Abstract:

The effects of solvent on the transition kinetics of polymorphic modifications of stearic acid were examd. in polar and nonpolar solns. Two typical polymorphs, B (low-temp. stable) and C (high-temp. stable), and the solvents (butanone, methanol, n-hexane, and decane) were studied. In all solns. the transitions from B to C and from C to B took place at temps. above and below 32°, at which the free energies of B and C have the same value, resp. The rates of the C → B transition were significantly dependent both on temp. and solvent. First, the transition rate was fastest between 22 and 26°. This was due to 2 conflicting factors; the free energy difference between B and C which decreases as the temp. approaches 32°, and the rates of dissoln. of C and growth of B which increase with temp.. Secondly, the solvent exclusively influenced the C → B transition; polar solvents, esp. methanol, caused a significantly more rapid transition than nonpolar solvents, the rates being relatively higher than those prediced by the soly. 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. [on SciFinder(R)]