Haining Li

Kinetic Effect on Pressure-Induced Phase Transitions of Room Temperature Ionic Liquid, 1-Ethyl-3-methylimidazolium Trifluoromethanesulfonate

Room temperature ionic liquids (RTILs) have intriguing high-pressure phase behavior, and investigation of how pressure affects phase transitions of RTILs might yield interesting results. We here present kinetically driven phase transitions of 1-ethyl-3-methylimidazolium trifluoromethanesulfonate ([Emim][CF3SO3]) at different rates of ∼0.3 and ∼1.2 GPa/h up to ∼5 GPa. Two crystalline phases formed at ∼1.3 and ∼1.7 GPa with increasing pressure at lower compression rate of ∼0.3 GPa/h; however, the amorphous phase solidified with superpressurized glass above ∼3.3 GPa at higher compression rate of ∼1.2 GPa/h. Notably, crystal polymorphism is discussed in view of the conformational isomerism of [Emim](+) cation and an unknown cation conformer is observed. These facts indicate that kinetic effect on pressure-induced phase transitions of [Emim][CF3SO3] might be dependent on compression rate, which needs to be considered as a non-negligible factor for phase transitions of RTILs under high pressure.

In situ crystallization of ionic liquid [Emim][PF6] from methanol solution under high pressure.

The solubility of 1-ethyl-3-methylimidazolium hexafluorophosphate ([Emim][PF6]) in methanol under high pressure is newly measured quantitatively according to the correlation between the ratios of Raman intensity and the concentrations. In situ crystallization and cation conformation of [Emim][PF6] from methanol solution under high pressure have been investigated by using Raman spectroscopy in detail. Remarkably, crystal polymorphism was observed and two crystalline phases (phases I and II) coexisted under high pressure up to ∼ 1.4 GPa. However, only phase II was obtained by recrystallization at ∼ 2 GPa. Our findings may facilitate the development of an effective way for crystallization and purification of ionic liquids under high pressure.

In situ solidification of ionic liquid [EMIM][EtOSO3] from melt under high pressure

In situ solidification of 1-ethyl-3-methylimidazolium ethyl sulfate [EMIM][EtOSO3] from melt under high pressure has been investigated by using Raman spectroscopy. The results indicate that [EMIM][EtOSO3] might experience a phase transition at about 2.4 GPa upon compression, which could be identified as solidification to a superpressurized glass by pressure broadening of the sharp ruby R1 fluorescence line. Upon cooling, it solidifies as a glassy state rather than crystallizes at low temperature down to 93 K. These facts are suggestive of a phase transition of liquid to a superpressurized glass induced by compression in [EMIM][EtOSO3], which is similar to the glassy state at low temperature.

In situ observation of gelation of methylcellulose aqueous solution with viscosity measuring instrument in the diamond anvil cell

Gelation of methylcellulose aqueous solution was investigated by a high-pressure viscosity measurement device which consisted of diamond anvil cell, microscope and CCD. And the temperature and pressure dependence of the viscosity of methylcellulose aqueous solution was measured utilizing a rolling-ball technique. The results showed that sol-gel thermal transition of methylcellulose solution occurred at the temperature of 53 °C under atmospheric pressure. Upon compression, it was indicated that the viscosity showed a dramatic change in the vicinity of the pressure of 500 MPa. Parabolic phase diagram of methylcellulose aqueous solution was constructed, and it showed that the melting point was an increasing function of pressure at the first stage and an decreasing function of pressure at the final stage. The mechanism of sol-gel transformation of methylcellulose aqueous solutions was also discussed, it might be assumed that both hydrogen and hydrophobic bonds were involved with the gel formation in the case of methylcellulose aqueous solution.

High pressure and low-temperature polymorphism in cyclopentanol

The polymorphism of cyclopentanol (C5H10O) has been investigated as a function of temperature at ambient pressure and as a function of hydrostatic pressures to 3.7 GPa at room temperature. Differential scanning calorimetry (DSC) and Raman spectra reveal that two plastic phases and two fully ordered crystalline phases are formed during cooling. High pressure Raman and infrared spectra show that cyclopentanol undergoes two-phase transformations. At around 0.6 GPa, the liquid cyclopentanol transforms to a solid plastic structure. On further compression to 1.9 GPa, one fully ordered crystalline phase is observed. Based on pieces of evidence such as peak splitting and emergence of new peaks, it can be concluded that the ordered crystalline structure has a lower symmetry. In addition, the decrease in the wavenumber of the O–H stretching modes at low temperature and high pressure suggests the ordered crystalline phases are characterized by the formation of hydrogen-bonded molecular chains.

Pressure-induced ionic liquid crystal in 1-dodecyl-3-methylimidazolium tetrafluoroborate

The phase behaviors of 1-dodecyl-3-methylimidazolium tetrafluoroborate ([C12MIM][BF4]) had been investigated by means of Raman spectroscopy and polarized optical microscopy under pressure values up to 2.0 GPa at the temperature of 80.0 °C. Upon compression, change in the ratio of peak heights of symmetric and asymmetric CH2 stretching modes of Raman spectra in [C12MIM][BF4] (r(CH2)ss/(CH2)as) indicated that it might experience two successive phase transitions. The structure evolution of the sample, which was investigated through the image analysis from polarized optical microscopy, was found to share many of the known quantitative properties of the smectic A phase of [C12MIM][BF4]. These facts were suggestive of ionic liquid crystal induced by compression in [C12MIM][BF4] under the pressure between 0.25 GPa to 0.60 GPa, which was similar to ionic liquid crystal upon cooling from its melt.