Takafumi Aizawa

Relation between Volume Expansion and Hydrogen Bond Networks for CO2−Alcohol Mixtures at 40 °C

We experimentally determined the density and mole fraction of CO(2) (x(CO(2))) for CO(2)-alcohol (methanol, ethanol, propanol, butanol, isopropyl alcohol, and tert-butyl alcohol) mixtures and performed molecular dynamics (MD) simulations to study the mechanisms of volume expansion at 40 °C. The volume as calculated by vapor-liquid equilibrium (VLE) data increased with decreasing alkyl chain length, although there was no effect of branched alkyl groups. Analysis of the hydrogen bond network showed that the average number of hydrogen bonds per alcohol molecule decreased with increasing branched methyl groups. At pure alcohol condition, large size hydrogen bond networks were made. With further addition of CO(2) molecules, it became difficult to contain the large hydrogen bond networks. Furthermore, the hydrogen bond networks changed to a cyclic pentamer or tetramer, and volume expansion occurred.

Formation of Self-Ordered TiO2 Nanotubes by Electrochemical Anodization of Titanium in 2-Propanol/ NH4F

Ti0 2 nanotubes (TiNTs) were formed by the electrochemical anodization of titanium in 2-propanol/water containing 0.14 M NH 4 F as the supporting electrolyte. The effects of the water content, time of anodization, and potential on the growth behavior of TiNT were studied. At the optimum solvent composition of 16 vol % water/84 vol % 2-propanol, the reaction was kinetically controlled. The TiNTs obtained under optimum conditions had heights of up to 1800 nm and inner diameters of about 90 nm. Increasing the potential beyond 20 V did not improve the quality of the nanotubes because of the introduction of disordering. The scanning electron microscopy and X-ray diffraction analysis revealed a preferred growth direction of the TiNTs relative to the substrate surface. A thin barrier layer existed at the nanotubes/metal interface, which was also confirmed by ellipsometry. This barrier layer was enriched with the rutile phase whereas the nanotubes were enriched with the anatase phase.

Determination of Kamlet-Taft solvent parameters π* of high pressure and supercritical water by the UV-Vis absorption spectral shift of 4-nitroanisole

Kamlet-Taft solvent parameters, pi*, of high pressure and supercritical water were determined from 16-420 degrees C based on solvatochromic measurements of 4-nitroanisole. For the measurements, an optical cell that could be used at high temperatures and pressures was developed with the specification of minimal dead space. The low dead space cell allowed us to measure the absorption spectra of 4-nitroanisole at high temperature conditions before appreciable decomposition occurred. The behavior of pi* in terms of water density (pi* = 1.77rho- 0.71) was found to be linear, except in the near critical region, in which deviations were observed that could be attributed to local density augmentation. Excess density, which was defined as the difference between local density and bulk density, showed a maximum near the critical density of water. The frequencies of UV-Vis spectra of 4-(dimethylamino)benzonitrile and N,N-dimethyl-4-nitroaniline were correlated with pi* based on a linear solvation energy relationship (LSER) theory. Local density augmentation around 4-nitroanisole and that around 4-(dimethylamino)benzonitrile were similar but the augmentation observed around N,N-dimethyl-4-nitroaniline was larger.

Local density augmentation from fluorescence lifetime for anthracene N,N-dimethylaniline exciplex in supercritical carbon dioxide

Abstract Local density augmentation around exciplex between anthracene and N , N -dimethylaniline in supercritical carbon dioxide was measured by fluorescence lifetime at 40 °C and at pressures from 9.3 to 19.1 MPa. In the near-critical (9.3–10.5 MPa) region, the exciplex was more stable than that predicted by Kirkwood analysis, which means strong influence of local density augmentation around the exciplex.

Determination of anisotropic solvation structure of octafluorotoluene in supercritical carbon dioxide by means of solvent-induced 19F NMR chemical shift

Abstract 19 F NMR chemical shifts of octafluorotoluene and hexafluorobenzene in dilute carbon dioxide solutions were precisely determined at a fixed temperature of 314.4 K over a wide range of pressure from 1 to 35 MPa. We newly present the analytical procedure to determine anisotropic solvation structure by means of a solvent-induced 19 F NMR chemical shift. The anisotropic solvation structure around octafluorotoluene in supercritical carbon dioxide was discussed in comparison with a more symmetric geometry of hexafluorobenzene.

Numerical simulation of two-dimensional piston effect and natural convection in a square cavity heated from one side

Piston effect and natural convection in two-dimensional square cavity heated from one side are solved numerically in which it is assumed that the temperature of the left-hand-side wall is suddenly increased at t=0. According to the piston effect, the bulk temperature suddenly rises and convection cells are generated near not only hot wall but also cold wall. However, the rise of temperature due to natural convection has no influence on the piston effect, because the difference in time scale between the natural convection and piston effect is very large

“Totsu”-window optical cell for absorption and emission studies of high-pressure liquids and supercritical fluids

Abstract A high-pressure optical cell was developed for studying absorption and fluorescence phenomena in high-pressure liquids or supercritical fluids. The characteristic features of the cell are a “totsu” (denoting the shape ) type window that enhances the effective excitation and a two-compartment sample chamber that facilitates sample preparation. Transient absorption measurements of the benzophenone (BP)– N , N -dimethylaniline (DMA) system in supercritical carbon dioxide are reported to demonstrate the system performance.

Pressure dependence of acetophenone N,N,N′,N′-tetramethylbenzidine exciplex in supercritical carbon dioxide

Abstract The pressure dependence of the formation and decay processes of exciplex between two neutral species acetophenone (AP) and N , N , N ′ , N ′ -tetramethylbenzidine (TMB) in supercritical carbon dioxide was investigated by a transient absorption technique at 40 °C and at pressures from 9.0 to 14.7 MPa. The exciplex formation rate constant could be described by the Stokes–Einstein/Debye (SE/D) equation. The trend of the exciplex decay rate constant showed local density augmentation around the exciplex in the low-pressure region.

Phase behavior of Xe confined in porous vycor glass probed by 129Xe NMR chemical shift

129 Xe NMR spectroscopy is applied for direct observation of Xe confined in the porous Vycor glass. The chemical shifts of bulk and confined Xe are obtained simultaneously in a wide range of density from the gas phase to the liquid phase, including the supercritical region. The temperature and pressure dependences of the chemical shift of confined Xe are different from those of bulk Xe, which are approximately expressed by a linear function of bulk density. The phase transition of confined Xe is detected by the chemical shift, of which the pressure dependence along isotherms shows hysteresis loops.

Cosolvent effect on enhancement of reaction rate constant in near-critical region

Abstract Triplet–triplet annihilation of anthracene in supercritical carbon dioxide with and without acetonitrile cosolvent was studied with a transient absorption technique. In pure CO2, enhancement of the reaction rate was observed. Extinction coefficients and quantum yields of triplet state anthracene in the near-critical region sharply increased and decreased, respectively. In the presence of cosolvent, however, the enhancement of the reaction rate constant in the near-critical region decreased with increasing amounts of cosolvent. Under these conditions, both the extinction coefficients and quantum yields of triplet state anthracene showed less variation providing strong evidence that the cosolvent shields specific interactions between solute and the CO2 solvent.