2025 AIChE Annual Meeting

(231c) Unraveling Strain-Driven Surface Morphology Transformations in Crystallization: A Kinetic Monte Carlo Analysis of Dual-Mechanism Growth Modifiers

Authors

Jeffrey Rimer, University of Houston
Joseph Kwon, Texas A&M University
Crystal growth modifiers (CGMs) have long been investigated for their ability to tune crystallization processes, particularly in solutions where precise control of crystal size, shape, and purity is paramount [1]. However, while conventional research often emphasizes the inhibitory aspects of CGMs [2] where molecular adsorption blocks step or kink sites, recent findings have revealed that modifiers such as hydroxycitrate (HCA) and citrate (CA) can exert far more complex influences on crystal surfaces. Specifically, these modifiers can induce localized strain, promote etch-pit formation [3], and ultimately reshape surface morphology, a phenomenon observed via atomic force microscopy (AFM) when the CGM coverage exceeded a critical threshold [4]. Below this threshold, the modifiers behave as classical site blockers by selectively occupying kink and step sites, thereby suppressing crystal growth. However, as CGM coverage increases, overlapping strain fields begin to lower the activation energy for desorption, triggering strain-mediated dissolution that manifests as triangular etch pits, negative step velocities, and other morphological features distinctly different from conventional layer-by-layer growth modes.

Motivated by these findings, we developed an advanced kinetic Monte Carlo (kMC) model [5] that captures this dual nature of CGMs from passive site blocking at low coverage to actively driving local dissolution once sufficient strain is induced in the crystal lattice. Using adaptive solid-on-solid (SOS) kMC framework, we incorporate thermodynamic parameters such as supersaturation and surface-specific binding energies alongside strain-dependent free energy contributions to simulate these varied growth behaviors in a unified manner. The model dynamically calculates site-specific attachment and detachment rates based on local CGM coverage and lattice distortion. When CGMs are adsorbed sparingly, growth inhibition ensues but a net growth rate is maintained. As modifier concentration crosses a critical threshold, the once-passive inhibitors trigger surface dissolution in supersaturated media by effectively destabilizing lattice sites via overlapping strain fields. The outcome is pitting, step recession, and in some cases, near-complete surface passivation at very high CGM coverage.

References

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[2] G. Zhu et al., “Crystal dissolution by particle detachment,” Nature Communications 2023 14:1, vol. 14, no. 1, pp. 1–11, Oct. 2023, doi: 10.1038/s41467-023-41443-y.

[3] A. C. Lasaga and A. Luttge, “Variation of Crystal Dissolution Rate Based on a Dissolution Stepwave Model,” Science (1979), vol. 291, no. 5512, pp. 2400–2404, Mar. 2001, doi: 10.1126/SCIENCE.1058173.

[4] J. Chung, I. Granja, M. G. Taylor, G. Mpourmpakis, J. R. Asplin, and J. D. Rimer, “Molecular modifiers reveal a mechanism of pathological crystal growth inhibition,” Nature 2016 536:7617, vol. 536, no. 7617, pp. 446–450, Aug. 2016, doi: 10.1038/nature19062.

[5] S. Nagpal, N. Sitapure, Z. Gagnon, and J. S. Kwon, “Advancing crystal growth prediction: An adaptive kMC model spanning multiple regimes,” Chem Eng Sci, vol. 299, p. 120472, 2024, doi: https://doi.org/10.1016/j.ces.2024.120472.