Effect of Growth Stages on Anthocyanins and Polyphenols in the Root System of Sweet Potato.

The storage roots of purple-fleshed sweet potato contain a variety of anthocyanins and polyphenols. Little is known about changes in the total content and composition of anthocyanins and polyphenols in the early growth stages of the root system. In this study, we investigated the changes in anthocyanins and polyphenols in the root system of purple-fleshed sweet potato cultivars at 15, 30, 45, and 60 days after transplant (DAT). Unexpectedly, the highest percentage of acylated anthocyanins in three purple-fleshed cultivars among all growth stages was at 15 DAT. On the other hand, the total polyphenol content in the early growth stages of the root system increased rapidly toward 45 DAT, just before the beginning of storage root enlargement, and then decreased rapidly as the storage roots began to enlarge. These data indicate that the early growth stage of the root system is a critical time. This timing may present a strategy to maximize the accumulation of polyphenols with high antioxidant activity, as well as acylated anthocyanins, to protect against abiotic and biotic stresses.

Thermally Induced Phase Transition of Cubic Structure II Hydrate: Crystal Structures of Tetrahydropyran-CO2 Binary Hydrate.

We report a thermally induced phase transition of cubic structure II hydrates of tetrahydropyran (THP) and CO2 below about 140 K. The phase transition was characterized by powder X-ray diffraction measurements at variable temperatures. A dynamical ordering of the CO2 guests in small pentagonal dodecahedral 512 host water cages, not previously observed in the simple CO2 hydrate, occurs simultaneously with the symmetry lowering transition from a cubic structure II (space group Fd-3m with cell dimensions a = 17.3202(7) Å at 153 K) to a tetragonal (space group I41/amd with cell dimensions a = 17.484(4) Å and c = 12.145(1) Å at 138 K) unit cell. The effect of guest molecules on the phase transition at low temperatures is discussed, which demonstrates that the clathrate hydrate structures and thermodynamic properties can be modified by adjusting the size and chemical structure of larger and smaller guest molecules.

Molecular dynamics simulations of interfacial structure, dynamics, and interfacial tension of tetrabutylammonium bromide aqueous solution in the presence of methane and carbon dioxide.

The interfacial behavior of tetrabutylammonium bromide (TBAB) aqueous solutions in the absence of gas and the presence of methane and carbon dioxide gases is studied by molecular dynamics simulations. The aqueous TBAB phase, at concentrations similar to the solid semiclathrate hydrate (1:38 mol ratio), has a smaller interfacial tension and an increase in the gas molecules adsorbed at the interface compared to that in pure water. Both these factors may contribute to facilitating the uptake of the gases into the solid phase during the process of semiclathrate hydrate formation. At similar gas pressures, CO2 is adsorbed preferentially compared to CH4, giving it a higher surface density, due to the stronger intermolecular interactions of CO2 molecules of the solution at the interface. The increase in relative adsorption of CH4 at the solution surface compared to that in pure water surface is due to the hydrophobic interactions between the n-alkyl chains of the TBA+ cation and methane gas.

Investigation of crystal growth of CO2 hydrate in aqueous fructose solution for the potential application in carbonated solid foods.

CO2 hydrate is applicable to solid carbonated foods. The hydrate crystal morphology, which represents the crystal size and shape, is an important characteristic that changes the texture of foods. We report an observational study of the crystal growth of CO2 hydrate in aqueous fructose solution. The difference between the phase equilibrium temperature and the experimental temperature, ΔTsub, is applied as an index of the driving force. Experiments were performed at ΔTsub range of 0.9 K to 5.4 K. At all ΔTsub, initial crystal formed at the gas-solution interface and grew along the interface. After covering the interface, the crystals grew in the liquid phase The individual crystals were identified as polyhedral with facets (ΔTsub = 0.9 K), skeletal crystals (ΔTsub = 2.0 K) and dendrites (ΔTsub = 3.0 K and 5.4 K). Based on these results, the potential effect of gas hydrate morphology on texture of foods has been discussed.

