(371e) Stearic Acid Polymorph Development and Nucleation Using Molecular Dynamic Simulations

Authors: 
Browning, A. R., University of California
Doherty, M. F., University of California


Nucleation and polymorph development have continued to be topics of both academic and industrial interest. The structure of organic crystals affects the physical and chemical properties of the crystal so the ability of most molecules to crystallize in two or more different structures (polymorphs) (Bernstein 2002) must be considered when designing a product or process. Recent experiments have suggested that nucleation, the first step in the crystallization process, plays an important role in polymorph development (Yau & Vekilov, 2001; Sato 1993). The ability of researchers to explore solid nucleation directly has improved as simulation and experimental techniques have improved. Previous work by the authors used molecular dynamic simulations of model particles to explore nucleation. The current work expands the tools used in that work to a real molecule system: stearic acid in hexane.

Stearic acid with its four polymorphs (six if polytypes are included) provides a broad range of conditions over which polymorph development can be explored. All four polymorphs can be observed in hexane, the solvent of choice for this work, but only two are stable. Form C is stable above 32 °C and form B is stable below 32 °C (Garti & Sato, 2001; Sato et. al. 1988). In both forms the stearic acid molecules are found in dimers that form bilayers in the crystal (Garti & Sato, 2001). Molecular dynamics was chosen to study structure changes of the stearic acid polymorphs and cluster development in a kinetic environment. The high computational cost of molecular simulations excludes the ability to observe homogenous nucleation in realistic molecular systems, but by using initial conditions containing small crystallites, energetic and kinetic factors related to nucleation can be explored. The small crystallites are of specified structure, size, and shape. By observing different crystallites grow and dissolve, new insights of stearic acid nucleation were gained.

Models of solvated molecular crystals, including stearic acid, contain many more parameters to explore in determining nucleation properties than idealized models such as the hard sphere system. For example in the stearic acid crystal (a monoclinic crystal), three parameters are needed to fully specify the minimum size of growth rather than one for model spherical particles (a FCC crystal). The minimum size of growth was found to vary with shape of the crystallite. The bilayer structure was also found to affect both the minimum size of growth and how the crystallite dissolves. Crystallites were seen to dissolve by a series of steps involving primarily the bilayer structure. Growth of the crystallites was also observed. The ability of this simulation method to observe the affect of structure, shape, and size indicate the continued usefulness of molecular dynamics simulations with pre-constructed crystallites in studying nucleation and polymorph development.

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