| Summary: | The shape of a crystalline organic solid has a major impact on its downstream processing and on its end-product quality, issues that are becoming increasingly important in the specialty chemical industry. In particular, shape affects the key washing and filtering steps in solution crystallization, and it determines solids agglomeration and dissolution characteristics. Traditional models for predicting crystal shape are based exclusively on the internal crystal structure. They are able to predict sublimation-grown crystals, but are not able to account for the effects of the environment (i.e., solvent and impurities) that dominate solution-grown shapes. Detailed kinetic theories of crystal growth—the spiral, two-dimensional nucleation, and rough growth mechanisms—have also been developed, but have not been widely employed. They require as input certain face-specific and solvent-specific properties that are generally unknown. This work explores all of the traditional methods and detailed kinetic theories in an attempt to develop a new method for predicting crystal shape. The detailed kinetic theories are simplified, yielding relative face growth rate expressions that depend primarily on kink and edge free energies—two microscopic properties of crystal faces. A method for estimating these properties is also proposed. It depends on the solvent's surface free energy, which is often known, and the crystal's internal energy, which can be determined from simple molecular mechanics calculations. The method is used to predict the shape of adipic acid grown from water, ibuprofen grown from polar and non-polar solvents, and biphenyl grown from toluene. An extension to this technique is developed in order to estimate hydrogen bonding between solvents and crystals: it is applied to succinic acid grown from water and isopropanol, and to paracetamol grown from water and acetone. This approach appears to be the first practical technique that can successfully predict solution-grown organic crystal shapes.
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