The distribution of conformational disorder in the high-temperature phases of the crystalline n-alkanes

The distributions of conformational defects that exist in the high-temperature phase II (also referred to as the hexagonal or rotator phase) of the crystalline n-alkanes C₂₁ and C₂₉ have been measured by an infrared CD₂-substitution technique and have been accounted for in terms of a lattice model that provides freedom for longitudinal displacement of the chains. The defects consist almost entirely of gtg′ kinks distributed nonuniformly along the chain. The uneven distribution is indicated in the variation in the concentration of gauche bonds measured at various sites along the chain. The highest concentration is at the chain ends, and the concentrations at interior sites decrease exponentially in going toward the middle. To explain the distribution we used a modification of a lattice model that had been successfully applied to the lipid bilayer. Comparison of observed distributions with those computed from the model indicates that the factors that determine the shape of the distribution are quite different in the n-alkane and bilayer cases. For the bilayer, the dominant factor is the variation in the lateral density of chains; for the n-alkane, the dominant factor is associated with longitudinal displacement of the chains.