Monotropic Transition Mechanism of m-Hydroxybenzoic Acid Investigated by Temperature-Resolved Second Harmonic Generation

Temperature-resolved second harmonic generation (TR-SHG) and SHG microscopy were used to study under normal pressure the solid-solid transition mechanism occurring between the two monotropically related polymorphic forms (metastable Pna2(1) and stable P2(1)/n) of 3-hydroxybenzoic acid (MHBA). The activation energy Ea (as a measure of the barrier energy) of the irreversible transition was determined via isothermal TR-SHG (137-144 kJ.mol(-1)). It fits well with that determined from differential scanning calorimetry (139 kJ.mol(-1)). Regarding the two crystal structures, optical microscopy observations, and kinetics parameters from TR-SHG, a destructive/reconstructive mechanism is proposed for the solid-solid transition. The present study dearly demonstrates that TR-SHG is a relevant and accurate technique for monitoring solid-solid phase transitions.