How crystals nucleate from dilute vapours and solutions


Although seemingly simple, the nucleation of crystals from the fluid phase is a complex phenomenon. In spite of numerous computational and experimental studies, its mechanism is not completely understood. Recent studies have shown that, contrary to the century-old classical nucleation theory, crystal nucleation follows a two-step mechanism where the molecules initially come together to form an aggregate, followed by crystal nucleation within the aggregate. In chemical process industries, an understanding of the nucleation mechanism is important for design and control of crystallisation processes. For example, in the pharmaceutical industry, the effectiveness of the drug that one ingests depends on the structure of the crystal (also called polymorph) containing the active pharmaceutical ingredient. Hence, polymorph control during crystallisation is of vital technological and commercial importance.

In such a scenario, Ravi Kumar Reddy Addula and Sudeep N Punnathanam from the Department of Chemical Engineering have developed a new theory to explain crystal nucleation from dilute phases, called the ‘molecular theory of nucleation’. A dilute phase is defined as one where the concentration of the component that forms the nucleus is low, such as vapours and dilute solutions. The molecular theory, with its basis in statistical mechanics, is able to provide the most complete description till date of the nucleation process from dilute phases. Going forward, the molecular theory is expected to provide important insights into the mechanism of crystal nucleation from solutions. This should enable scientists and engineers to make improvements in the process design and control of the crystallisation process.


Ravi Kumar Reddy Addula and Sudeep N Punnathanam, Molecular Theory of Nucleation from Dilute Phases: Formulation and Application to Lennard-Jones Vapor, Physical Review Letters (2021).