Isomeric Differences in the Nucleation of Crystalline Hydrocarbons from Their Melts

Large liquid clusters of the globular hydrocarbon 2,2,3-trimethylbutane were generated in supersonic expansions and frozen, in flight, by evaporative cooling while being monitored by electron diffraction. The nucleation rate rose to 8 × 10 28 m -3 s -1 by the time half of the clusters were frozen. Nucleation properties are contrasted with those observed several decades ago by Turnbull and Cormia for three n -alkanes in emulsions. Despite the 15 order of magnitude difference between the rates of nucleation for the globular and linear hydrocarbons, it was possible to make valid comparisons and corroborate Turnbull's inferences that the flexible, linear molecules nucleate by a rather different mechanism. Nucleation of the globular hydrocarbon was similar to that of other simple, rather rigid molecules studied in this laboratory and implied a solid−liquid interfacial free energy of about 4.6 mJ/m 2, a value fairly typical for a substance of its (low) heat of fusion. On the other hand, the linear alkanes exhibit interfacial free energies that are twice as great and kinetic prefactors that are considerably greater, presumably because they nucleate in small molecular segments rather than by the association of entire molecules. Unlike several other simple substances which froze to metastable solids when cooled at extremely fast rates in this laboratory, 2,2,3-trimethylbutane froze to its thermodynamically stable form.