Crystallization by single diffusion through agar gel.

Abstract

Crystallization is an important process in nature, various biological systems, and in industrial processes ranging from pharmaceuticals to material synthesis. In the industrial crystallization processes, common methods of crystallization include evaporative and cooling crystallization, and less frequently varying pH and addition of anti-solvents. The crucial driving force in all these techniques involves bringing the concentration of solutions to supersaturation to trigger the initial nucleation and growth of crystals. These conventional modes may not be applicable in systems where localized nucleation is required as these methods cause supersaturation throughout the bulk of solution which makes it challenging to affect crystallization at a specific desired location. In addition, these modes of reaching supersaturation cannot be used to explain or influence the crystallization in certain scenarios e.g., biological systems and in cases where the scope of temperature and a systemic pH change is very minimal. Another existing mode of crystallization involves the separation of ions into two different chambers and double diffusion through gels which is also not widely practical. Here we propose a mechanism in which we place salts in different solubility conditions on two sides of a permeable gel medium held at a steady state. Our steady state diffusion calculations predict that in certain pairings of different solubility conditions, the concentration of ions in the gel will always be above saturation concentration despite the absolute concentration decreasing along the length of the medium. We perform experiments in systems with variable solubilities in two different scenarios (pH and solvent mixtures) and we observe that the crystallization takes place in the gel indicating the supersaturation of the ions. This method offers an alternative way to get supersaturated solutions without evaporating solvents, changing temperatures, or separating the constituent ions into two different chambers across the diffusive medium.