(238g) Heterogeneous Nucleation of Urea from Aqueous Solution: A Combined Experimental and Simulation Approach

Crystallisation is essential in many applications across the food, chemical and pharmaceutical industries. Current understanding of nucleation mechanisms is limited making control of 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 showed that contact with PTFE and tridecane significantly increase the nucleation rate of glycine from solution (1,2). This was attributed to an interfacial concentration enhancement, caused by dispersion interactions, which has observed using fully atomistic molecular dynamics simulations (MD) (2,3).

Following these studies, we aim to develop a new approach to control nucleation rate and polymorphic outcomes, based on tunable surface interactions. Crystal nucleation at solid interfaces will be investigated using both MD simulations and small-scale, high-throughput experiments. MD simulations are limited to short time and length scales by computational power, making direct simulation of nucleation very unlikely. Therefore choices about system setup and aims are important for obtaining meaningful and useful results. We have selected urea-water as a model system, based on its small molecular size and fast growth mechanisms making it well suited to both MD simulations and laboratory experiments.

Simulations of urea aqueous solutions at interfaces including PTFE and graphite will be presented, to simplify the simulations only the dispersion interactions of the material will be modelled. Previous work identified the most suitable urea force field considering both crystal and solution properties (4). Lennard-Jones (LJ) interactions are commonly used to model atom-atom dispersion interactions in MD systems. Walls with at LJ potential will be used for the interfaces, since they have successfully been used to simulate interfacial concentration enhancements (3).

Preliminary results in Figure 1 show that LJ interactions also induce a concentration enhancement of aqueous urea solutions near the interface. This indicates that dispersion interactions cause these interfacial effects and that this is not specific to glycine. The simulations will be used to make predictions of how various interfaces affect nucleation, which will be validated against the experimental results.

Experimental results of heterogeneous induction times with control interfaces (glass vial surface and air), PTFE surfaces (coated magnetic stirrer bars) and diamond surfaces will be presented. As shown in Figure 2, PTFE significantly increases the nucleation rate in aqueous urea solutions whereas diamond does not have an affect. We will also present a vial-by-vial analysis of the variability/repeatability of nucleation experiments to help adress the discussion about the influence of impurities on heterogeneous nucleation.

Future simulation work will investigate heterogeneous nucleation by probing the effect of various interfaces, including those studied experimentally, on crystal ‘nuclei’ seeds at the interface. 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. Vesga MJ, McKechnie D, Mulheran PA, Sefcik J, Johnston K. Conundrum of γ glycine nucleation revisited: to stir or not to stir? CrystEngComm, 2019, 21, 2234
2. McKechnie D, Anker S, Zahid S, Mulheran PA, Sefcik J, Johnston K. Interfacial concentration effect facilitates heterogeneous nucleation from solution. J Phys Chem Lett, 2020, 11, 2263
3. McKechnie D, Mulheran PA, Sefcik J, Johnston K. Tuning interfacial concentration enhancement through dispersion interactions to facilitate heterogeneous nucleation. J Phys Chem C, 2022, 38, 16387
4. Anker S, McKechnie D, Mulheran PA, Sefcik J, Johnston K. Reproducibility of crystal and solution properties for various GAFF and OPLS force fields for urea. (in preparation)