A Structured Approach To Cope with Impurities during Industrial Crystallization Development

Studying the effects of polymers on therapeutic deep eutectic solvents' formation and stability: A thermal analysis-based approach to optimise polymer selection

Therapeutic deep eutectic solvents (THEDES) are the liquids produced upon mixing two solid materials, where at least one of them is an active pharmaceutical ingredient. The strong hydrogen bonding (HB) between the parent materials is the reason for such profound depression in their melting points. THEDES formation can improve drugs' solubility and permeation characteristics. However, this can be limited by their encapsulation within drug delivery platforms e.g., polymeric matrices which might disrupt the HB network of THEDES by introducing new HB active sites into the mixture. Despite this, the effects of polymers on THEDES stability are not well-documented. In addition, the polymers' impact on THEDES formation during end-to-end production has not been studied. In this work, these issues were addressed by employing thermal processing of polymers and drugs. The dually active lidocaine: flurbiprofen (1:1) THEDES was utilized in addition to model polymers, namely polyethylene-co-vinyl acetate (PEVA), polyethylene oxide (PEO) and Eudragit® RL PO (EuRLPO). Firstly, probing the interaction between the polymers and the individual components of THEDES revealed that PEVA has no affinity towards both drugs while PEO can interact with flurbiprofen, and EuRLPO can interact with lidocaine. Then, to study the effect of the polymers on THEDES formation and stability, a valid in-situ method was developed to quantify THEDES in its mixture using modulated temperature differential scanning calorimetry (MTDSC). MTDSC data showed that the three polymers retarded THEDES formation in the following order PEVA < EuRLPO < PEO. This retardation was attributed to increased medium viscosity and the subsequent reduction in THEDES formation rate. Increasing the heating time led to complete THEDES formation in the case of PEVA and EuRLPO but not with PEO. This result was explained based on the polymers' interaction with THEDES parent drugs and inputs from polymers' viscoelastic properties. On the other hand, introducing the polymer after THEDES formation mitigated their viscosity effect but their interaction with the parent drugs remained an issue, where PEO and EuRLPO were able to destabilize the pre-formed THEDES to variable extents. Long-term storage stability study further confirmed this conclusion. Therefore, this study will provide valuable information regarding the interaction (or lack of) of THEDES with model pharmaceutical polymers that have been thermally processed and will provide recommendations regarding the rational choice of polymers to maintain THEDES stability.

Developing a Vial-Scale Methodology for the Measurement of Nucleation Kinetics Using Evaporative Crystallization: A Case Study with Sodium Chloride

Understanding nucleation kinetics is vital for designing crystallization processes, yet traditional measurement methods based on cooling are unsuitable for compounds with temperature-independent solubility. This study introduces an experimental procedure to measure the nucleation kinetics based on evaporative crystallization and applies it to sodium chloride (NaCl) in water. By systematically varying the experimental conditions such as temperature and evaporation gas flow rate, we obtained a comprehensive data set for the nucleation of NaCl crystals that allowed estimating kinetic parameters using a rate expression derived from classical nucleation theory (CNT). This work demonstrates the robustness of evaporation as a method for measuring the nucleation kinetics that is applicable regardless of how the solubility of a compound depends on the temperature.

Sensitizing Explosives Through Molecular Doping

Cocrystallization assembles multicomponent crystals in defined ratios that are held together by intermolecular interactions. While cocrystals have seen extensive use in the pharmaceutical industry for solving issues with stability and solubility, extension to the field of energetic materials for improved properties has proven difficult. Predicting successful coformers remains a challenge for systems lacking well-understood synthons that promote reliable intermolecular assembly. Herein, an alternative method is investigated for altering energetic properties that operates in the absence of well-defined interactions by molecular doping. An impact sensitive primary explosive, cyanuric triazide (CTA), was selected as the dopant to test if less impact sensitive secondary explosives could gain increased sensitization to impact when CTA is inserted into their crystal lattices. Molecular doping was successful in sensitizing three melt-castable energetics: 2,4,6-trinitrotoluene (TNT), 2,4-dinitroanisole (DNAN), and 1,3,3-trinitroazetidine (TNAZ). CTA could also be incorporated as a stabilized inclusion to sensitize DNAN further. These results demonstrate how the judicious choice of dopant can lead to specific property improvements, providing a method for creating energetic materials with new properties to access metal-free primary explosives and physical hot spot models for explosive ignition.

