Dielectric Measurements at High Pressures and Low Temperatures VIII: Dielectric and Thermodynamic Properties of Cycloheptanol

Abstract

The thermodynamic and dielectric behaviours of liquid and solid cycloheptanol have been investigated using differential thermal analysis and capacitance measurements up to 10 MHz. The phase behaviour has been studied between 80 and 340 K and pressures up to 300 MPa. The well‐known phase transformations at 1 atm have also been observed at elevated pressures that enables to calculate the associated volume changes. The static and complex permittivities have been determined in the temperature range 240–310 K and pressures up to 300 MPa. Activation parameters derived from the dielectric relaxation occurring in the two plastic phases are similar to previously reported data for cyclopentanol and cyclohexanol.

Solid phases in cycloheptanol: Dynamic disorder and glass transitions

“…From an earlier work [27], it seems to be clear that three different solid phases (denoted on cooling from the liquid state as I, II and III) could be identified down to about 170 K. Their transition temperatures (earlier consigned in Table 1 of the Ref. [27]) seem to be consistent with those obtained from several experimental techniques, in previous published works [21][22][23]25]. Preliminary X-ray diffraction experiments [27] revealed that phase I is simple cubic with a unit-cell parameter of 11.54(1) Å at 273 K (Z = 8) and phase II is tetragonal with unit-cell parameters of a = 19.487(2) Å and c = 11.7805(2) Å at 248.15 K (Z = 24).…”

“…The molar volume changes at the III-to-II and II-to-I phase transitions, obtained from X-ray technique, are 0.07 and 1.15 cm 3 mol À1 , respectively. They are in accord with those obtained from pressure-temperature data using the Clausius-Clapeyron equation [23].…”

“…X-ray diffraction measurements performed up to 273 K confirm the published [27] stable polymorphism above 175 K. According to the reported lattice symmetries for phases III, II and I, lattice parameters were refined at each measured temperature by means of the pattern-matching option of the FULLPROF program. The knowledge of the volume change at the III-to-II phase transition from pressure-temperature data [23] along with the calculated molar volume of phase II from the lattice parameters allows to calculate the number of molecules per unit cell (Z) for phase III, resulting in 24. According to this data and the refined lattice parameters, the molar volume of phases III, II and I has been calculated and plotted in Fig.…”

“…There exists a wide consensus in the scientific literature about three stable solid phases, although, some evidences for an additional stable solid phase have been reported [20,22]. The first two solid phases (phases I and II) appearing on cooling the liquid state are undoubtedly OD-states [20,21,23] and they can be supercooled, phase I much more easily than phase II, giving rise to two different OG-states. It is clear that C 7 -OH is a double OGforming material, but the question that arises now is, could the C 7 -OH be considered as a CG-forming material such as its homologue C 8 -OH?…”

“…As for its solid state polymorphism, a great amount of works has been published up to now, in which several experimental techniques have been used [20][21][22][23][24][25][26][27]. Surprisingly, next to nothing devoted to the structural features of the solid states has been published [27].…”