The growth of α-glycine crystals from aqueous solution is investigated at constant supersaturations by utilizing the constant chemical potential molecular dynamics method. The study considers two faces ( 010) and ( 011) that predominantly determine the α-glycine crystal morphology. The general Amber force field (GAFF) with two different charge sets derived from semi-empirical calculations using the complete neglect of differential overlap method (CNDO) and from density functional calculations using the double-numerical plus d-and p-polarization basis set (DNP) is applied to describe α-glycine. The extended simple point charge model is used to simulate water. It is observed that the GAFF/DNP set leads to a much slower integration of glycine molecules into the crystal structure than the GAFF/CNDO set. The GAFF/CNDO set, however, causes the growth even at concentrations well below the experimental solubility. For the GAFF/DNP set, the influence of potassium chloride (KCl) and sodium chloride (NaCl) on the face growth rates is investigated. The parameters recently proposed by Yagasaki et al. [J. Chem. Theory Comput. 2020, 16, 2460−2473 are used to describe salt ions, as standard GAFF parameters lead to the unexpected formation of salt clusters at a concentration lower than the experimental solubility value. According to our simulation results, both salts suppress the growth of the ( 011) and ( 010) faces. The inhibiting effect of NaCl is much stronger than that of KCl for the (011) face, while both salts have a similar inhibiting effect on the (010) face. The results are in line with the experimental observations of the impact of salt ions on the α-glycine growth rates for the (011) face reported in literature.
The interaction between
crystal growth and abrasion is essential
during crystallization. Although each of these phenomena has been
independently studied in the literature in detail, their interaction
is not well understood. Here, we present a method to track the growth
of abraded potash alum crystals in three dimensions. The method is
based on micro-computed tomography. This technique distinguishes between
different growth domains in three dimensions and is used to track
the growth of crystal faces and abraded regions. We observed how abraded
regions grew faster than crystal faces. Therefore, growth leads to
ideally facetted crystals. Further, growth rates in all directions
were derived and used to parametrize a growth model. The model is
able to describe the size and shape evolution of abraded potash alum
crystals in three dimensions.
The integration of a flow-through cell into a Mach–Zehnder interferometer offers the possibility to study the dissolution of crystals in detail. The influence of flow on the displacement velocity of a specific crystal facet and the distribution of the solute concentration around the crystal are measured simultaneously in a time-resolved manner. The disintegration from the crystal surface and the mass transfer into the solvent can be separated. We aim to establish an in vitro experiment that improves the quality of prediction for the bioavailability of active pharmaceutical ingredients. In the presented feasibility study, glycine was used as a model substance. It was successfully demonstrated that the set-up is suitable for observing disintegration and mass transfer separately. The description of the dissolution rate in terms of the Sherwood number as a function of Reynolds, Schmidt and Grashof numbers clearly shows that with increasing flow rate there is a transition from natural to forced convection as the dominant mass transfer mechanism. Temporal and spatial resolved concentration fields visualize the convective mass transfer and also show the influence of convection on the diffusive boundary layer. No limitation of the dissolution by surface disintegration could be found in the examined range of flow rates.
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