A vanishing nucleation barrier for the n-alkane rotator-to-crystal transformation

Abstract

Alkanes exhibit intermediate plastic–crystalline rotator phases between the liquid and the low-temperature fully ordered crystal. Here we report measurements of the liquid-to-rotator and rotator-to-crystal homogeneous nucleation kinetics using x-ray scattering on emulsified samples. These are the first reported studies of rotator-to-crystal homogeneous nucleation, and they show an anomalous temperature dependence of nucleation. Through use of mixed alkanes, we probe this over a wide temperature range. For three compositions along the C19H40–C20H42 binary, we find that the liquid-to-rotator interfacial free energy, calculated from the nucleation barrier in classical nucleation theory, is the same for all samples. However, the rotator-to-crystal interfacial free energy is both considerably smaller and strongly dependent on composition and temperature. As the temperature range of the rotator phase widens due to chain mixing, both the thermodynamic transition temperature as well as the interfacial energy for the rotator-to-crystal transition decrease. The interfacial energy reaches half its initial value at the lowest point measured, and it extrapolates to zero at a temperature about 5 K lower. Surprisingly, this corresponds to a situation where the latent heat itself is still finite. Typically, first-order phase transitions exhibit hysteresis due to a kinetic barrier. Our data suggests the unusual case of zero hysteresis for a first-order transition due to the disappearance of the homogeneous nucleation barrier.