Controlling urea crystallisation via heterogeneous nucleation

Nucleation is likely to be heterogeneous and occur at solution-container or solution-impurity interfaces. Previous work has shown that tridecane and PTFE interfaces significantly increase the nucleation rate of aqueous glycine solutions (1,2). This has been attributed to an interfacial concentration enhancement, observed using fully atomistic simulations with molecular dynamics (MD) (1). This study aims to develop a new approach to control nucleation rate and polymorphic outcomes, based on tunable surface interactions. Crystal nucleation at solid or liquid interfaces will be investigated using MD simulations and small-scale, high-throughput experiments, with urea-water as a model system. Preliminary, force field testing for urea aqueous solutions are presented, and GAFF and SPC/E are selected for urea and water, respectively. The selected force fields are used to simulate urea aqueous solutions at Lennard-Jones (LJ) walls, which represent dispersion interactions between the solution and the interface (3). The LJ wall is shown to induce a concentration enhancement of urea near the interface, showing that dispersion interactions cause these interfacial effects. Future simulation work will study the behaviour and effects of heterogeneous nucleation by placing seed crystal ‘nuclei’ at the interface. Preliminary results will be presented of heterogeneous nucleation rates in small-scale, high-throughput experiments. The initial interfaces investigated are PTFE (coated stirrer bars) and diamond, with a control (glass vial and air). Concentrated stock solutions are prepared and pipetted into glass vials at a high temperature (60°C). The samples are cooled to room temperature (25°C) and monitored for 3 days with webcams to determine nucleation times. Our studies will lead to an increased understanding of how different interfaces impact nucleation, which will enable design of nucleants to enhance heterogeneous nucleation or design of process equipment to prevent fouling. References 1. McKechnie D, Anker S, Zahid S, Mulheran PA, Sefcik J, Johnston K, J Phys Chem Lett, 2020, 11, 2263, 10.1021/acs.jpclett.0c00540 2. Vesga MJ, McKechnie D, Mulheran PA, Sefcik J, Johnston K, CrystEngComm, 2019, 21, 2234, 10.1039/C8CE01829D 3. McKechnie D, Mulheran PA, Sefcik J, Johnston K, (in preparation