Solid-solid transformation in racemic Ibuprofen.

Purpose: To clarify the polymorphism of racemic Ibuprofen and to determine the kinetic of the phase transformation that follows crystallisation of phase II.

Methods: Differential Scanning Calorimetry (DSC), X-ray powder diffraction and Hot Stage Microscopy are complementarily used to perform a kinetic investigation of the particular temperature range where competition between the occurrence of phases I and II is suspected.

Results: Experiments performed with the three techniques reveal that at 273 K the crystallisation to phase II is then followed by a solid-solid transition towards phase I. This transformation is exothermic (conversion enthalpy of 8.0 ± 0.5 kJ/mol), which proves that the two phases form a monotropic set. The kinetics of conversion deduced from X-Ray experiments follows a Johnson-Mehl-Avrami equation and the Hot Stage Microscopy allows us to establish that the transformation proceeds by the growth of some nuclei of phase I probably formed at lower temperature.

Conclusions: These results allow us to precise the stability pattern of racemic Ibuprofen and to establish the kinetic conditions of appearance and interconversion of the different phases. Therefore such real time resolved investigations would help if applied in the screening of polymorphs when competitive crystallisations occur.