Energetics of rotation and translation in hexagonal aggregates of extended chains

Abstract

Potential energy calculations have been made for hexane aggregates in the hexagonal arrangement as models of paraffin-like solids. As a part of the general energy hypersurface the feasibility of translation and rotation of extended chains in aggregate has been examined for various degrees of packing. The pure translation is favoured over the pure rotation at all interchain separations. The energy map of coupled rotation-translation motion has been developed for the intermediate degree of packing. The energy minima on the map correspond to the parallel orientation of chain backbones and are presumed to represent the prevailing short-range structure of the rotator phase in n-alkane crystals. The calculated barriers are used for the rationalization of chain dynamics in the rotator phase, and in the disorded phases with the similar alkyl chain packing as found in crystals, monolayers and bilayers of amphiphiles. Probability of formations of the translational, rotational and conformational defects in the above systems is discussed.