LIGHT-SCATTERING-STUDIES ON THE THERMALLY-INDUCED CRYSTALLIZATION TRANSITION OF BETA-CYCLODEXTRIN

Employing light scattering, we have examined the thermally induced crystallization transition of beta-cyclodextrin in the range between 343.2 and 283.2 K with the critical temperature T-c = 307.3 K. The observed cluster evolution resembles the transition from subcritical to postcritical nucleation. The initial cluster size at 343.2 K is 125 +/- 21 nm, and the final cluster size at 283.2 K is 400 +/- 72 nm with a size of 236 +/- 38 nm at T-c. This information can be exploited by means of a simple two-state model so that tentative estimates of the parameters governing beta-cyclodextrin nucleation can be extracted according to the classical nucleation theory. The free energy of crystallization at the barrier is Delta G(nc) = -126 +/- 26 kJ/mol, and the critical number of seeding particles involved in the formation of such a crystallite is 5. Static light-scattering measurements indicate that subcritical nuclei are more compact than postcritical nuclei. A pronounced hysteresis detected between cooling and heating cycles was further established by light microscopy and digital image analysis.