Spatial hydration maps and dynamics of naphthalene in ambient and supercritical water

The hydration structures and dynamics of naphthalene in aqueous solution are examined using molecular-dynamics simulations. The simulations are performed at several state points along the coexistence curve of water up to the critical point, and above the critical point with the density fixed at 0.3 g/cm(3). Spatial maps of local atomic pair-density are presented which show a detailed picture of the hydration shell around a bicyclic aromatic structure. The self-diffusion coefficient of naphthalene is also calculated. It is shown that water molecules tend to form pi-type complexes with the two aromatic regions of naphthalene, where water acts as the H-bond donor. At ambient conditions, the hydration shell of naphthalene is comprised, on average, of about 39 water molecules. Within this shell, two water molecules can be identified as pi-coordinating, forming close to one H-bond to the aromatic rings. With increasing temperature, the hydration of naphthalene changes dramatically, leading to the disappearance of the pi-coordination near the critical point.     

Noncatalytic kinetic study on site-selective H/D exchange reaction of phenol in sub- and supercritical water

The site-selective H/D exchange reaction of phenol in sub- and supercritical water is studied without added catalysts. In subcritical water in equilibrium with steam at 210-240 degrees C, the H/D exchange proceeds both at the ortho and para sites in the phenyl ring, with no exchange observed at the meta site. The pseudo-first-order rate constants are of the order of 10(-4) s(-1); 50% larger for the ortho than for the para site. In supercritical water, the exchange is observed also at the meta site with the rate constant in the range of 10(-6)-10(-4) s(-1). As the bulk density decreases, the exchange slows down and the site selectivity toward the ortho is enhanced. The enhancement is due to the phenol-water interaction preference at the atomic resolution. The site selectivity toward the ortho is further enhanced when the reaction is carried out in benzene/water solution. Using such selectivity control and the reversible nature of the hydrothermal deuteration/protonation process, it is feasible to synthesize phenyl compounds that are deuterated at any topological combination of ortho, meta, and para sites.     

Dispersion stability of colloids in sub- and supercritical water

Dispersion stability of colloids has been investigated in sub- and supercritical water by measuring the hydrodynamic diffusion coefficients of the particles by means of dynamic light scattering. It is interestingly found that coagulation of the colloids in sub- and supercritical water is a universal phenomenon irrespective of the material of the colloids. Highly charged colloids were found to be more stable in water against high temperature. Numerical analysis reveals that the stability of the colloids at elevated temperature and pressure is primarily governed by the temperature dependence of the dielectric constant of the medium. The effect of the temperature dependence of the ion product of water (pKw) was found to be very little. Surface charge density and Stern potential may change with respect to temperature due to the readjustment of the ion concentration in the diffuse layer through the enhanced ion product and reduced dielectric constant of water. These are the secondary causes of the particle coagulations in sub- and supercritical water.     

Radial distribution functions of sub- and supercritical water according to the nonempirical molecular dynamics data

The Car-Parrinello nonempirical molecular dynamics method was used to obtain radial distribution functions of water at the critical point and in six sub- and supercritical states. The influence of changes in state parameters on radial distribution functions was found to be much stronger close to the saturation curve than in the region of high pressures. The reproduction of radial distribution functions by classical and quantum molecular dynamics methods was analyzed. The positions of radial distribution peaks and the ratios between their heights were found to be almost identical and to correspond to the experimental data, but, as concerns quantitative estimates of peak heights, the same contradictions are observed as between the data of various experimental studies.     

Uncatalyzed partial oxidation of p-xylene in sub- and supercritical water

In sub- and supercritical water, partial oxidation of p-xylene was performed in a batch reactor without a catalyst at 240-500^oC, 220-300 bar. The loaded amount of hydrogen peroxide was set to 0-100% of the theoretically required oxygen amount. Conversion of p-xylene was reached over 99% within 15-20 min. In sub- and supercritical water, we propose two parallel pathways and major products that consist of p-tolualdehyde, p-toluic acid, terephthalic acid, toluene and benzaldehyde. Yields of major products in subcritical conditions were higher than in a supercritical conditions.     

The structure of H-bonded clusters in sub- and supercritical water

The structure of H-bonded complexes in sub- and supercritical water in regions close to and remote from the saturation curve was studied. The Car-Parrinello method was used to calculate water dipole moment distributions in 11 thermodynamic states. The dependence of the mean dipole moment of water molecules on the size of clusters and number of H-bonds was obtained.     

Kinetic study for liquefaction of tar in sub- and supercritical water

Liquefaction of tar from oil distillation was studied under sub- and supercritical water conditions using a batch reactor at 623 and 673 K and 25-40 MPa. The reaction scheme for tar liquefaction was determined as follows: the liquefaction process of tar occurs first and then intermediate chemical compounds are transformed into lighter molecular weight species. The effects of pressure and treatment time were combined into a single severity parameter that was used to monitor the conversion of tar. The main products from the liquefaction of tar were phenol (3.44 wt%), biphenyl (2.23 wt%), diphenylether (13.70 wt%) and diphenylmethane (1.30 wt%), respectively. Liquefaction of tar clearly increased with increasing water density at the same temperature reaction. It indicates that hydrolysis was important in the cleavage of the macromolecular structure of tar under sub- and supercritical conditions. Based on the results, this method could become an efficient method for tar liquefaction, producing high yields of valuable chemical intermediates.     

