Microscopic Studies of Calcium Sulfate Crystallization and Transformation at Aqueous–Organic Interfaces

The calcium sulfate crystal system is of considerable fundamental and practical interest, consisting of the three hydrates, gypsum (CaSO₄·2H₂O), bassanite (CaSO₄·0.5H₂O), and anhydrite (CaSO₄). Each has significant applications; however, synthesis of anhydrite via conventional aqueous methods requires elevated temperatures and therefore high energy costs. Herein, we investigate calcium sulfate crystal growth across a nonmiscible aqueous–organic (hexane or dodecane) interface. This process is visualized via in situ optical microscopy, which produces high magnification videos of the crystal growth process. The use of interferometry, Raman spectroscopy, and X-ray diffraction allows the full range of calcium sulfate morphologies and hydrates to be analyzed subsequently in considerable detail. In the case of dodecane, gypsum is the final product, but the use of hexane in an open (evaporating) system results in anhydrite crystals, via gypsum, at room temperature. A dissolution–precipitation mechanism between neighboring microcrystals is responsible for this transformation. This work opens up a simple new crystal synthesis route for controlling and directing crystallization and transformation.