Solution rate of crystals at fluid-fluid interface.

A model system was developed in which the dissolution behavior of a single crystal of potassium ferricyanide was studied at a liquid paraffin-water interface. Since the equilibrium position of a crystal at the interface is independent of its size, the lifetime of a crystal dissolving at the interface is determined entirely by its initial size and its dissolution rate in the water phase. The dimensions of every crystal were measured microscopically before dissolution. A continuous-flow recording dissolution apparatus was used to measure spectrophotometrically the mass flow of dissolved potassium ferricyanide. The dissolution cell in this system was mounted in an optical bench, making it possible to follow dissolving crystals visually by projecting them on a screen. The results show that the lifetime of a crystal is proportional to the shortest length of the crystal face in contact with the liquid paraffin and is rather independent of its form. Furthermore, crystal shape changes during dissolution, which is explained partly by the nonisometric dissolution of potassium ferricyanide crystal faces and partly by the nonconstancy of film thickness.