Scaling of Glycine Nucleation Kinetics with Shear Rate and Glass–Liquid Interfacial Area

Abstract

The scaling of the nucleation kinetics of glycine was investigated in supersaturated aqueous solutions under isothermal conditions. Induction times were measured in a Couette cell with a wide range of average shear rates γ̇a vg (25−250 s −1 ) and a range of glass−liquid interfacial areas A (2.5−10 cm 2 per mL solution). The probability distributions of induction times were found to scale with shear rate and glass−liquid interfacial area, with the characteristic time scale (γ̇a vg A) −1 . Primary nucleation rates and growth times to reach detection (estimated from the probability distributions) were both dependent on this time scale. In-situ dynamic light scattering revealed mesoscale clusters in the solutions that increased in size over time at rates which also depended on this time scale. The increase in size was thought to be due to the shear-enhanced aggregation or coalescence of mesoscale clusters leading to a higher number of larger mesoscale clusters, resulting in higher rates of primary nucleation. ■ INTRODUCTIONFluid shear has been reported to influence the nucleation of crystals from solution. It is well-known to have an effect on secondary nucleation since shear forces imposed on a crystal face can be sufficient to produce secondary nuclei from the crystal surface, 1,2 but the effect that fluid shear can have on primary nucleation from solution is not well understood to date, particularly for small organic molecules. 3,4 Studies have reported that fluid shear can influence the primary nucleation of various organic compounds from solution, including protein molecules such as lysozyme 5−7 and insulin 8 and smaller molecules such as glycine 4,9 carbamazepine, 10 butyl paraben, 3 and m-hydroxybenzoic acid. 11 In some cases, fluid shear can enhance primary nucleation rates, 3,4,9−11 while in others, it can suppress them, 3,5−7 and the mechanisms behind the role of fluid shear on primary nucleation therefore remain unclear.In many studies on the influence of fluid shear on nucleation, the shear rates were not well quantified so in our previous work, 4 Couette and capillary flow devices were used to achieve controlled, quantifiable flow conditions. It was shown that this controlled fluid shear could be used to significantly enhance primary nucleation rates in supersaturated glycine solutions. 4 Controlled fluid shear in a Couette cell has been found to enhance primary nucleation in other systems, including butyl paraben 3 and m-hydroxybenzoic acid. 11 While it is recognized that complex flow fields are present in industrial crystallization processes, it is useful to understand the effect of simple, wellcontrolled flow fields on various aspects of the crystallization process, such as primary nucleation, and then map this information on more complex equipment through knowledge of shear rate distributions obtained for example by computational fluid dynamics.In addition to fluid shear, exposure of solutions to a solid surface such as the walls of the glass Couette cell is known to have an influence on nucleation. ...

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“…However, we note that nucleation was likely to be heterogeneous at the glass vial surface. 36 The interfacial concentration at the glass surface is unknown and, as we have seen from our results for the oil and air interfaces, it cannot be directly inferred from the bulk concentration.…”

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“…Liu and Rasmuson have used a Taylor-Couette cell to show that nucleation rates increase for increasing shear rates. Forsyth et al found similarly that shearing a supersaturated aqueous glycine solution (in a Taylor-Couette flow cell and a capillary crystallizer) resulted in lower induction times 153,154 . These results may be explained by the existence of agitation-enhanced mesoscale clusters 51,153,154 .…”

“…Forsyth et al found similarly that shearing a supersaturated aqueous glycine solution (in a Taylor-Couette flow cell and a capillary crystallizer) resulted in lower induction times 153,154 . These results may be explained by the existence of agitation-enhanced mesoscale clusters 51,153,154 . Similar observations have been made in microfluidic crystallizers.…”

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