Molecular Mechanism of Organic Crystal Nucleation: A Perspective of Solution Chemistry and Polymorphism

Crystal nucleation determining the formation and assembly pathway of first organic materials is the central science of various scientific disciplines such as chemical, geochemical, biological, and synthetic materials. However, our current understanding of the molecular mechanisms of nucleation remains limited. Over the past decades, the advancements of new experimental and computational techniques have renewed numerous interests in detailed molecular mechanisms of crystal nucleation, especially structure evolution and solution chemistry. These efforts bifurcate into two categories: (modified) classical nucleation theory (CNT) and non-classical nucleation mechanisms. In this review, we briefly introduce the two nucleation mechanisms and summarize current molecular understandings of crystal nucleation that are specifically applied in polymorphic crystallization systems of small organic molecules. Many important aspects of crystal nucleation including molecular association, solvation, aromatic interactions, and hierarchy in intermolecular interactions were examined and discussed for a series of organic molecular systems. The new understandings relating to molecular self-assembly in nucleating systems have suggested more complex multiple nucleation pathways that are associated with the formation and evolution of molecular aggregates in solution.

The heterogeneous nucleation of pimelic acid under the effect of a template: experimental research and molecular simulation

The nucleation experiments of pimelic acid were investigated in the absence and presence of a template.

Salts of 2-hydroxybenzylamine with improvements on solubility and stability: Virtual and experimental screening

2-Hydroxybenzylamine (2-HOBA) is a drug used to effectively treat oxidative stress. To improve its aqueous solubility and thermal stability, salt screening and synthesis was carried out. The conductor-like screening model for the real solvents model (COSMO-RS) was applied to virtual screening of coformers among 200 commonly used candidates for salification of 2-HOBA. As a result, 40 hit compounds were subjected to experimental liquid-assisted grinding (LAG) with 2-HOBA, then 21 systems were characterized as new solid phases by PXRD. Nine multicomponent single crystals of 2-HOBA with succinic acid, p-aminobenzoic acid, p-nitrobenzoic acid, o-nitrobenzoic acid, p-toluic acid, 2,3-dihydroxybenzoic acid, 3,4-dihydroxybenzoic acid, p-nitrophenol, and 5-hydroxyisophthalic acid were obtained and characterized by single-crystal X-ray diffraction, powder X-ray diffraction, differential scanning calorimetry, and thermogravimetric analysis. All of them were salts and exhibited higher decomposition temperatures compared with pure 2-HOBA. The apparent aqueous solubility of three new salts, i.e., those with succinic acid, p-aminobenzoic acid, and p-nitrophenol were higher than the equilibrium solubility of 2-HOBA. The accelerated stability test indicated that all salts show excellent stability under conditions (40 °C and 75% RH) for 4 weeks. Overall, this work introduced a protocol that combined the virtual screening tool based on the COSMO-RS model and the experimental LAG method to screen new salts for a target compound. The feasibility of this protocol was confirmed in the case of 2-HOBA whose new salts were successfully obtained and represented an improvement for aqueous solubility and thermal stability.

Molecular Dynamic Simulation of D-Mannitol Polymorphs in Solid State and in Solution Relating With Spontaneous Nucleation

In this article, the polymorphic nucleation of D-mannitol was studied in depth by molecular simulating the solid state of different forms and the aqueous solution both in unsaturated and supersaturated conditions. In our previous work, it was found that different polymorphs of D-mannitol could nucleate in various supersaturations. Here in this work, we try to use molecular dynamic simulation as a tool to explore the system to microscopically understand the phenomenon. First, the molecular structure of the 3 D-mannitol polymorphs in the solid unit cell is studied, and it is found that the molecular conformation directly affects the stability of the polymorphs. Then, the properties of D-mannitol molecules in unsaturated and supersaturated aqueous solutions are investigated. The results reveal that D-mannitol molecules are mostly monomers in unsaturated aqueous solution, while dimers start to show as the increase of concentration and also in the supersaturated aqueous solutions. Moreover, the form of the dimer varies with the degree of supersaturation, which might be the reason why the final crystal forms are different under different supersaturations. It seems that the I-dimers in supersaturated aqueous solutions is beneficial to the formation of the delta form of D-mannitol, and the T-dimers might be likely to form alpha crystal form, while the H-type dimers should be more favorable for to nucleate the beta form.

Revealing the role of a surfactant in the nucleation and crystal growth of thiamine nitrate: experiments and simulation studies

This work combines experiments and simulations to investigate the inhibition effects of a surfactant on the nucleation and crystal growth rate of thiamine nitrate.