Polymorphism of even saturated carboxylic acids from n-decanoic to n-eicosanoic acid

Abstract

The polymorphism of normal saturated even carboxylic acids from n-decanoic to n-eicosanoic acid is discussed. Seven crystal modifications, including polymorphs and polytypes, were identified and fully characterized by the combination of calorimetric measurements (DSC) at atmospheric and high pressures, X-ray powder diffraction, FT-IR spectroscopy and scanning electron microscopy (SEM). All seven crystal forms, including polymorphs and polytypes, are observed at room temperature. Forms A₂ and A_super are triclinic, form C is monoclinic and forms E and B show both a monoclinic and an orthorhombic polytype. The triclinic modifications A₂ and A_super predominate for acids up to n-tetradecanoic acid (C₁₄H₂₇O₂H). The orthorhombic and the monoclinic forms predominate for acids from n-hexadecanoic (C₁₆H₃₁O₂H) up to n-eicosanoic acid (C₂₀H₃₉O₂H). When the temperature is increased, all the crystal modifications transform irreversibly to the C form. In the first part of this paper, cell parameters for the different forms are given, the observed temperatures and enthalpies of the transitions are reported and the stability of the different forms is discussed. In the second part, we state the main contribution of each technique for the identification and interpretation of the polymorphism of even numbered carboxylic acids.