Engineering Oriented Gases: The Mechanism of Dyeing Potassium Sulfate

Potassium sulfate was crystallized from solutions containing mM quantities of a variety of sulfonated dye molecules that became encapsulated within particular growth sectors. Linear dichroism measurements of the crystals coupled with a knowledge of the absorption anisotropy of the dyes obtained from semi-empirical molecular orbital theory, enabled the determination of the orientation of at least one dye molecule within each of the principal growth sectors ({010}, {021}, {110}, {001}, {111}) of K2SO4. The ensemble of orientations suggested a general mechanism of mixed crystal formation in which the dyeʹs sulfonate groups matched the separation between sulfate ions in the lattice. The corresponding experiments with K2SO4 isomorphs as well as the habit modification accompanying dye inclusion crystal (DIC) formation are discussed. We further demonstrate how sector specific recognition enables growing crystals to separate molecules from complex solutions.