Computational assessment of the potential of cross-catalytic coprecipitating systems for the bottom-up design of nanocomposites

The production of nanocomposites is often economically and environmentally costly. Silica-witherite biomorphs, known for producing a wealth of life-like shapes, are nanocomposites entirely formed through self-organization processes. Behind these precipitates are two precipitation reactions that catalyze each other. Using a simple computational approach, we show here that this type of chemical system - defined here as Cross-Catalytic Coprecipitating Systems (CCCSs) - is of great interest to material design. Provided that cross-catalytic effects are sufficient to overcome the precipitation thresholds for each phase, all CCCSs can be expected to self-organize into nanocomposite materials through a one-pot, one-step synthesis protocol. Symmetry-breaking events generating various complex, ordered textures are predicted in CCCSs involving crystalline phases. While high levels of stochasticity lead to a loss of ordering, coprecipitation is found to be robust to diffusion or advection in the solution. This model shows that a couple of chemical reactions can generate a range of complex textures - with possibly distinct physical/chemical properties. Cross-catalytic coprecipitating systems consequently represent a promising avenue for producing nanocomposites with complex textures at reduced economic and environmental costs.

Crystallization of nickel sulfate and its purification process: towards efficient production of nickel-rich cathode materials for lithium-ion batteries

NiSO4·6H2O is an important salt for the battery-making industry. The extraction of nickel sulfate relies on the hydrometallurgical processing of nickel ores as well as the recycling of nickel-containing products. The last step in hydrometallurgical processing is the crystallization of nickel sulfate. Because of the similar ionic radius and ionic charge between nickel and magnesium ions, magnesium undergoes isomorphous substitution and replaces nickel ions in the crystal lattice structure of NiSO4·6H2O. This poses a challenge as achieving the desired metal salt purity is difficult, resulting in an inferior cathode material for nickel-containing batteries. In this work, the removal of magnesium during the purification process of NiSO4·6H2O crystals via a repulping process was thoroughly investigated. Moreover, the impurity uptake mechanisms of magnesium into NiSO4·6H2O crystals were investigated. The results indicated that repulping NiSO4·6H2O crystals with a saturated NiSO4 solution results in 77% removal of magnesium. Using a second-stage repulping process is less effective with only 26% magnesium removal. The purification efficiency of the two repulping stages was quantified by the equilibrium distribution coefficient, which corroborates the trend of decreased removal of magnesium in the second stage of repulping compared with the first stage. The primary impurity uptake mechanisms of magnesium into NiSO4·6H2O crystals were identified to be surface adsorption and lattice substitution (isomorphous substitution).

Understanding the Degradation of Methylenediammonium and Its Role in Phase-Stabilizing Formamidinium Lead Triiodide

Formamidinium lead triiodide (FAPbI₃) is the leading candidate for single-junction metal-halide perovskite photovoltaics, despite the metastability of this phase. To enhance its ambient-phase stability and produce world-record photovoltaic efficiencies, methylenediammonium dichloride (MDACl₂) has been used as an additive in FAPbI₃. MDA²⁺ has been reported as incorporated into the perovskite lattice alongside Cl^-. However, the precise function and role of MDA²⁺ remain uncertain. Here, we grow FAPbI₃ single crystals from a solution containing MDACl₂ (FAPbI₃-M). We demonstrate that FAPbI₃-M crystals are stable against transformation to the photoinactive δ-phase for more than one year under ambient conditions. Critically, we reveal that MDA²⁺ is not the direct cause of the enhanced material stability. Instead, MDA²⁺ degrades rapidly to produce ammonium and methaniminium, which subsequently oligomerizes to yield hexamethylenetetramine (HMTA). FAPbI₃ crystals grown from a solution containing HMTA (FAPbI₃-H) replicate the enhanced α-phase stability of FAPbI₃-M. However, we further determine that HMTA is unstable in the perovskite precursor solution, where reaction with FA⁺ is possible, leading instead to the formation of tetrahydrotriazinium (THTZ-H⁺). By a combination of liquid- and solid-state NMR techniques, we show that THTZ-H⁺ is selectively incorporated into the bulk of both FAPbI₃-M and FAPbI₃-H at ∼0.5 mol % and infer that this addition is responsible for the improved α-phase stability.

Continuous and simultaneous conversion of phosphogypsum waste to sodium sulfate and potassium sulfate using quaternary phase diagram

In this present work, the transformation of the Moroccan phosphogypsum (PG) waste, considered a potential source of sulfate, into potassium sulfate compound could help reduce environmental impact and create a new value chain for the phosphate industry. Generally, solid-liquid equilibria are frequently applied in chemical industries. They are a valuable aid in visualizing the precipitation, separation, and purification of a solid phase and the pathways by which crystallization can occur. This process aims to produce potassium sulfate (K₂SO₄), a high-value fertilizer, from sulfate solutions obtained after dissolving PG in a NaOH medium. The quaternary phase diagram Na⁺, K⁺//Cl^-, SO₄^2--H₂O at 25 °C was especially used to determine the operating conditions and the design of a crystallization process during the PG conversion into K₂SO₄. The Jänecke representation of this system enables the determination of the optimal trajectory in the phase diagram for the double decomposition reaction. X-ray fluorescent (XRF) and X-ray diffraction (XRD) techniques were conducted to identify the crystalline phases formed during our process. In summary, the results of this study could contribute to the development of a sustainable valorization PG. Furthermore, K₂SO₄ represents a good alternative to potassium chloride for chloride-sensitive crops.

