The role of a mixture of DMSO : water in the crystallization of α-lactose monohydrate (α-LM) single crystals with desired morphology

Abstract

In this study, DMSO–water solvent mixtures (from 10 to 90% v/v) were used for the growth of alpha-lactose monohydrate (α-LM) single crystals. In the mixture of solvents (i.e. DMSO and water in the ratio from 9D : 1W to 1D : 9W), the solubility of α-LM in these solvent mixtures shows a parabolic relationship. With the adoption of two crystallization methods, slow and fast evaporation, the growth rate depends on the evaporation of water in water-rich solutions. On the other hand, in DMSO-rich solutions, it depends on the absorption of moisture which reduces the solubility of α-LM. The water absorption nature of DMSO and the evaporation nature of water at 33 °C induced an important increase in the mean crystal size, which can be explained by taking into account the role of solvent in the growth in the environment. Bulk single crystals of α-LM with a tomahawk morphology having different sizes were grown by slow evaporation. Among these 9 mixing compositions, 9D : 1W and 1D : 9W mixtures only produced the industrially preferred needle-like morphology of α-LM with narrow crystal size distribution upon adopting the fast evaporation method. Powder X-ray diffraction (PXRD) and differential scanning calorimetry (DSC) analyses confirm the form of crystallization of pure α-LM with different morphologies obtained by fast and slow evaporation methods. The percentages of α-L present in the grown crystals having different morphologies were determined using an automatic digital polarimeter. The fast evaporation method employed led to the achievement of crystals with desired morphology, narrow crystal size distribution, shorter nucleation period and high purity, compared to the slow evaporation method.