Length-dependent segregation in crystallization of n-alkanes: MD simulations

Abstract

We studied the crystallization behavior of pure n-alkanes and their binary and polydisperse mixtures by using Molecule Dynamics (MD) simulations. We found strong length dependent segregations during the crystallization of binary mixtures and weaker segregation in a polydisperse mixture. The segregation is strongly related with the growth kinetics of crystallization. In general, a higher crystallization temperature related with a slower crystal growth rate results in a stronger segregation. With faster growth rate, the shorter chains ally get kinetic entrapped by fast crystallization of longer chains which results in weaker segregation. At higher crystallization temperatures, segregations in both binary and polydisperse mixtures are stronger than at lower temperatures, due to weaker entrapment effect. There is an optimal chain length which has the maximum normalized segregation factor (most likely to crystallize) in the crystallization of a polydisperse mixture. The competition between temperature selection and kinetic entrapment effect of different growth rates is the possible reason to explain the existence of this optimal chain length, which value increases with the increase of crystallization temperature. We observed the formation of large layered crystalline structures by lateral growth of crystals as well as by merging of small crystals.