Vibrational spectroscopic study on the polytypism of n-alkanes and fatty acids

An important characteristic of n-alkanes and fatty acids is the great diversity of solid-state structures, which we can hardly imagine from their simple molecular structures. The variety of crystalline states can be categorized with two concepts: polymorphism and polytypism. Polytypism refers to higher-order structural variation caused solely by the stacking mode of molecular layers. Two basic polytypic structures, single-layer and double-layer, were confirmed in several modifications of n-alkanes and fatty acids having an O⊥ subcell. During the last decade, the two basic polytypes of n-alkanes have been intensively studied mainly by vibrational spectroscopy, with a focus on their formation conditions, thermodynamic stability, and molecular motions. This review summarizes new progress in the context of research on the polytypism of long-chain compounds.