Supersaturation controlled morphology and aspect ratio changes of benzoic acid crystals

Highlights

• Supersaturation effects on crystal habit and aspect ratio of benzoic acid.

• Benzoic acid crystal morphology changes from needle-like shape to rectangular sheet and then to hexagonal particles with the increasing supersaturation.

• Aspect ratio of benzoic acid crystal decreases with the increase of supersaturation.

• Modified attachment energy (MAE) model was used to evaluate the shape changes.

• Solvents interact with the crystal surface and selectively slow down the growth rate of exposed faces.

Abstract

Supersaturation is a factor of great industrial importance to the crystal growth by affecting the final aspect ratio and morphology of organic compounds. In this work, the qualitative relationship between aspect ratio and supersaturation of benzoic acid (BA) was elucidated for the first time by experimental and simulative study. Experimentally, it was found that the crystal shape of BA changes from needle-like crystal to rectangular sheet and then to hexagonal particles with the increasing supersaturation ranging from 1.029 to 2.941. The increment of supersaturation decreases the average aspect ratio of crystallized particles from ∼20.5 to ∼1.3. Furthermore, a higher supersaturation (σ = 1.618) leads to more isotropic hexagonal crystals due to less face discrimination at high crystallization rates. Additionally, we predicted the supersaturation-dependent crystal habit by the modified attachment energy (MAE) model, which yield good agreement with the experimental observed crystals at medium and high supersaturations.

Introduction

Crystallization from solution, as a typical separation and purification process, has been widely used for the manufacture of pharmaceutical compounds. (Fujiwara et al., 2005) Optimization and control of crystal shape and size is an important objective in crystal preparation as different crystal shapes can induce both chemical and physical properties. (Liu et al., 2013, Lovette et al., 2008, Ma et al., 2002) Hence, it is necessary to understand factors that affect crystal shape and control approach during the crystallization process in order to obtain products with desired characteristics, which has driven intensive research toward this direction. Many factors, such as solvent, (Karunanithi et al., 2009) supersaturation, (Jung and Kim, 2011a, Jung and Kim, 2011b) seed, (Liu et al., 2010) and impurities, (Yang et al., 2012) have effects on product qualities in terms of crystal morphology, crystal forms, and crystal size distribution. Supersaturation, as a driving force to determine the nucleation process and crystal growth, affects growth kinetics for each crystallographic facet so as to the formation of different morphologies in crystallization processes. (Kang et al., 2011) The supersaturation is important not only in controlling crystal morphology but also in designing strategy for particle size, (Jim et al., 2013) especially in industrial crystallization process. (Larsen et al., 2006)

Over the past decades, computer simulation provides a helpful approach for crystallization solvent design, crystal morphology prediction and crystal habit evaluation in solvents to have a better understanding of the crystal growth. (Karunanithi et al., 2006, Schmidt and Ulrich, 2012a) Many models were reported and applied to predict the crystal habit by considering the supersaturation, which is a very crucial factor in crystallization. Deij et al. (2005) predicted the crystal morphology for a yellow isoxazolone by using Monte Carlo (MC) simulation dye depending on the supersaturation. Lovette and Doherty (2012) proposed a first-principles model to predict steady-state crystal shapes of naphthalene at different supersaturations. Schmidt and Ulrich (2012b) predicted the crystal morphology of benzoic acid (BA) by using the modified attachment energy (MAE) model in supersaturation solution. Recently, different solvent controlled crystal growth morphology of BA at the infinitely low supersaturation (saturated solution) has been studied by combining experimental crystallization and MAE simulation (Liang et al., 2014a, Liang et al., 2014b). Among these methods, the MAE model has been considered as an effective model to predict the crystal morphology in supersaturated solution.

Additionally, the supersaturation controlled morphology evolution has attracted numerous attentions. For example, Ristic et al. (2001) described the morphology of paracetamol crystals grown from water at different level of supersaturations. Lu and Ulrich (2005) studied the crystal habit of caprolactam from water at low and high supersaturations. Schmidt et al. (2011) calculated the crystal morphology of BA from aqueous solution with different water models. However, it is worth noting that the relationship between supersaturation and the resulting aspect ratio of crystal morphology still requires further clarification both in experiments and theoretical explanations. For example, the aspect ratio of naphthalene crystal from cyclohexane increases with the increase of supersaturation (Tilbury et al., 2016). In contrast, the aspect ratio of L-alanine crystal decreases with the supersaturation increasing (Tan et al., 2016). Therefore, highly effective method and supersaturation controlled aspect ratio are required for better understanding the nucleation and crystal growth process.

Herein, BA crystals at different level of supersaturations in aqueous solution were prepared via natural cooling method. The modified attachment energy (MAE) method was used to predict the crystal habit in different supersaturations to confirm the crystal shape evolution. Furthermore, the adsorption of water molecule on the most exposed surfaces was evaluated. The key aim of this work is to achieve a better understanding of supersaturation determined crystal growth habit and quantify aspect ratio changes of BA with the increasing supersaturation.