MOLECULAR-DYNAMICS STUDY OF HOMOGENEOUS NUCLEATION IN SUPERCOOLED CLUSTERS OF TERT-BUTYL CHLORIDE - TRANSITION FROM TETRAGONAL TO MONOCLINIC PHASE

In a series of molecular dynamics runs on 188-molecule clusters of ter t-butyl chloride spontaneous transitions from the orientationally diso rdered tetragonal phase to the ordered monoclinic phase occurred durin g cooling. Nucleation rates exceeding 10(36) critical nuclei m(-3) s(- 1) were determined at 100, 90, and 80 K. These were found to be consis tent with a rate 8 orders of magnitude lower measured at 156 K in expe riments with much larger clusters, when compared on the basis of the c lassical theory of nucleation. Interfacial free energies of the tetrag onal-monoclinic boundaries derived from the rates were similar to 3.5 mJ/m(2), or approximately 0.3 of the heat of transition of the phase c hange (per unit area of a molecular layer), just as Turnbull. had obse rved for a series of freezing transitions. Rotational jump frequencies of molecules from the disordered to the ordered orientation were infe rred from rotational diffusion in the tetragonal phase and appeared to be satisfactory in the treatment of nucleation. They were far too low , however, to account for the growth rate of postcritical nuclei. A ra tionale for this discrepancy is proposed.