Crystal growth of clathrate hydrate formed with H2 + CO2 mixed gas and tetrahydropyran.

Hydrate-based gas separation technology is applicable to the CO2 capture and storage from synthesis gas mixture generated through gasification of fuel sources including biomass. This paper reports visual observations of crystal growth dynamics and crystal morphology of hydrate formed in the H2 + CO2 + tetrahydropyran (THP) + water system with a target for developing the hydrate-based CO2 separation process design. Experiments were conducted at a temperature range of 279.5-284.9 K under the pressure of 4.9-5.3 MPa. To simulate the synthesis gas, gas composition in the gas phase was maintained around H2:CO2 = 0.6:0.4 in mole fraction. Hydrate crystals were formed and extended along the THP/water interface. After the complete coverage of the interface to shape a polycrystalline shell, hydrate crystals continued to grow further into the bulk of liquid water. The individual crystals were identified as hexagonal, tetragonal and other polygonal-shaped formations. The crystal growth rate and the crystal size varied depending on thermodynamic conditions. Implications from the obtained results for the arrangement of operating conditions at the hydrate formation-, transportation-, and dissociation processes are discussed.

Characterization of the Clathrate Hydrate Formed with Fluoromethane and Pinacolone: The Thermodynamic Stability and Volumetric Behavior of the Structure H Binary Hydrate.

To reveal the relation of guest dynamics within the structure H clathrate hydrate and its macroscopic physical properties, experimental and computational works have been conducted on the system of fluoromethane (HFC-41) and pinacolone coexisting with water. The phase boundaries of the hydrate formed from HFC-41 and pinacolone within the pressure range of (0.25-2.48) MPa and the temperature range of (277-293) K were measured. The equilibrium hydrate formation pressure incorporating HFC-41 was lowered by adding the pinacolone as a large guest molecule compound to form a sH phase compared to the HFC-41 single hydrate. Powder X-ray diffraction measurements confirmed the formation of the structure H hydrate with the HFC-41 and pinacolone binary hydrate. The lattice constants of the sH hydrate were also measured to see the effect of the help guest molecular size, which showed a different trend from that of the previous studies of sH pinacolone hydrates. Molecular dynamics simulations of the binary sH phase indicate weak hydrogen bonding of the pinacolone molecules with the water in the cages in the phase with HFC-41. The oblate HFC-41 molecules showed strong orientational preference to the equatorial planes of the D' cages, which may explain some of the trends in the behavior of this phase.

X-Ray attenuation and image contrast in the X-ray computed tomography of clathrate hydrates depending on guest species.

In this study, X-ray imaging of inclusion compounds encapsulating various guest species was investigated based on the calculation of X-ray attenuation coefficients. The optimal photon energies of clathrate hydrates were simulated for high-contrast X-ray imaging based on the type of guest species. The proof of concept was provided by observations of Kr hydrate and tetra-n-butylammonium bromide (TBAB) semi-clathrate hydrate using absorption-contrast X-ray computed tomography (CT) and radiography with monochromated synchrotron X-rays. The radiographic image of the Kr hydrate also revealed a sudden change in its attenuation coefficient owing to the K-absorption edge of Kr as the guest element. With a photon energy of 35 keV, X-ray CT provided sufficient segmentation for the TBAB semi-clathrate hydrate coexisting with ice. In contrast, the simulation did not achieve the sufficient segmentation of the CH4 and CO2 hydrates coexisting with water or ice, but it revealed the capability of absorption-contrast X-ray CT to model the physical properties of clathrate hydrates, such as Ar and Cl2 hydrates. These results demonstrate that the proposed method can be used to investigate the spatial distribution of specific elements within inclusion compounds or porous materials.

Thermophysical Property Measurements of Tetrabutylphosphonium Oxalate (TBPOx) Ionic Semiclathrate Hydrate as a Media for the Thermal Energy Storage System.

