(496a) Crystal Product Engineering Through Dissolution

Crystal shape is an important product quality with direct implications for the pharmaceutical, food product and dye industries. It affects both down stream processes such as filtering, washing and drying as well as material properties such as mechanical strength, bioavailability and surface functionality. Needle-like crystals, for instance, present difficulties in washing and drying, but can be favored for systems where a large surface area to volume ratio is needed. Also, particular crystal surfaces can either be specifically desired or undesired due to the surface characteristics. For example, for a crystal with a single face of particularly high catalytic activity, the area of that face should be maximized; however, for a crystal such as adipic acid where one crystal face's hydrophobicity causes agglomeration during storage and transportation of the product, the area of that face should be minimized.

Most industrial crystallizations take place in solution; therefore, many existing efforts in engineering enhanced crystal shapes rely directly upon changing the chemical nature of the crystallization. Different solvents lead to different crystal shapes; mixed solvents can lead to even more flexibility, and additives, both tailor made and surfactants, have been used to chemically alter the shapes that crystals can obtain. Each of these methods relies on the growth of the crystal to manipulate the crystal shape. Dissolution, however, can be a viable option to enhance the crystal product. While some processes, such as recipes for catalyst manufacturing, already use dissolution to enhance the product properties, a systematic method to predict the crystal shape as a result of dissolution would enhance the ability of the crystal engineer to develop processes that exhibit the desired crystal product properties.

In this talk, we will highlight our recently developed model (Snyder and Doherty, 2007) that incorporates the prediction of the relative dissolution rates of the crystal faces for organic crystals with a faceted crystal shape evolution model. The model provides for a comprehensive, a priori, prediction for the shape of a crystal as it dissolves. Additionally, we will demonstrate the validity of the model's predictions, by comparing the predictions to video microscopy results for the evolution of crystals dissolving in a Peltier viewcell. The shape evolution model combines a face selection methodology with algebraic and differential equations describing the dynamics. For a growing crystal, selecting the important planes for consideration in a shape evolution is a straightforward process, since the few, slow growing planes are those that are going to be important to the shape. However, for a dissolving crystal, the fast dissolving planes are most important, and an infinite set of these planes exist on the crystal. Thus, a methodology that incorporates the internal crystal interactions has been developed to predict the appropriate set of planes to consider for the dissolution evolution. These planes are then entered into the model which incorporates a set of differential equations (one for each face) coupled to an algebraic condition describing whether a plane will appear or disappear. The results of the model predictions show excellent agreement with the experimental measurements. The model also demonstrates that crystals will evolve towards their stable steady state shape in growth; however, crystals will dissolve away from the unstable steady state shape in dissolution. Finally, we will discuss the practical applications of these predictions for applications in both process cycling and polymorphic phase transformations.

Snyder RC, Doherty MF. Crystal shape evolution during dissolution and growth. AICHE J. 2007;53:1337-1348.