Collective dynamics of sub- and supercritical methanol by inelastic X-ray scattering

Inelastic X-ray scattering experiments have been performed on methanol as a function of density from ambient to the supercritical state. Positive dispersion of the sound velocity, as compared to the hydrodynamic values, is 50% in the ambient condition and decreases to zero at 0.50 g cm⁻³ over the momentum transfer Q = 1–10 nm⁻¹ with lowering density; however, it increases again with a further decrease in density down to 0.20 g cm⁻³in the supercritical state only in the Q-range above 5 nm⁻¹. These results have been interpreted as the formation of small oligomers in the low-density supercritical methanol.     

Features of ion hydration and ion association in sub- and supercritical aqueous solutions of rubidium bromide

Results of investigations by the integral equation method in the XRISM approximation of features of structure formation in sub- and supercritical 0.5 M aqueous solution of RbBr. As the system passes from the sub- to supercritical state, the tetrahedral network of the solvent is destroyed, which is typical of water and aqueous solutions; thermal dehydration of the cation and anion are also observed. Dehydration is accompanied by contraction of the first hydration sphere of Rb⁺ and extension of the first hydration sphere of Br⁻. Both in sub- and supercritical solutions conditions a minor amount of contact ion pairs is present whose fraction is Higher by 15% compared to that in standard conditions.     

Intermediates and kinetics for phenol gasification in supercritical water

We processed phenol with supercritical water in a series of experiments, which systematically varied the temperature, water density, reactant concentration, and reaction time. Both the gas and liquid phases were analyzed post-reaction using gas chromatographic techniques, which identified and quantified the reaction intermediates and products, including H(2), CO, CH(4), and CO(2) in the gas phase and twenty different compounds--mainly polycyclic aromatic hydrocarbons--in the liquid phase. Many of these liquid phase compounds were identified for the first time and could pose environmental risks. Higher temperatures promoted gasification and resulted in a product gas rich in H(2) and CH(4) (33% and 29%, respectively, at 700 °C), but char yields increased as well. We implicated dibenzofuran and other identified phenolic dimers as precursor molecules for char formation pathways, which can be driven by free radical polymerization at high temperatures. Examination of the trends in conversion as a function of initial water and phenol concentrations revealed competing effects, and these informed the kinetic modeling of phenol disappearance. Two different reaction pathways emerged from the kinetic modeling: one in which rate ∝ [phenol](1.73)[water](-16.60) and the other in which rate ∝ [phenol](0.92)[water](1.39). These pathways may correspond to pyrolysis, which dominates when there is abundant phenol and little water, and hydrothermal reactions, which dominate in excess water. This result confirms that supercritical water gasification of phenol does not simply follow first-order kinetics, as previous efforts to model phenol disappearance had assumed.     

Ultrafast hydration dynamics in the lipidic cubic phase: Discrete water structures in nanochannels

We report here our studies of hydration dynamics of confined water in aqueous nanochannels (approximately 50 A) of the lipidic cubic phase. By systematically anchoring the hydrocarbon tails of a series of tryptophan-alkyl ester probes into the lipid bilayer, we mapped out with femtosecond resolution the profile of water motions across the nanochannel. Three distinct time scales were observed, revealing discrete channel water structures. The interfacial water at the lipid surface is well-ordered, and the relaxation dynamics occurs in approximately 100-150 ps. These dynamically rigid water molecules are crucial for global structural stability of lipid bilayers and for stabilization of anchored biomolecules in membranes. The adjacent water layers near the lipid interface are hydrogen-bonded networks and the dynamical relaxation takes 10-15 ps. This quasi-bound water motion, similar to the typical protein surface hydration relaxation, facilitates conformation flexibility for biological recognition and function. The water near the channel center is bulklike, and the dynamics is ultrafast in less than 1 ps. These water molecules freely transport biomolecules near the channel center. The corresponding orientational relaxation at these three typical locations is well correlated with the hydration dynamics and local dynamic rigidity. These results reveal unique water structures and dynamical motions in nanoconfinements, which is critical to the understanding of nanoscopic biological activities and nanomaterial properties.     

Molecular dynamic simulation of sub- and supercritical water with new interaction potential

Molecular dynamic simulation of water under sub- and supercritical conditions was performed with a new form of O···H pairwise potential with an additional non-electrostatic term corresponding hydrogen bond formation. Some structural properties of water and hydrogen bonds characteristics were calculated in a wide temperature range and pressure 50 MPa. Dependences of these values on temperature were analyzed.     

Chiral separations in sub- and supercritical fluid chromatography

Sub- and supercritical fluid chromatography (SFC) received more and more attention in pharmaceutical analysis during the last years. Especially for chiral separations, this technique is becoming increasingly popular. This review gives an overview of most chiral separation applications using SFC, covering the literature from 2000 on.     

