Sonocrystallization: Monitoring and controlling crystallization using ultrasound

Crystallization is an extensively used unit operation for production of solid forms of inorganic and organic compounds, pharmaceuticals, cosmetics, and polymers etc. However, the stochastic nature of crystallization makes it difficult to control the product characteristics such as crystal size, size distribution, morphology, polymorphism, and chirality. Use of ultrasound has been shown to greatly influence crystallization and help in gaining control over product characteristics. Ultrasonic waves generate cavitation in the solution resulting in enhanced micromixing, increased mass transfer rates and uniform supersaturation which then lowers induction time, reduces metastable zone width, increases nucleation rates and narrows down particle size and size distribution. This review attempts to present a coherent understanding of the mechanism of sonocrystallization. Further, a newly emerging field of ultrasonic spectroscopy, which is relatively unexplored but holds a tremendous potential for online monitoring of crystallization, has also been discussed.