With increasing global power demand, thermal energy storage technology could play a role ensuring a sustainable energy supply in power generation from renewable energy sources and power demand concentration. Hydrates have high potential as phase change materials (PCMs) for the use as a thermal energy storage medium. To develop thermal energy storage technology using a hydrate-based material, further investigation of thermophysical properties and the selection of a suitable hydrate are required. Tetrabutylphosphonium oxalate (TBPOx) ionic semiclathrate hydrate contains oxalic acid in salt form, as a guest compound, which is classified as carboxylic acid group with low environmental impact. In the present study, the phase equilibrium temperature and the dissociation heat of TBPOx hydrate were measured. The highest equilibrium temperature of the solid hydrate formed was 9.4°C at the mass fraction 0.35 of TBPOx in aqueous solution. The largest dissociation heat was 186.0 ± 0.5 kJ·kg-1 at the mass fraction of 0.35. Comparing with other PCMs with close phase equilibrium temperatures, TBPOx hydrate is superior in safety and sustainability. These results indicate that TBPOx hydrate would be suitable as the thermal storage medium for the general air conditioning systems.

Molecular dynamics simulations of interfacial properties of the CO2-water and CO2-CH4-water systems.

Molecular dynamics simulations were performed to study the interfacial behavior of the pure carbon dioxide-water system and a binary 40:60 mol. % gas mixture of (carbon dioxide + methane)-water at the temperatures of 275.15 K and 298.15 K and pressures near 4 MPa for CO2 and up to 10 MPa for methane. The simulations are used to study the dynamic equilibrium of the gases at the water-gas interface, to determine the z-density profiles for the gases and water, and calculate the interfacial tension γ under the different temperature/pressure conditions close to those of the formation of clathrate hydrates of these gases. At the same hydrostatic gas phase pressure, the CO2-water interface has a lower interfacial tension than the CH4-water interface. A greater number of CO2 molecules, as much as three times more than methane at the same pressure, were adsorbed at the interfacial layer, which reflects the stronger electrostatic quadrupolar and van der Waals interactions between CO2 and water molecules at the interface. The water surfaces are covered by less than a monolayer of gas even when the pressure of the system goes near the saturation pressure of CO2. The surface adsorbed molecules are in dynamic equilibrium with the bulk gas and with exchange between the gas and interface regions occurring repeatedly within the timescale of the simulations. The effects of the changes in the CO2-water interfacial tension with external temperature and pressure conditions on the formation of the clathrate hydrates and other CO2 capture and sequestration processes are discussed.

Synthesis of Methane Hydrate from Ice Powder Accelerated by Doping Ethanol into Methane Gas.

Clathrate hydrate is considered to be a potential medium for gas storage and transportation. Slow kinetics of hydrate formation is a hindrance to the commercialized process development of such applications. The kinetics of methane hydrate formation from the reaction of ice powder and methane gas doped with/without saturated ethanol vapor at constant pressure of 16.55 ± 0.20 MPa and constant temperature ranging from -15 to -1.0 °C were investigated. The methane hydrate formation can be dramatically accelerated by simply doping ethanol into methane gas with ultralow ethanol concentration (<94 ppm by mole fraction) in the gas phase. For ethanol-doped system 80.1% of ice powder were converted into methane hydrate after a reaction time of 4 h, while only 26.6% of ice powder was converted into methane hydrate after a reaction time of 24 h when pure methane gas was used. Furthermore, this trace amount of ethanol could also substantially suppress the self-preservation effect to enhance the dissociation rate of methane hydrate (operated at 1 atm and temperatures below the ice melting point). In other words, a trace amount of ethanol doped in methane gas can act as a kinetic promoter for both the methane hydrate formation and dissociation.

Interfacial properties of hydrocarbon/water systems predicted by molecular dynamic simulations.

