In this article, the high-pressure optical cells used for measurements of the non-linear laser spectroscopic methods under supercritical conditions are described in details. The high-pressure optical cells with a large angle aperture have been developed for the transient grating measurement and resonance Raman measurement in supercritical water. The study on the vibrational energy relaxation of the ground-state azulene molecule in supercritical fluids by the acoustic signal of the transient grating measurement is described in detail. The study revealed that the V-V energy transfer from the photo-excited azulene molecule to the solvent ethane molecules plays an important role in the energy transfer. The results on measurements of the resonance Raman spectra of p-nitroaniline and N,N-dimethyl-p-nitroaniline are also presented.
A number of studies on collective dynamics of hydrogen bonded liquids, such as water and methanol, with high-resolution inelastic X-ray scattering (IXS) have been reported for the last decade. The IXS technique has become a powerful tool owing to third-generation X-ray synchrotron radiation facilities. The dynamical transition of water from hydrodynamic behavior to viscoelastic one at the Q that was predicted by MD simulation was experimentally revealed by the IXS technique. The positive dispersion of dynamical velocity of sound of supercritical water (SCW) and methanol (SCM) drastically changes with density, reflecting cluster formation in them. The IXS results reveal the dynamic features in SCW and SCM, which are consistent with the previous structure data obtained from the diffraction and spectroscopic methods.
Acid-base equilibria in supercritical water and alcohols are of great importance in controlling chemical processes in the high-temperature and high-pressure media. Recent advances in spectroscopic measurement of acid-base equilibria in supercritical water are reviewed together with those in conductmetric and potentiometric methods. I also present our recent study of acid-base equilibria in supercritical methanol using a flow-through apparatus and 2,5-dichlorophenol (DCP) as an indicator. It is explained how to determine the degree of dissociation of DCP in ambient to supercritical methanol from the absorption spectra at various concentrations of base.
Solvation dynamics and rotational relaxation in supercritical fluids have been examined using the techniques of the subpicosecond laser spectroscopy. The fluorescence up-conversion technique has been employed to detect the fluorescence anisotropy decay of coumarin 153 in supercritical CO2 (310 K) and CHF3 (302, 310 K) over a wide density range. The rotational correlation times in supercritical CHF3 exhibit the remarkable increase at 0.6-0.7 times ρc, which implies that the local solvent density augmentation slows down the rotational dynamics in supercritical fluids. This conclusion is supported by computer simulations based on a 2-site model of fluoroform. The solvation dynamics of the probe 4-dimethylamino-4'-cyanostilbene have been measured in supercritical CHF3 at 310 K using the optical Kerr-gated emission technique, which appears to have captured most (∼90%) of the dynamics occurring over the accessible density range between 1.4-2.0ρc. The measured spectral response exhibits a bimodal decay with ∼0.6 ps and ∼9 ps components and only a modest density dependence.
The increasing use of supercritical fluids in a wide range of practical applications has motivated a number of resent attempts to understand the fundamental aspects of fluid structure. In this article, recent advances in experimental studies on local structure of supercritical fluids are reviewed. Especially, the local structures probed by three spectroscopic methods, i.e. dynamic light scattering, terahertz absorption, and vibrational Raman spectroscopies, are reported. These experimental results and theoretical analysis allow us to understand the local structure from the point of view on molecular motion, i.e. translational diffusion, rotational, and vibrational motions, by changing continuously density from the gas-like to liquid-like conditions.
High-pressure NMR spectroscopy enables us direct observation of fluids confined in nanospace. Since two distinct signals due to bulk and confined fluids are observed, the chemical shifts and the longitudinal relaxation times are obtained simultaneously. The temperature and pressure dependences of 129Xe chemical shift of confined Xe strongly depend on the pore size and the surface properties of the matrices, while those of bulk Xe are approximately a linear function of density. The mean density in nanopore, which is estimated from the integrated signal intensities, accounts well for the 19F longitudinal relaxation times of confined SF6 and CHF3.
The ultrahigh-pressure technique in multimegabar pressures range has been developed using a diamond anvil cell (DAC) and the feasible range of static pressure is extended beyond 300 GPa. By synchrotron radiation x-ray diffraction and Raman scattering experiments on the base of this technique, high-pressure behaviors of elemental materials, that is, O2, S, Se, Te, P, Mg, Al, Ti and Sc have been investigated, and unique and/or unexpected high-pressure phenomena, such as structure phase transitions, metallization and molecular dissociation are revealed at ultra-high pressures. These results will help scientists to enrich their understanding (physical view) of materials under the extremely high-density condition.
We have developed a new method to grow single crystals of transition metal oxides under high pressures of several GPa, using a cubic anvil press. Here in situ observations of the reactions under high pressures are performded by synchrotron powder X-ray diffrraction, to find out the optimum crystal growth conditions. We have succeeded in growing single crystals of various transition metal oxides such as PrNiO3 and the high pressure phase of (VO)2P2O7. We thus expect that the present high pressure technique plays more and more important roles to develop studies using single crystalline samples.
Natural-gas hydrate field under the deep-ocean floor has become the object of public attention as a potential unconventional energy resource. In addition, the storage and transport systems of natural-gas or hydrogen using gas hydrates, and high-pressure separation of hydrogen from gas-mixtures containing carbon dioxide have been a focus of constant attention as new techniques. In order to proceed toward the realization of them, the studies on physical and chemical properties of gas hydrate crystal have been accelerated. In this article, recent thermodynamic and Raman spectroscopic studies on pure and mixed gas hydrate systems are reviewed with particular emphasis on the cage occupancy of guest species and the structural phase transition.
The commercial-scale feasibility of high-pressure treatment to fermented foods was investigated, with attention given to the fact that the pressure resistance varies with the kind of microorganism. Kimchi was chosen because lactic acid bacteria had been found to be separated from yeast by pressure application of 300 MPa (20°C), and subjected to pressure treatment to investigate the microbiological, physicochemical and sensory changes with the progress of fermentation. Pressure application of 300 MPa (20°C) to Kimchi for 5 minutes slightly decreased the number of lactic acid bacteria, while completely inactivated the yeast to prevent expansion of the Kimchi-containing package during the storage. The pressure treatment also reduced the generation of lactic acid, the drop in pH value and the decrease of glucose and fructose contents. Although the pressure-treated Kimchi showed increased color difference and translucence, the breaking stress was not changed. The sensory tests demonstrated the advantages of pressure-treated Kimchi, to confirm the improvement of shelf life of Kimchi.