(694f) Prediction of Crystal Habits of Urea Using Molecular Dynamics Simulations and Comparision to Experimental Results

Crystals grown at moderate and low levels of supersaturation typically exhibit a faceted shape that reflects the ordered arrangement of molecules within the crystal lattice and the environment in which the crystal was grown. In solution crystallization, solvents and additives (or impurities) can significantly change crystal shape by selectivley discouraging (or promoting) the growth of individual crystal faces. While these effects manifest in habit changes on the scale of whole crystals, their underlying reason is the interaction between solvent and additive molecules with the crystal on a molecular scale (Salvalaglio et al., 2012).

Investigating crystal growth at the molecular level is therefore of great importance in order to enhance our current understanding of crystallization processes. Moving towards this goal is a multi-scale (and multi-disciplinary) challenge that can greatly benefit from the interplay between theory, simulations and experimental techniques. In this work we therefore combine advanced molecular simulation methods (Laio and Parrinello, 2002, Barducci et al., 2008, Bonomi et al., 2009) with a theoretical crystal growth model that describes rough crystal growth and growth by the birth and spread (2D nucleation) mechanism (Ohara and Reid, 1973) on a molecular scale. Using molecular simulations that explicitly include solute, solvent and additive molecules allows us to obtain the physical parameters in our theoretical crystal growth model, which allows calculating relative growth rates of different crystal faces. The relative growth rates can then be used to predict crystal habits that originate from the simulated conditions.

Here we examine the paradigmatic case of urea, which grows from vapor exposing three well defined crystal faces: {001}, {110} and {111} (Docherty et al., 1993). Experimental evidence (Vetter, 2012) highlighted that the solvent and additives in solution caused a wide variety of crystal habits that all consisted of combinations (or subsets) of these faces. In order to rationalize this finding we have developed a representation tool, the Shape Diagram (SD), in which all possible steady state morphologies of urea crystals for different growth rate ratios can be assessed.

The rational display of the accessible crystal habits allowed comparing experimental results with computational predictions of the surprisingly wide ensemble of urea growth shapes obtained from different solvents and additives. The experimentally found and computationally predicted crystal habits were found to be in good agreement, thus demonstrating that the developed simulation methodology and theoretical model are able to predict crystal habits from a variety of supersaturations, solvents and additives (Salvalaglio et al., 2013).

References:

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M. Salvalaglio, T. Vetter, M. Mazzotti, and M. Parrinello (2013). Controlling and Predicting Crystal Shapes: The Case of Urea. Submitted to Nature Materials.

T. Vetter (2012). PhD thesis, ETH Zurich, 2012. (http://e-collection.library.ethz.ch/view/eth:6639)