Effects of solvent,supersaturation ratio and silica template on morphology and polymorph evolution of vanillin during swift cooling crystallization

Swift cooling crystallization of vanillin was investigated in water, ethanol, isopropanol and ethyl acetate. Morphology and polymorph evolution of vanillin were discussed in terms of solvent, supersaturation ratio and silica template. PXRD, DSC, FTIR and microscope were used to identify polymorphs of vanillin. Results showed that the nucleated polymorphs of vanillin depended largely on the solvent, supersaturation ratio and silica template. Low supersaturation ratios favor the nucleation of stable form I in water, and high supersaturation ratio exceeds 7 generating 100% metastable form II. However, if the supersaturation ratio is too high (S>8), liquid–liquid phase separation will occur, and no crystals could be obtained. In other solvents such as ethanol, isopropanol and ethyl acetate, only form I was obtained. However, it should be noted that the morphology of form I prepared in ethanol, isopropanol and ethyl acetate is distinct from that obtained in water, the former is flake-like and the latter is rod-like. The nucleation of vanillin from different solution was also studied with the presence of SiO₂, SiO₂–NH₂ and SiO₂−COOH templates, which did not change the nucleated polymorph of vanillin, but changed the nucleation and growth rate of stable form I.