(680b) Influence of Controlled Fluid Shear on Nucleation Kinetics in Glycine Aqueous Solutions

Understanding crystal nucleation from solution is important for rationally designing and controlling manufacturing processes that involve crystallization. Both batch and continuous industrial crystallization processes typically involve fluid flow where supersaturated solutions and/or suspensions of growing crystals are pumped or agitated. It is well known that fluid shear can induce secondary nucleation and it has been experimentally demonstrated that primary nucleation can be affected by fluid shear as well (1,2). In order to gain further experimental evidence to facilitate improved understanding of effects of fluid shear on primary nucleation, we investigated nucleation in aqueous glycine solutions under well-defined flow conditions at a constant temperature.

Supersaturated glycine solutions were prepared by nanofiltering and slow cooling, and they were then subjected to controlled fluid shear in Couette cells under isothermal conditions. Average shear rates between 25s^-1 and 1000s^-1 were studied. Crystallization in the Couette cell was monitored using a custom built instrument that allowed transmission and light scattering measurements to be taken simultaneously in situ; the onset of nucleation was associated with a rapid increase in scattering and decrease in transmission, so induction times were readily obtained from these measurements. The role of surface was also studied by changing the diameter of the inner cylinder of the Couette cell; this allowed surfaces of 2.5cm²/cm³ solution, 5 cm²/cm³ solution and 10 cm²/cm³ solution to be investigated. Due to the stochastic nature of nucleation, experiments were repeated multiple times and a statistical analysis was performed to show that the number of repetitions was sufficient for accurate trends to be deduced.

Nucleation induction times in sheared solutions were found to be considerably lower than those in unsheared solutions, while a great care was taken to avoid seeding either in solution or at solid-liquid interfaces. This suggested that fluid shear had a profound influence on primary nucleation glycine from aqueous solutions. In sheared solutions, a large number of small crystals formed in close succession, causing solutions to rapidly go turbid, so it is proposed that shear enhanced the formation of primary nuclei which then initiated extensive secondary nucleation (3). Increasing the average shear rate was found to reduce the induction time in a power law relationship with the exponent of -0.98±0.1. This indicated that the total strain applied had an important effect on nucleation kinetics. Increasing the surface area was also found to notably decrease the induction times, with a power law relationship with the exponent of -0.88±0.2, which indicated that the surface played an important role in nucleation kinetics as well. In situ dynamic light scattering measurements showed that colloidal scale liquid-like clusters (nanodroplets) observed in mesostructured amino acid aqueous solutions (4) had their mean size increasing upon continued shearing of solutions. This is consistent with these mesoscale clusters (in the bulk liquid phase and/or at the solid-liquid interface) playing a direct role in the primary nucleation pathway in this system as proposed recently (1), where larger clusters correlated with shorter nucleation induction times.

(1) Jawor-Baczynska, Sefcik and Moore, Crystal Growth & Design, 13, 470 (2013)

(2) Liu and Rasmuson, Crystal Growth & Design, 13, 4385 (2013)

(3) Kadam, Kramer and ter Horst, Crystal Growth & Design, 11, 1271 (2011)

(4) Jawor-Baczynska, Moore, Lee, McCormick and Sefcik, Faraday Discussions, 167, 425 (2013)