Investigating hydration dependence of dynamics of confined water: monolayer, hydration water and Maxwell-Wagner processes

The dynamics of water confined in silica matrices MCM-41 C10 and C18, with pore diameter of 21 and 36 A, respectively, is examined by broadband dielectric spectroscopy (10(-2)-10(9) Hz) and differential scanning calorimetry for a wide temperature interval (110-340 K). The dynamics from capillary condensed hydration water and surface monolayer of water are separated in the analysis. Contrary to previous reports, the rotational dynamics are shown to be virtually independent on the hydration level and pore size. Moreover, a third process, also reported for other systems, and exhibiting a saddlelike temperature dependence is investigated. We argue that this process is due to a Maxwell-Wagner process and not to strongly bound surface water as previously suggested in the literature. The dynamics of this process is strongly dependent on the amount of hydration water in the pores. The anomalous temperature dependence can then easily be explained by a loss of hydration water at high temperatures in contradiction to previous explanations.     

Hydrogen bond and dipole moment in sub- and supercritical water close to the saturation curve

The Car-Parrinello molecular dynamics method was used to determine the distributions of water dipole moments under normal conditions, at the critical point, and in six thermodynamic states in sub- and supercritical phase diagram regions. Dipole moment changes along the saturation curve, the 650 K isotherm, and the 30 MPa isobar were analyzed.     

Study of H-bond characteristics in sub- and supercritical methanol

Classical molecular dynamics simulations of various methanol phase lines near the saturation curve and the critical point have been performed to study the changes in H-bonded clusters structure at transition of methanol to supercritical state. Analysis of H-bonds statistics with combined distance-energy H-bond criterion showed that the correlations between topological characteristics of H-bonds and the mole fraction of H-bonded molecules have unique functional representation despite the phase path applied. In the present study, an attempt has been also made to evaluate the degree of hydrogen bonding by combining the DFT computations on classical MD configurations with the natural bond orbital analysis of the waves functions obtained.     

Non-catalytic and selective alkylation of phenol with propan-2-ol in supercritical water

Phenol can be alkylated with propan-2-ol without catalyst in supercritical water at 673 K with mainly ortho substituted alkylphenols being obtained and alkylation reaction rate increasing with increasing water density.     

Entropy and dynamics of water in hydration layers of a bilayer

We compute the entropy and transport properties of water in the hydration layer of dipalmitoylphosphatidylcholine bilayer by using a recently developed theoretical scheme [two-phase thermodynamic model, termed as 2PT method; S.-T. Lin et al., J. Chem. Phys. 119, 11792 (2003)] based on the translational and rotational velocity autocorrelation functions and their power spectra. The weights of translational and rotational power spectra shift from higher to lower frequency as one goes from the bilayer interface to the bulk. Water molecules near the bilayer head groups have substantially lower entropy (48.36 J/mol/K) than water molecules in the intermediate region (51.36 J/mol/K), which have again lower entropy than the molecules (60.52 J/mol/K) in bulk. Thus, the entropic contribution to the free energy change (TΔS) of transferring an interface water molecule to the bulk is 3.65 kJ/mol and of transferring intermediate water to the bulk is 2.75 kJ/mol at 300 K, which is to be compared with 6.03 kJ/mol for melting of ice at 273 K. The translational diffusion of water in the vicinity of the head groups is found to be in a subdiffusive regime and the rotational diffusion constant increases going away from the interface. This behavior is supported by the slower reorientational relaxation of the dipole vector and OH bond vector of interfacial water. The ratio of reorientational relaxation time for Legendre polynomials of order 1 and 2 is approximately 2 for interface, intermediate, and bulk water, indicating the presence of jump dynamics in these water molecules.     

A new high-temperature multinuclear-magnetic-resonance probe and the self-diffusion of light and heavy water in sub- and supercritical conditions

A high-resolution nuclear-magnetic-resonance probe (500 MHz for 1H) has been developed for multinuclear pulsed-field-gradient spin-echo diffusion measurements at high temperatures up to 400 degrees C. The convection effect on the self-diffusion measurement is minimized by achieving the homogeneous temperature distributions of +/-1 and +/-2 degrees C, respectively, at 250 and 400 degrees C. The high temperature homogeneity is attained by using the solid-state heating system composed of a ceramic (AlN) with high thermal conductivity comparable with that of metal aluminium. The self-diffusion coefficients D for light (1H2O) and heavy (2H2O) water are distinguishably measured at subcritical temperatures of 30-350 degrees C with intervals of 10-25 degrees C on the liquid-vapor coexisting curve and at a supercritical temperature of 400 degrees C as a function of water density between 0.071 and 0.251 gcm3. The D value obtained for 1H2O is 10%-20% smaller than those previously reported because of the absence of the convection effect. At 400 degrees C, the D value for 1H2O is increased by a factor of 3.7 as the water density is reduced from 0.251 to 0.071 gcm3. The isotope ratio D(1H2O)D(2H2O) decreases from 1.23 to approximately 1.0 as the temperature increases from 30 to 400 degrees C. The linear hydrodynamic relationship between the self-diffusion coefficient divided by the temperature and the inverse viscosity does not hold. The effective hydrodynamic radius of water is not constant but increases with the temperature elevation in subcritical water.