Mechanistic study of the formation of arbutin polymorphs and solvates

Abstract

In this study, the complicated solvation behaviors of arbutin were studied from both experiment investigations and theoretical calculations. Six solid forms of arbutin, namely the anhydrous form I and five solvates, were successfully obtained. The structures of form I, solvate S1, solvate S4, and solvate S5 are reported for the first time. The obtained forms of arbutin were thoroughly characterized and distinguished by PXRD, PLM, DSC, TGA, Raman, and FTIR spectroscopy techniques. It was found that solvate S4 and solvate S5 were isostructural solvates. Further, the packing arrangements and pore structures were analyzed to illustrate the formation of the isostructural solvates. Moreover, analyses of the crystal structures showed that arbutin exhibited flexible conformations in various solid forms, and there were strong correlations between the crystal lattice energy and the thermal stability of various solvates. Finally, the formation mechanism of arbutin solvates was proposed and rationalized and the conversion relationship between different solid forms was also established.