Seeking Rules Governing Mixed Molecular Crystallization

Mixed crystals result when components of the structure are randomly replaced by analogues in ratios that can be varied continuously over certain ranges. Mixed crystals are useful because their properties can be adjusted by increments, simply by altering the ratio of components. Unfortunately, no clear rules exist to predict when two compounds are similar enough to form mixed crystals containing substantial amounts of both. To gain further understanding, we have used single-crystal X-ray diffraction, computational methods, and other tools to study mixed crystallizations within a selected set of structurally related compounds. This work has allowed us to begin to clarify the rules governing the phenomenon by showing that mixed crystals can have compositions and properties that vary continuously over wide ranges, even when the individual components do not normally crystallize in the same way. Moreover, close agreement of the results of our experiments and computational modeling demonstrates that reliable predictions about mixed crystallization can be made, despite the complexity of the phenomenon.

NaCl Crystals as Carriers for Micronutrient Delivery

Iron deficiency leading to anemia is one of the most severe and important nutritional deficiencies in the world today. To combat this deficiency, the fortification of food products with iron is a natural way to increase the global iron uptake. Here, we report a novel strategy for iron encapsulation in NaCl crystals via microscopic inclusions containing dissolved iron salt. The liquid inclusions embedded in the crystal insulate the reactive iron salts from their environment while assuring that iron is in a soluble and bioavailable form. While the size distribution of inclusions remains independent of the evaporation conditions, their density increases during crystallization at lower relative humidity. Using Raman confocal microspectroscopy, we have been able to analyze the morphology, length/thickness ratio, of inclusions and show that inclusions evolve toward a plate-like structure with the increase in size. By growing a pure NaCl shell around the iron-containing NaCl crystals, the stability of the composite crystals can be even further enhanced. The role of halite crystals as a carrier for iron fortification opens the way for the delivery of other types of micronutrients by including them in table salt.

Direct Image Feature Extraction and Multivariate Analysis for Crystallization Process Characterization

Small-scale crystallization experiments (1-8 mL) are widely used during early-stage crystallization process development to obtain initial information on solubility, metastable zone width, as well as attainable nucleation and/or growth kinetics in a material-efficient manner. Digital imaging is used to monitor these experiments either providing qualitative information or for object detection coupled with size and shape characterization. In this study, a novel approach for the routine characterization of image data from such crystallization experiments is presented employing methodologies for direct image feature extraction. A total of 80 image features were extracted based on simple image statistics, histogram parametrization, and a series of targeted image transformations to assess local grayscale characteristics. These features were utilized for applications of clear/cloud point detection and crystal suspension density prediction. Compared to commonly used transmission-based methods (mean absolute error 8.99 mg/mL), the image-based detection method is significantly more accurate for clear and cloud point detection with a mean absolute error of 0.42 mg/mL against a manually assessed ground truth. Extracted image features were further used as part of a partial least-squares regression (PLSR) model to successfully predict crystal suspension densities up to 40 mg/mL (R ² > 0.81, Q ² > 0.83). These quantitative measurements reliably provide crucial information on composition and kinetics for early parameter estimation and process modeling. The image analysis methodologies have a great potential to be translated to other imaging techniques for process monitoring of key physical parameters to accelerate the development and control of particle/crystallization processes.

TOF-SIMS analysis of curcuminoids and curcumin crystals crystallized from their pure and impure solutions

Impurities are frequently encountered during the crystallisation of active pharmaceutical compounds. Impurities can either adsorb onto active sites or replace atoms of the crystal lattice. Locating the impurities, especially structurally-similar...

Impurity incorporation in solution crystallization: diagnosis, prevention, and control

This work highlights recent advances in the diagnosis, prevention, and control of impurity incorporation during solution crystallization.

