to have a greater effect on retarding the growth of {111} compared to {110}, resulting in a more compact and equant morphology. These results are rationalised with computational visualisation and modelling of the surface chemistry of the crystal habit planes together with the energetics of the impurity binding to them.