A New Phase of Hydroquinone and Its Thermodynamic Properties

Abstract

Hydroquinone crystals have been found to undergo first-order phase transition to a denser polymorph with an enthalpy increment, as does resorcinol. The phase stable at room temperature (called R) transforms to the new phase (designated δ) only under elevated pressures. P-V-T relations of the new phase have been measured, and characteristics of the transitions and the triple point among the R phase, the δ phase, and the liquid have been estimated from the thermodynamic consistency. The thermal expansion coefficient, the internal pressure, and the heat capacities of the δ phase are significantly larger than those of the R phase. Rigid-body vibrations and/or librations may play an important role in the Rδ transition. Contributions of hydrogen bonds and the structure of the δ phase are discussed.

Thermal Properties and Transition Behavior of Host-Guest Compounds Under High Pressure

“…During isothermal pressurization in a diamond anvil cell at room temperature, they observed changes in the Raman spectra suggesting two transitions at 3.3 GPa and 12 GPa, respectively. The 3.3 GPa feature was attributed to a new phase, but considering the results of Naoki et al [42], it may correspond to the α to δ transition, which they observed at lower pressures and high temperatures. [44] found that the 3.3 GPa transition was reversible upon pressure decrease, but with a significant pressure hysteresis, and the ambient pressure phase was recovered below 1.3 GPa.…”

“…Yet another form of HQ, the δ-form, has been detected at elevated temperatures and pressures by Naoki et al [42]. In their differential thermal analysis (DTA) and volume measurements, they observed an endothermic transition from the α-form to a slightly denser phase (1.355 g cm -3 at 443.3 K and 78.5 MPa) on isobaric heating above 0.03 GPa.…”

“…Although the δ-form has a large molar heat capacity close to that of the liquid their entropies differ more than 6R [42].…”

“…In their differential thermal analysis (DTA) and volume measurements, they observed an endothermic transition from the α-form to a slightly denser phase (1.355 g cm -3 at 443.3 K and 78.5 MPa) on isobaric heating above 0.03 GPa. Naoki et al [42] used their DTA and volume results under pressure together with atmospheric pressure results to calculate the high pressure properties of the δ-form from thermodynamic relations. The temperature-pressure T-p phase line between the α-form and the new δ-form has a negative slope, as shown in Fig.…”

Structure of organic solids at low temperature and high pressure

“…Hydroquinone is an example of an inclusion compound which 55 exhibits a different crystal form under pressures exceeding ~20MPa. 42 At ambient pressure and temperature, hydroquinone has three forms: the clathrate-forming α and β and a more densely packed form, γ, which is accessible only via sublimation. The β and γ forms both spontaneously transform to the α form, 60 the most stable phase at room temperature.…”