Solutions to crystal-growth mysteries.

Solutions to crystal-growth mysteries

Crystallizing chemical compounds serves as a simple rite of passage for chemistry students. But even professional chemists admit that controlling how molecules pack together and dictating the shape of the crystals that emerge still falls largely under the category of black magic.

If scientists could demystify the physics behind that magic, they might discover ways of making drug-laced crystals that disintegrate at specific rates in the body, for instance, or crystals that process photons the way silicon-based chips process electrons, creating a new generation of communication and computing technologies, notes chemist Bruce M. Foxman of Brandeis University in Waltham, Mass.

Electrostatic, geometric and other physical interactions between a liquid solvent and the surface of a growing crystal play crucial, though poorly understood, roles in determining whether that crystal will assemble into a flat flake, a thin needle or a bulkier, more complex form. In 1985, Leslie Leiserowitz, Meir Lahav and Lia Addadi and others at the Weizmann Institute of Science in Rehovot, Israel, found that "tailor-made" chemical additives could bind to specific faces of a growing crystal, thereby influencing its shape.

Now, they and their co-workers report that the same principles hold for additive-free solvents. In the Aug. 15 JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, the team describes how specifically formulated solvents control crystal growth and disintegration.

When grown in water, crystals of asparagine (an amino acid) monohydrate can have as many as 18 faces. When the researchers replace some of the water with methanol or ethanol, the surface areas of some faces increase at the expense of others, resulting in a flatter, less complicated shape. Crystals of rhamnose (a sugar) monohydrate undergo comparable changes. The researchers attribute this behavior to the relative ease with which alcohol molecules bind to specific sites normally occupied by water molecules. This binding, they say, inhibits growth of these faces but not of others.

But when solvent molecules bind to specific surface sites on some crystals, they can enhance growth by a dynamic "relay" mechanism, the Israeli team suggests. To test this solvent effect, they examined crystals of the amino acids alanine or glycine, which form with positively charged amino groups and negatively charged carboxylate groups exposed at different ends.

Changing the solvent's alcohol/water ratio alters the relative rates of growth or dissolution at these ends, they found. Enhanced growth of the carboxylate ends of alanine crystals, for example, arises from a cyclic process in which new alanine molecules dock into surface pockets that are not blocked by overlying water molecules. Once in place, the newcomer alanine molecules temporarily get capped by water molecules, while repulsive forces drive away adjacent water molecules, opening new sites to the next wave of alanine molecules. "The repetitive succession of these steps results in a 'relay'-type mechanism of crystal growth," the researchers conclude.