Solvent effects on kinetics of solution-mediated transition of stearic acid polymorphs

The effects of solvent on the transition kinetics of polymorphic modifications of stearic acid have been examined in polar and nonpolar solutions. Two typical polymorphs, B (low-temperature stable) and C (high-temperature stable), and the solvents (butanone, methanol, n-hexane and decane) were studied. In all solutions the transitions from B to C and from C to B took place at temperatures above and below 32°C, at which the free energies of B and C have the same value, respectively. The rates of the C → B transition were significantly dependent both on temperature and solvent. First, the transition rate was fastest between 22 and 26°C. This was due to two conflicting factors: the free energy difference between B and C which decreases as the temperature approaches 32°C, and the rates of dissolution of C and growth of B which increase with temperature. Secondly, the solvent exclusively influenced the C → B transition; polar solvents, especially methanol, caused a significantly more rapid transition than nonpolar solvents, the rates being relatively higher than those predicted by the solubility values. It was inferred that the different growth units (monomers in the polar solvents and dimers in the nonpolar ones) and the twisted lateral interface structures of the B polymorph would be responsible for the present solvent effect.