The presence of small hydrocarbons is known to reduce the interfacial tension of the gas-water interface, and this phenomenon can affect the formation of the clathrate hydrates of these gases. In this work, the interfacial behavior of the pure methane-, ethane-, and propane-water, and the ternary 90:7:3 mol. % gas mixture of (methane + ethane + propane)-water were studied with molecular dynamics simulations. The interfacial tension, γ, and z-density profiles for the gases and water from simulations of the gas-water systems were determined at the temperatures of 275.15 and 298.15 K, and pressures up to 10 MPa for methane and up to near the experimental saturation pressures of ethane and propane. The goal is to accurately calculate the interfacial tension for the hydrocarbon/water systems and to analyze the molecular behaviors at the interfaces which lead to the observed trends. At the same hydrostatic gas phase pressure, propane, ethane, and methane reduce the gas-water interfacial tension in that order. The local density of the gas molecules at the interface is enhanced relative to the bulk gas, and it was determined that about 13%-20%, 33%-40%, and 54%-59% of the gas molecules in the simulation congregated at the interfaces for the CH4-, C2H6-, and C3H8-water systems, respectively, at the different simulated hydrostatic pressure ranges. For all gases in the pressure range studied, a complete monolayer of gas had not formed at the water interface. Furthermore, a dynamic equilibrium with fast exchange between molecules at the interface and in the gas phase was observed. For the gas mixture, deviations were observed between total calculated interfacial tension, γmix, and the "ideal mixture" value, ∑xiγi,pure, calculated from the interfacial tensions of the pure gases, where xi is the mole fraction of each substance in the simulation. Some possible implications of the results on the mechanism of clathrate hydrate formation are discussed.

Understanding decomposition and encapsulation energies of structure I and II clathrate hydrates.

When compressed with water or ice under high pressure and low temperature conditions, some gases form solid gas hydrate inclusion compounds which have higher melting points than ice under those pressures. In this work, we study the balance of the guest-water and water-water interaction energies that lead to the formation of the clathrate hydrate phases. In particular, molecular dynamics simulations with accurate water potentials are used to study the energetics of the formation of structure I (sI) and II (sII) clathrate hydrates of methane, ethane, and propane. The dissociation enthalpy of the clathrate hydrate phases, the encapsulation enthalpy of methane, ethane, and propane guests in the corresponding phases, and the average bonding enthalpy of water molecules are calculated and compared with accurate calorimetric measurements and previous classical and quantum mechanical calculations, when available. The encapsulation energies of methane, ethane, and propane guests stabilize the small and large sI and sII hydrate cages, with the larger molecules giving larger encapsulation energies. The average water-water interactions are weakened in the sI and sII phases compared to ice. The relative magnitudes of the van der Waals potential energy in ice and the hydrate phases are similar, but in the ice phase, the electrostatic interactions are stronger. The stabilizing guest-water "hydrophobic" interactions compensate for the weaker water-water interactions and stabilize the hydrate phases. A number of common assumptions regarding the guest-cage water interactions are used in the van der Waals-Platteeuw statistical mechanical theory to predict the clathrate hydrate phase stability under different pressure-temperature conditions. The present calculations show that some of these assumptions may not accurately reflect the physical nature of the interactions between guest molecules and the lattice waters.

Phase Transition of a Structure†II Cubic Clathrate Hydrate to a Tetragonal Form.

The crystal structure and phase transition of cubic structure II (sII) binary clathrate hydrates of methane (CH4 ) and propanol are reported from powder X-ray diffraction measurements. The deformation of host water cages at the cubic-tetragonal phase transition of 2-propanol+CH4 hydrate, but not 1-propanol+CH4 hydrate, was observed below about 110 K. It is shown that the deformation of the host water cages of 2-propanol+CH4 hydrate can be explained by the restriction of the motion of 2-propanol within the 5(12) 6(4) host water cages. This result provides a low-temperature structure due to a temperature-induced symmetry-lowering transition of clathrate hydrate. This is the first example of a cubic structure of the common clathrate hydrate families at a fixed composition.

Selective occupancy of methane by cage symmetry in TBAB ionic clathrate hydrate.

