Marinova, Veselina Ivaylova; (2020) Molecular-Level Characterisation of Crystal-Solution Interfaces. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The shape of solution-grown crystal particles is largely dependent on the relative growth rate of the morphologically dominant crystal faces, which is known to be affected by the solvent. Developing accurate models for predicting crystal morphologies requires a molecular-level understanding of the solid-liquid interface. Using a combination of molecular dynamics simulations and enhanced sampling methods, this work carries out a comprehensive study on the dynamics and thermodynamics of crystal-solution interfaces for the case of ibuprofen, focusing on aspects often neglected in mesoscopic models for crystal growth. An investigation on the conformational isomerism of ibuprofen shows that conformational rearrangements at the crystal-solution interface are governed by specific surface-solvent interactions and can have a non-negligible impact on the surface growth/dissolution kinetics. An unsupervised clustering algorithm is proposed to extend the study of conformational isomerism for systems with a large number of conformationally relevant degrees of freedom. By assessing thermodynamic and kinetic information on the solvent in contact with crystal surfaces, surface-solvent interactions are found to be solvent- and face-specific. Following this analysis, a computational screening procedure is proposed for identifying solvents which can significantly affect the relative growth rate of the crystal facets and hence, the growth morphology of the crystal. To gain an in-depth understanding into the role of the solvent on the ease of association/dissociation of solute molecules at the crystal surface, a study on the formation of a vacancy on the morphologically dominant crystal faces of ibuprofen is carried out. Thermodynamics of the process reveal a distinct solvent-dependency for several faces, indicating in such cases desolvation-dominated defect formation. The research subject of this dissertation contributes to developing general and computationally-affordable workflows necessary to obtain a comprehensive and quantitative understanding of molecular processes, impacting the solid-liquid interface, which will contribute towards the formulation of detailed mesoscopic growth and dissolution models.

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