Kinetically Driven Crystallization of a Pure Polymorphic Phase of Stearic Acid from CO₂-Expanded Solutions

Abstract

The polymorphic nature of microcrystalline solids of stearic acid prepared by the Depressurization of an Expanded Liquid Organic Solution (DELOS) and Gas Antisolvent (GAS) crystallization techniques, both based on CO2-expanded solvents as solvent media, has been studied. The crystalline solids produced by those techniques from “CO2-expanded ethyl acetate” with different contents of CO2 have been analyzed in detail and compared with those prepared by conventional cooling crystallizations, either from pure liquid ethyl acetate solutions or from the melt. It is shown that the monoclinic E form of stearic acid is more favored when using kinetically controlled crystallizations, like the DELOS one, in which a high supersaturation level is rapidly achieved. Remarkably, the DELOS process provides for the first time a pure polymorphic monoclinic E form, without the presence of traces from other polymorphs that always are present when using other kinetically driven methods, such as the conventional fast cooling. On the contrary, the C polymorph is preferentially obtained by the thermodynamically controlled GAS technique in which the increase of the solution supersaturation is slower and/or low supersaturation levels are attained.