Arrangement of Rotating Molecules in the High-temperature Form of Normal Paraffins

Abstract

NORMAL paraffins undergo a phase transition near the melting point into a crystal form which has been described as hexagonal. This form was first found by Müller¹, who obtained hexagonal dimensions from X-ray powder data and therefore concluded that the molecules rotate in the lattice. Later, Bernal² reported a hexagonal cell for dodecanol in which the molecules lay along triad axes. Crystal forms with the chains arranged according to the same type of chains have then been found in other long-chain compounds, for example, simple esters and glycerides, and they have been termed α-forms. The name α will also be used for the high temperature crystal form of n-paraffins here, since it is inadequate to term it hexagonal as will be shown below. The nature of the molecular motion in this crystal form of normal paraffins has been studied extensively, and the results indicate that the molecules aie undergoing some kind of concordant rotation^3–5.