The stable solid polymorphism of cyclooctanol (C 8 H 16 O, for short C 8-OH) is revealed to be a complex problem and only two stable solid phases, denoted on cooling from the liquid as phases I and II, are found using static ͑thermodynamic and x-ray diffraction͒ as well as dynamic ͑dielectric spectroscopy͒ experimental techniques. Both solid phases are known to exhibit glass transitions if they are cooled down fast enough to prevent transition to ordered crystalline states. Although glass transitions corresponding to both phases had been well documented by means of specific heat measurements, x-ray measurements constitute, as far as we know, the first evidence from the structural point of view. In addition, a great amount of dielectric works devoted to phase I and its glass transition, were published in the past but next to nothing relating to the dielectric properties of phase II and its glass transition. The nature of the disorder of phase II will be discussed.

The two-component system cycloheptanol (C7) + cyclooctanol (C8) has been studied by means of thermal analysis, X-ray powder diffraction, and dielectric spectroscopy. In a first step, the polymorphism of pure C7 and C8 has been revised, and new unpublished crystallographic data of the stable phases have been reported. Evidence for the isomorphism relationship between the simple cubic (SC) phases of both of the pure components has been seen through the continuous evolution of the lattice parameters, the continuous evolution of the dielectric strength, and the existence of a two-phase equilibrium [SC + L]. Despite the solid polymorphism of the pure components, the SC state has been observed to be the sole mixed solid state between 170 K and [SC + L] equilibrium. To perform a complete thermodynamic analysis, measurements of the excess enthalpy in the liquid state were undertaken. These measurements together with the experimental melting-phase diagram allow us to determine the excess thermodynamic properties of the mixed SC state via a compensation law. Dielectric data have been used to interpret the excess properties, and the existence of short-range orientatonal order for the mixed SC state has been inferred. In addition, this OD mixed state has been classified as extraordinary in light of the log T C−log T m plot.

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The polymorphism of cyclopentanol (C5H10O) has been further investigated by X-ray powder diffraction experiments and it has been found to agree with the most recent thermodynamic study [J. Chem. Thermodyn. 1995, 27, 953]. In addition to the previously reported orientationally disordered hexagonal phases I and II, the lattice symmetry of the low-temperature ordered phases III and IV have been determined by means of pattern-matching analyses both as monoclinic with Z = 24 (Cc and C2/c, respectively). The patterns revealed no significant differences between phases III and IV and the transition between both phases was found through careful determination of the variation of the lattice parameters as a function of the temperature. The strength of intermolecular interactions as well as the anisotropy in the solid phases has been analyzed by the study of the isobaric thermal-expansion tensor and the results are discussed in the light of previously reported literature covering dynamics disorder.

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