Impact of Impurities on Crystallization and Product Quality: A Case Study with Paracetamol

A thorough, systematic study into the effect that structurally related impurities have on both the process and product quality during the crystallization of an active pharmaceutical ingredient is presented. The presence of acetanilide and metacetamol influences the crystallization and product quality of paracetamol. Where high concentrations of either impurity were present in the crystallization feed, product recovery decreased by up to 15%. Acetanilide is included in the final product through adsorption onto the particle surface in concentrations up to 0.79 mol%, which can be reduced to acceptable levels through product reslurrying. The presence of metacetamol results in much higher concentrations—up to 6.78 mol% in the final product, of which approximately 1 mol% is incorporated into the crystal lattice, resulting in the perturbation of the unit-cell dimensions. The incidental crystallization and subsequent isolation of metastable Form II paracetamol increased product purity in the presence of a low metacetamol concentration. This metastable product converts to stable paracetamol Form I through reslurrying, offering an efficient metacetamol impurity rejection route. The morphology of the product is modified consistently by both impurities. An elongation of the normal prismatic shape is observed, which in the extreme case of high metacetamol contamination results in the isolation of fine, fragile needles. This problematic morphology is also improved by a reslurrying of the crystallization product to give a more equilateral shape. This systematic study of the influence of acetanilide and metacetamol on the crystallization of paracetamol builds a well-rounded picture of the concomitant impact of impurities on the principal quality attributes of a crystallization product.

UNGAP best practice for improving solubility data quality of orally administered drugs

An important goal of the European Cooperation in Science and Technology (COST) Action UNGAP (UNderstanding Gastrointestinal Absorption-related Processes, www.ungap.eu) is to improve standardization of methods relating to the study of oral drug absorption. Solubility is a general term that refers to the maximum achievable concentration of a compound dissolved in a liquid medium. For orally administered drugs, relevant information on drug properties is crucial during drug (product) development and at the regulatory level. Collection of reliable and reproducible solubility data requires careful application and understanding of the limitations of the selected experimental method. In addition, the purity of a compound and its solid state form, as well as experimental parameters such as temperature of experimentation, media related factors, and sample handling procedures can affect data quality. In this paper, an international consensus developed by the COST UNGAP network on recommendations for collecting high quality solubility data for the development of orally administered drugs is proposed.

The Behaviour of Rare Earth Elements from South African Coal Fly Ash during Enrichment Processes: Wet, Magnetic Separation and Zeolitisation

Rare earth elements (REEs) are essential raw materials in a variety of industries including clean energy technologies such as electric vehicles and wind turbines. This places an ever-increasing demand on global rare earth element production. Coal fly ash (CFA) possesses appreciable levels of REEs. CFA, a waste by-product of coal combustion, is therefore a readily available source of REEs that does not require mining. CFA valorisation to zeolites has been achieved via various synthesis pathways. This study aimed to evaluate one such pathway by monitoring how REEs partition during CFA processing by the wet, magnetic separation process and zeolitisation. South African CFA was subjected to wet, magnetic separation and subsequent zeolitisation of the nonmagnetic fraction (NMF); solid products were characterised by XRD, SEM, XRF and LA-ICP-MS. The wet, magnetic separation process resulted in the partitioning of a specific set of transition metals (such as Fe, Mn, Cr, V, Ni, Zn, Cu, Co and Mo) into the magnetic fraction (MF) of CFA, while REEs partitioned into the NMF with a total REE content of 530.2 ppm; thus, the matrix elements of CFA were extracted with ease. Zeolitisation resulted in a solid zeolite product (hydroxysodalite) with a total REE content of 537.6 ppm. The process of zeolitisation also resulted in the selective enrichment of Ce (259.1 ppm) into the solid zeolite product (hydroxysodalite), while other REEs were largely partitioned into the liquid phase. CFA valorisation by wet, magnetic separation and zeolitisation therefore allowed for the partitioning of REEs into various extraction products while recovering the matrix elements of CFA such as Fe, Si and Al. The findings of this study highlight the geopolitical importance of REEs in terms of the development of alternative processes for REE recovery from waste and alternative sources, which may potentially give countries that employ and develop the technology a key advantage in the production of REEs for the global market.

Switching polymorph stabilities with impurities provides a thermodynamic route to benzamide form III

Almost 200 years ago, benzamide was reported as polymorphic with two of its forms (II and III) found to be difficult to crystallise. In a recent study, it was shown that benzamide form I can easily convert into benzamide form III using mechanochemistry in the presence of nicotinamide. Here we show, experimentally and computationally, that this transformation is the result of a thermodynamic switch between these two polymorphic forms driven by the formation of solid solutions with small amounts of nicotinamide. The presence of nicotinamide in the crystallisation environment promotes the robust and exclusive crystallisation of the elusive form III. These results represent a promising route to the synthesis and utilisation of elusive polymorphs of pharmaceutical interest.

A robust new tool for online solution-phase sampling of crystallizations

Dynamically flushed filter allows for sampling of crystallizations.

Crystallographic tomography and molecular modelling of structured organic polycrystalline powders

Novel combination of crystallographic tomography and molecular modelling is used to examine the powder packing behaviour and crystal interactions for an organic polycrystalline powder bed.