Heterogeneous nucleation of urea from aqueous solutions: a combined experimental and simulation approach

Crystallisation is an essential separation process with many applications across the food, chemical and pharmaceutical industries. Current understanding of crystal nucleation mechanisms is limited, which makes controlling nucleation rate and polymorphism a major challenge for developing better crystallisation processes. Primary nucleation is likely to be heterogeneous, and occur at solution-container or solution-impurity interfaces. Previous work on glycine aqueous solutions found that PTFE and tridecane interfaces significantly increased nucleation rates^1,2, which was attributed to an interfacial concentration enhancement, which was observed in molecular dynamics (MD) simulations^1,3.

We present a study of crystal nucleation of urea from aqueous solution at three solid interfaces (glass, PTFE and diamond) using both small-scale, high-throughput experiments and MD simulations⁴. PTFE was found to significantly increase the urea nucleation rate, with diamond having only a moderate effect compared to the control (glass vials). MD simulations showed that the interfacial concentration enhancement was strongest for PTFE and weakest for glass, indicating that this interfacial concentration enhancement effect plays a key role in heterogeneous nucleation kinetics. Increased understanding of how surfaces influence nucleation will enable design of nucleants to enhance desired crystal nucleation or to design process equipment to prevent fouling.

1. McKechnie D, Anker S, Zahid S, Mulheran PA, Sefcik J, Johnston K, J Phys Chem Lett 2020, 11, 2263

2. Vesga MJ, McKechnie D, Mulheran PA, Sefcik J, Johnston K, Cryst Eng Comm, 2019, 21, 2234

3. McKechnie D, Mulheran PA, Sefcik J, Johnston K, J Phys Chem C, 2022, 38, 16387

4. Anker S, McKechnie D, Mulheran PA, Sefcik J, Johnston K, under review