Dielectric and Calorimetric Study of Orientationally Disordered Phases in Two Unusual Two-Component Systems

Abstract

In the present communication, investigations of two interesting (two-component) solid solutions are reported where one is a hydrogen (H-)-bonded pair and the other is a non-H-bonded pair. The former is the two-component system cyclooctanol (COOL) + cycloheptanol (CHOL), which forms a simple cubic phase [Rute, M. A.; Salud, J.; Negrier, P.; López, D. O.; Tamarit, J. Ll.; Puertas, R.; Barrio, M.; Mondieig, D. J. Phys. Chem. B 2003, 107, 5914]. This solid phase has been investigated at low temperatures and for several concentrations by means of low-frequency dielectric spectroscopy and differential scanning calorimetry (DSC). Depending upon the concentration, this phase reveals a glass transition in the temperature range of 138-172 K and a pronounced relaxation process identifiable with the so-called alpha process characteristic of a single-component orientationally disordered crystal. The dielectric spectra are found to follow the Havriliak-Negami (HN) equation. The analysis of the various parameters obtained show an isomorphic relationship between the simple cubic phases of both pure components through a continuous change of parameters. In addition, a sub-T(g) process, which is Arrhenius, is found. The kinetic freezing of the various dielectric processes has been critically examined in relation to the T(g) found in the DSC experiments. The non-H-bonded pair that has been studied is cis-1,2-dimethylcyclohexane (DMCH) and cyclohexylchloride (CHC). The liquid mixture of DMCH and CHC upon lowering the temperature forms a solid solution on the DMCH-rich side, which is an orientationally disordered crystal. This phase demonstrates a pronounced alpha process in the dielectric measurements that follows the HN equation. The results are discussed in the context of the solid-liquid phase diagram of this binary system. The observed deviations from Arrhenius and Debye behaviors in the solid solutions studied in this paper are shown to follow the "strong-fragility" pattern of Angell.