Using Two-Dimensional Colloidal Crystals To Understand Crystallography

X-ray crystallography is an essential technique for modern chemistry and biochemistry, but it is infrequently encountered by undergraduate students owing to lack of access to equipment, the time-scale for generating diffraction-quality molecular crystals, and the level of mathematics involved in analyzing the resulting diffraction patterns. Herein, we describe a simple experiment that avoids all three of these limitations by using micrometer-sized latex spheres to form two-dimensional colloidal crystals. Starting with a solution of latex spheres suspended in water, students are able, in only a few minutes, to grow crystals that diffract visible light. Diffraction patterns formed by passing a laser beam through these crystals reveal their symmetry and, by using trigonometry, allow the determination of the size of the particles that make up the crystal. This experiment allows students to learn the basic principles of crystallography in a hands-on fashion within one or two laboratory periods.