Translational and rotational disorder in solid <i>n</i>-alkanes: Constant temperature–constant pressure molecular dynamics calculations using infinitely long flexible chains

Abstract

Molecular dynamics calculations are used to study the effect of temperature on the interchain packing in solid n-alkanes. The model used consists of infinite chains initially arranged in a centered orthorhombic structure with the lateral packing found in the low temperature phase of n-alkanes with an odd number of carbon atoms. An atom–atom interchain potential is employed and the chains have flexible backbones. The problem of equilibrating the inter- and intramolecular motions is overcome using the technique of massive stochastic collisions proposed by Andersen. The calculated isobaric lateral thermal expansion of the orthohombic a and b cell parameters is in excellent agreement with experimental data over a wide temperature range. For T&amp;gt; 250 K translational (jump) diffusion is observed along the chain axis (c direction) without any accompanying rigid body chain rotation. At higher temperatures, the diffusion becomes liquid- like and rotational diffusion sets in between four well defined sites. Possible implications of our results for understanding of real alkanes are explored.