The interplay between nucleation and patterning during shear-induced crystallization from solution in a parallel plate geometry

Cooling crystallization of small organic molecules from solution is an important operation for the separation and purification of drug products. In this research, shear-induced nucleation from a supersaturated solution is studied in a parallel plate geometry. Under conditions of shear and small gap sizes, narrow mesoscale circular bands of small crystals appeared spontaneously and reproducibly on the plate's surface. We have investigated the connection between nucleation and the emergence of these circular patterns. Our results show that nucleation occurs preferably in zones with high local shear rate (located at the outer edges of the plates), compared to zones with low local shear rate (at the center of the plates). The time before nucleation occurs decreases significantly for increasing mean shear rate and time. The circular crystalline patterns appear at the plate's surface, where heterogeneous nucleation first occurs. Multiple hypotheses are explored to understand the pattern formation in crystallization. Since no satisfactory explanation is found, a new mechanism is proposed. This hypothesis involves crystals initially forming on the surface of the plates and undergoing stick-slip motion, which influences the local nucleation kinetics. This results in an interplay between (secondary) nucleation and stick-slip motion at the start of the crystallization process. By modifying the surface of the plates, their ability to act as a heterogeneous nucleation site can be altered, allowing control over the formation of patterns.

How Does Ion Exchange Construct Binary Hexacyanoferrate? A Case Study

In this work, using electrochemical active Fe as an ion-exchange element (attack side) and the Na _x MnFe(CN)₆ slurry with a high solid content (MnHCF) as a template (defensive side), a series of binary hexacyanoferrates are prepared via a simple Mn/Fe ion-exchange process, in which Na _x FeFe(CN)₆ (FeHCF) and solid solution Na _x (FeMn)Fe(CN)₆ are concentrated on the shell and the core, respectively. The proportions of the two structures are mainly controlled by the competition between the ion-exchange rate in the bulk material and dissolution-reprecipitation rate. Slowing down the attacking rate, such as the use of a chelating agent complexed with the attacker Fe, is advantageous to form a thermodynamically metastable state with homogeneous distribution of elements since the diffusion of Fe²⁺ in the solid MnHCF is relatively fast. The shell FeHCF could be adjusted by the dissolution-reprecipitation rate, which is driven by the solubility difference. Adding the chelating agent in the defensive side will promote the dissolution of MnHCF and reprecipitation of FeHCF on the surface. Meanwhile, with the increase of Fe sources, the thickness of the shell FeHCF increases, and correspondingly the content of solid solution decreased due to FeHCF is more stable than solid solutions in thermodynamics. Finally, such a design principle in this case study could also be generalized to other ion-exchange processes. Considering the difference of two components in solubility, the larger difference can make the core/shell structure more clear due to the enhancement of dissolution-reprecipitation route.

Effect of ultrasound on the kinetics of anti-solvent crystallization of sucrose

The effect of ultrasound on the kinetics of anti-solvent crystallization of sucrose was studied. The influence of temperature, stirring rate, supersaturation and ultrasonic power on the anti-solvent crystallization of sucrose was investigated. The relationship between infrared spectral characteristic band of sucrose and supersaturation was determined with an online reaction analyzer. The crystal size distribution of sucrose was detected by a laser particle-size analyzer. Ultrasound accelerated the crystallization process, and had no impact on the crystal shape. Abegg, Stevens and Larson model was fitted to the experimental data, and the results were the following: At 298.15 K, the average size of crystals was 133.8 μm and nucleation rate was 4.87 × 10⁹ m^-3·s^-1 without ultrasound. In an ultrasonic field, the average size was 80.5 μm, and nucleation rate was 1.18 × 10¹¹ m^-3·s^-1. Ultrasound significantly reduced the average size of crystals and improved the nucleation rate. It was observed that the crystal size decreased with the increase of stirring rate in silent environment. When the stirring rate increased from 250 to 400 rpm, the average size decreased from 173.0 to 132.9 μm. However, the stirring rate had no significant impact on the crystal size in the ultrasonic field. In addition, the activation energy of anti-solvent crystallization of sucrose was decreased, and the kinetic constant of nucleation rate was increased due to the effect of ultrasound. In the ultrasonic field, the activation energy was reduced from 20422.5 to 790.5 J·mol^-1, and the kinetic constant was increased from 9.76 × 10² to 8.38 × 10⁸.