Investigation of Oiling‐Out Phenomenon of Small Organic Molecules in Crystallization Processes: A Review

Chemically Specific Coherent Raman Imaging of Liquid-Liquid Phase Separation and Its Sequelae

Liquid-liquid phase separation (LLPS) plays a pivotal role in processes ranging from cellular structure reorganization to the formation of crystalline structures in materials science. In the pharmaceutical field, it has been demonstrated to impact drug crystallization and delivery. To date, characterization of LLPS has been limited to nonspatially resolved or nonchemically resolved analyses. In this study, we employed chemically specific stimulated Raman scattering (SRS), combined with second harmonic generation (SHG), for the first time to image crystallization in the presence of LLPS. Using the model compound ibuprofen, we examined the interplay between LLPS and crystallization, and explored the influence of both dissolution medium and enantiomeric form on this behavior. In doing so, we also discovered and partially characterized a new polymorph of (S)-ibuprofen. Our results demonstrate the potential of correlative SRS and SHG for monitoring phase separation and crystallization in real time, giving mechanistic insights into the spatial distribution, chemical composition and structure of phase-separated domains and newly formed crystals. Such insights could benefit not only pharmaceutical development, but also the biomedical, food and chemical sectors.

Oiling-Out: Insights from Gibbsian Surface Thermodynamics by Stability Analysis

The crystallization process is a significant stage in the pharmaceutical industry. During the process of crystallization with cooling, it is possible for a secondary liquid phase to appear before the formation of crystals. This phenomenon is called "oiling out" or liquid-liquid phase separation (LLPS). In this article, we explore the oiling-out phenomenon in a binary system of water and vanillin using stability analysis based on Gibbsian surface thermodynamics. To obtain the full picture of oiling out, we investigated three cases: droplet-solute-lean liquid equilibrium (DLE), crystal-solute-rich liquid equilibrium (CL'E), and crystal-solute-lean liquid equilibrium (CLE). The phase diagram of the system is plotted using the NRTL model for activity coefficients, along with considering the effect of the interfacial curvature on the phase diagram. From the phase boundaries and free-energy diagram of each case, we showed that the occurrence of the oiling-out phenomenon is justified based on the lower energy barrier of the droplet formation compared to that of the crystal formation. However, the energy level of a stable crystal is significantly lower and hence more stable than that of a stable droplet. Finally, we have determined different regions for droplet and crystal formation in the metastable phase diagram based on their supersaturation and provide insight for the oiling-out phenomenon.

Mesoscale Clusters in the Crystallisation of Organic Molecules

The early stages of the molecular self-assembly pathway leading to crystal nucleation have a significant influence on the properties and purity of organic materials. This mini review collates the work on organic mesoscale clusters and discusses their importance in nucleation processes, with a particular focus on their critical properties and susceptibility to sample treatment parameters. This is accomplished by a review of detection methods, including dynamic light scattering, nanoparticle tracking analysis, small angle X-ray scattering, and transmission electron microscopy. Considering the challenges associated with crystallisation of flexible and large-molecule active pharmaceutical ingredients, the dynamic nature of mesoscale clusters has the potential to expand the discovery of novel crystal forms. By collating literature on mesoscale clusters for organic molecules, a more comprehensive understanding of their role in nucleation will evolve and can guide further research efforts.

Nonclassical Crystallization and Core-Shell Structure Formation of Ibuprofen from Binary Solvent Solutions

Liquid-liquidphase separation (LLPS) or dense liquid intermediates during the crystallization of pharmaceutical molecules is common; however, their role in alternative nucleation mechanisms is less understood. Herein, we report the formation of a dense liquid intermediate followed by a core-shell structure of ibuprofen crystals via nonclassical crystallization. The Raman and SAXS results of the dense phase uncover the molecular structural ordering and its role in nucleation. In addition to the dimer formation of ibuprofen, which is commonly observed in the solution phase, methyl group vibrations in the Raman spectra show intermolecular interactions similar to those in the solid phase. The SAXS data validate the cluster size differences in the supersaturated solution and dense phase. The focused-ion beam cut image shows the attachment of nanoparticles, and we proposed a possible mechanism for the transformation from the dense phase into a core-shell structure. The unstable phase or polycrystalline core and its subsequent dissolution from inside to outside or recrystallization by reversed crystal growth produces the core-shell structure. The LLPS intermediate followed by the core-shell structure and its dissolution enhancement unfold a new perspective of ibuprofen crystallization.

