Crystal nucleation in normal alkane liquids

Abstract

The homogeneous nucleation of crystals in normal alkane liquids has been studied using a droplet technique for a range of alkanes between C16H34 and C5H12. The critical undercooling for nucleation, ΔTN, relative to the melting point TE, increases significantly with decreasing length of the alkane chain n: ΔTN/TE increases from 0.05 for C16H34 to 0.20 for C5H12, with separate variations noted for the odd-n and even-n alkanes. Use of a spherical nucleus model gives molar surface energies σM relative to the molar heat of fusion, ΔHM, which also increase significantly with decreasing n but show little odd–even variation: σM/ΔHM increases from 0.07 for C16H34 to 0.28 for C5H12. Because of the pronounced increase in surface energy with decreasing chain length, the model of a spherical nucleus is suggested to be inappropriate for the alkane liquids (at least for the high-n alkanes). Using a preferred cylindrical nucleus model with separate side- and end-surface energies, a side-surface energy of about 6 erg cm−2 is estimated from the present data, at least for the high-n alkanes. Using this model, the end-surface energies are found to increase with decreasing chain length. This increase is taken to reflect the increasing importance of chain localization and chain end effects with decreasing chain length. For the small-n alkanes, the side surface energy is also suggested to increase with decreasing chain length, again associated with chain localization effects in the liquid adjacent to the crystal (nucleus) surface.