Preferential crystallization is a technique used to separate enantiomers and is usually performed in batch mode. For a continuously operated preferential crystallization process from a supersaturated racemic solution, however, nucleation and growth of the unwanted counter enantiomer eventually becomes inevitable, and a controlling measure should be taken. Through the use of polarimetry as an effective monitoring tool to detect the crystallization of the unwanted enantiomer, a novel strategy to eliminate the unwanted enantiomer crystals in a continuous cooling preferential crystallization process is presented. The strategy involves switching from the racemic feed solution to an enantiopure feed solution upon detection of the counter enantiomer crystals. This allows selective dissolution of the counter enantiomer crystals while the preferred enantiomer crystals continue to crystallize. After all of the counter enantiomer crystals are dissolved by decreasing the counter enantiomer solution concentration sufficiently below its solubility, the feed is switched back to the racemic solution. Through the use of this modelfree controlling action the continuous process does not have to be terminated. Instead, this method rectifies the situation to the initial metastable steady state by using a portion of the produced enantiomer product. The process can therefore operate at higher supersaturations compared with existing processes for longer periods of time since the control action does not rely on the dissolution kinetics of the system but rather on the thermodynamics of the phase diagram. We show that this new approach is an effective and scalable control strategy for achieving enantiopure product in a continuous preferential crystallization process.
The in vitro degradation of biodegradable polymer/ceramic composites was assessed in two different environments under both static and pseudodynamic conditions. The blends, consisting of polycaprolactone, poly(lactic-co-glycolic acid), and hydroxyapatite, have potential use in bone tissue engineering applications, thus it is essential to establish a standardized method of characterizing the degradation of new biomaterials. In this study, the variation in polymer blend ratio was examined to observe a change in degradation rate. The porous blends were degraded in water and serum-containing media. A previous study examined in vitro degradation in serum-free buffer. Molecular weight loss, gravimetric weight loss, pH changes and morphological changes were evaluated. The changes in porosity were observed with scanning electron microscopy and quantitatively assessed using image analysis. There was a significant difference in molecular weight loss and gravimetric weight loss between the blends after 10 weeks in vitro. Blends containing the greatest amount of poly(lactic-co-glycolic acid) degraded most rapidly.
Directional crystallization from the melt has been used as a tool to grow parallel crystalline stripes of p-and n-type molecular semiconductors. To start, the phase behavior of 2,7dioctyl[1]benzothieno [3,2-b][1]benzothiophene (C8-BTBT-C8): tetracyanoquinodimethane (TCNQ) blends (molar ratio from 20:1 to 1:1) has been investigated by variable-temperature X-ray diffraction, differential scanning calorimetry, and polarized optical microscopy. The partial charge transfer between the C8-BTBT-C8 donor and the TCNQ acceptor as a function of temperature has been studied. Blends of 10:1 and 20:1 have been selected for directional crystallization because they show similar thermotropic and phase behavior comparable to that of pure C8-BTBT-C8. Directional crystallization results suggest that moderate cooling rates (6 and 12 °C min −1 ) leads to a digitated growth mode that gives rise to parallel crystalline stripes of C8-BTBT-C8 and C8-BTBT-C8-TCNQ charge-transfer complexes (C8-BTBT-C8-TCNQ CT), as confirmed by confocal Raman imaging. X-ray diffraction reveals high preferential orientation and good in-plane alignment for both C8-BTBT-C8 and C8-BTBT-C8-TCNQ CT crystallites.
In the pharmaceutical industry the separation of chiral molecules is important due to the different physiochemical properties that the enantiomers of a chiral drug possess. Therefore, resolution techniques are used to separate such enantiomers from one another. In particular, preferential crystallization is a common technique used to separate conglomerate forming compounds, due to its high selectivity. However, efficient separation of enantiomers in a batchwise preferential crystallization process through seeding with the preferred enantiomer alone is still inefficient since unwanted nucleation of the counter enantiomer is inevitable. Here we demonstrate a novel method for the separation of enantiomers for a conglomerate forming compound (asparagine monohydrate), by using mechanical separation by sieving after crystallization, whereby the separation is enabled by a designed bias in the crystal size distributions of each enantiomer. This bias is created by a concomitant crystallization of both enantiomers using optimized seeding and cooling profiles obtained from a population balance model. In this way, a high level of control is achieved over a batch-wise preferential crystallization process since the crystallization of both enantiomers is controlled. We show that through this separation method, material with impurity levels as low as 6 wt% can be obtained.To our knowledge this is the first demonstration of modelling such a process to separate enantiomers of a conglomerate forming compound.
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