In-line measurement of liquid-liquid phase separation boundaries using a turbidity-sensor-integrated continuous-flow microfluidic device

Liquid-liquid phase separation (LLPS), also known as oiling-out, is the appearance of the second liquid phase preceding the crystallization. LLPS is an undesirable phenomenon that can occur during the crystallization of active pharmaceutical ingredients (APIs), proteins, and polymers. It is typically avoided during crystallization due to its detrimental impacts on crystalline products due to lowered crystallization rate, the inclusion of impurities, and alteration in particle morphology and size distribution. In situ monitoring of phase separation enables investigating LLPS and identifying the phase separation boundaries. Various process analytical technologies (PATs) have been implemented to determine the LLPS boundaries prior to crystallization to prevent oiling out of compounds. The LLPS measurements using PATs can be time-consuming, expensive, and challenging. Here, we have implemented a fully integrated continuous-flow microfluidic device with a turbidity sensor to quickly and accurately evaluate the LLPS boundaries for a β-alanine, water, and IPA mixture. The turbidity-sensor-integrated continuous-flow microfluidic device is also placed under an optical microscope to visually track and record the appearance and disappearance of oil droplets. Streams of an aqueous solution of β-alanine, pure solvent (water), and pure antisolvent (IPA or ethanol) are pumped into the continuous-flow microfluidic device at various flow rates to obtain the compositions at which the solution becomes turbid. The onset of turbidity is measured using a custom-designed, in-line turbidity sensor. The LLPS boundaries can be estimated using the turbidity-sensor-integrated microfluidic device in less than 30 min, which will significantly improve and enhance the workflow of the pharmaceutical drug (or crystalline material) development process.

Modeling Diffusive Mixing in Antisolvent Crystallization

Diffusion controls local concentration profiles at interfaces between segregated fluid elements during mixing processes. This is important for antisolvent crystallization, where it is intuitively argued that local concentration profiles at interfaces between solution and antisolvent fluid elements can result in significant supersaturation overshoots over and above that at the final mixture composition, leading to poorly controlled nucleation. Previous work on modeling diffusive mixing in antisolvent crystallization has relied on Fickian diffusion, where concentration gradients are the driving force for diffusion. This predicts large overshoots in the supersaturation at interfaces between solution and antisolvent, as is often intuitively expected. However, chemical potential gradients provide a more physically realistic driving force for diffusion, and in highly nonideal solutions, such as those in antisolvent crystallization, this leads to nonintuitive behavior. In particular, as solute diffusion toward antisolvent is severely hindered, it can diffuse against its concentration gradient away from antisolvent. We apply thermodynamically consistent diffusion model based on the multicomponent Maxwell-Stefan formulation to examine diffusive mixing in a nonideal antisolvent crystallization system. Large supersaturation overshoots above that at the final mixture composition are not found when a thermodynamically consistent approach is used, demonstrating that these overshoots are modeling artifacts and are not expected to be present in physical systems. In addition, for certain conditions, localized liquid-liquid spinodal demixing is predicted to occur during the diffusive mixing process, even when the final mixture composition is outside the liquid-liquid phase separation region. Intermittent spinodal demixing driven by diffusive mixing may provide a novel explanation for differences of nucleation behaviors among various antisolvents.

The crystallization of decanoic acid/dopamine supramolecular self-assemblies in the presence of coacervates

HYPOTHESIS

Supramolecular self-assemblies involving non-covalent interactions play important roles in material science as well as living systems as they result in unique properties and/or functions. However, understanding of their self-assembly mechanism and crystallization has remained rudimentary.

EXPERIMENT

Here, we focus on biomolecular fatty acid and dopamine, which commonly exist in biological systems and closely related to neurodegenerative diseases, and investigate their self-assembly pathway by optical and fluorescence microscopy, DLS, SAXS, TEM, 2D-NMR, etc. FINDINGS: It is found that they could form the crystalline plates in solution or via a metastable liquid - liquid phase separation (LLPS). The nucleation and growth of crystalline plates observed occurs in solution or the dilute phase of LLPS, and not within the concentrated coacervate phase. This is because in coacervate, dopamine intercalates into fatty acid through hydrophobic and electrostatic interaction, which hinders the rearrangement of molecules and nucleation process, whereas in solution or dilute phase, they have the mobility to arrange into ordered structures to maximize electrostatic, hydrogen bonding and π-π interactions, leading to nucleation and crystallization. Moreover, the transitions between the coacervates and crystalline phase can be realized by adjusting the temperature. Our results shed light on the multistep nucleation in the presence of LLPS, as well as molecular mechanisms involved, thus further extending the nucleation-growth mechanisms.

pH-Dependent-Oiling-out During the Polymorphism Transformation of Disodium Guanosine 5′-Monophosphate

Oiling-out occurs frequently in industrial crystallization, and strongly influences the morphology and quality of crystals. In this study, the influence of oiling-out with higher pH during the polymorph transformation of...

Effect of Process Conditions on Particle Size and Shape in Continuous Antisolvent Crystallisation of Lovastatin

Lovastatin crystals often exhibit an undesirable needle-like morphology. Several studies have shown how a needle-like morphology can be modified in antisolvent crystallisation with the use of additives, but there is much less experimental work demonstrating crystal shape modification without the use of additives. In this study, a series of unseeded continuous antisolvent crystallisation experiments were conducted with the process conditions of supersaturation, total flow rate, and ultrasound level being varied to determine their effects on crystal size and shape. This experimental work involved identifying acetone/water as the most suitable solvent/antisolvent system, assessing lovastatin nucleation behaviour by means of induction time measurements, and then designing and implementing the continuous antisolvent crystallisation experiments. It was found that in order to produce the smallest and least needle-like particles, the maximum total flow rate and supersaturation had to be combined with the application of ultrasound. These results should aid development of pharmaceutical manufacturing processes where the ability to control particle size and shape would allow for optimisation of crystal isolation and more efficient downstream processing.