Investigation into the subambient behavior of aqueous mannitol solutions using temperature-controlled Raman microscopy

The aim was twofold; to demonstrate the ability of temperature-controlled Raman microscopy (TRM) to locate mannitol within a frozen system and determine its form; to investigate the annealing behavior of mannitol solutions at −30 °C. The different polymorphic forms of anhydrous mannitol as well as the hemihydrate and amorphous form were prepared and characterized using crystal or powder X-ray diffractometry (XRD) as appropriate and Raman microscopy. Mannitol solutions (3% w/v) were cooled before annealing at −30 °C. TRM was used to map the frozen systems during annealing and was able to differentiate between the different forms of mannitol and revealed the location of both β and δ polymorphic forms within the structure of the frozen material for the first time. TRM also confirmed that the crystalline mannitol is preferentially deposited at the edge of the frozen drop, forming a rim that thickens upon annealing. While there is no preference for one form initially, the study has revealed that the mannitol preferentially transforms to the β form with time. TRM has enabled observation of spatially resolved behavior of mannitol during the annealing process for the first time. The technique has clear potential for studying other crystallization processes, with particular advantage for frozen systems.