Methane trapped in the two distinct dodecahedral cages of the ionic clathrate hydrate of TBAB was studied by single crystal XRD and MD simulation. The relative CH4 occupancies over the cage types were opposite to those of CO2, which illustrates the interplay between the cage symmetry and guest shape and dynamics, and thus the gas selectivity.

Disorder of Hydrofluorocarbon Molecules Entrapped in the Water Cages of Structure†I Clathrate Hydrate.

Water versus fluorine: Clathrate hydrates encaging hydrofluorocarbons as guests show both isotropic and anisotropic distributions within host water cages, depending on the number of fluorine atoms in the guest molecule; this is caused by changes in intermolecular interactions to host water molecules in the hydrates.

Preservation of carbon dioxide clathrate hydrate in the presence of trehalose under freezer conditions.

To investigate the preservation of CO2 clathrate hydrate in the presence of sugar for the novel frozen dessert, mass fractions of CO2 clathrate hydrate in CO2 clathrate hydrate samples coexisting with trehalose were intermittently measured. The samples were prepared from trehalose aqueous solution with trehalose mass fractions of 0.05 and 0.10 at 3.0 MPa and 276.2 K. The samples having particle sizes of 1.0 mm and 5.6-8.0 mm were stored at 243.2 K and 253.2 K for three weeks under atmospheric pressure. The mass fractions of CO2 clathrate hydrate in the samples were 0.87-0.97 before the preservation, and CO2 clathrate hydrate still remained 0.56-0.76 in the mass fractions for 5.6-8.0 mm samples and 0.37-0.55 for 1.0 mm samples after the preservation. The preservation in the trehalose system was better than in the sucrose system and comparable to that in the pure CO2 clathrate hydrate system. This comparison indicates that trehalose is a more suitable sugar for the novel frozen carbonated dessert using CO2 clathrate hydrate than sucrose in terms of CO2 concentration in the dessert. It is inferred that existence of aqueous solution in the samples is a significant factor of the preservation of CO2 clathrate hydrate in the presence of sugar.

A molecular dynamics study of guest-host hydrogen bonding in alcohol clathrate hydrates.

Clathrate hydrates are typically stabilized by suitably sized hydrophobic guest molecules. However, it has been experimentally reported that isomers of amyl-alcohol C5H11OH can be enclosed into the 5(12)6(4) cages in structure II (sII) clathrate hydrates, even though the effective radii of the molecules are larger than the van der Waals radii of the cages. To reveal the mechanism of the anomalous enclathration of hydrophilic molecules, we performed ab initio and classical molecular dynamics simulations (MD) and analyzed the structure and dynamics of a guest-host hydrogen bond for sII 3-methyl-1-butanol and structure H (sH) 2-methyl-2-butanol clathrate hydrates. The simulations clearly showed the formation of guest-host hydrogen bonds and the incorporation of the O-H group of 3-methyl-1-butanol guest molecules into the framework of the sII 5(12)6(4) cages, with the remaining hydrophobic part of the amyl-alcohol molecule well accommodated into the cages. The calculated vibrational spectra of alcohol O-H bonds showed large frequency shifts due to the strong guest-host hydrogen bonding. The 2-methyl-2-butanol guests form strong hydrogen bonds with the cage water molecules in the sH clathrate, but are not incorporated into the water framework. By comparing the structures of the alcohols in the hydrate phases, the effect of the location of O-H groups in the butyl chain of the guest molecules on the crystalline structure of the clathrate hydrates is indicated.

Water proton configurations in structures I, II, and H clathrate hydrate unit cells.

Position and orientation of water protons need to be specified when the molecular simulation studies are performed for clathrate hydrates. Positions of oxygen atoms in water are experimentally determined by X-ray diffraction analysis of clathrate hydrate structures, but positions of water hydrogen atoms in the lattice are disordered. This study reports a determination of the water proton coordinates in unit cell of structure I (sI), II (sII), and H (sH) clathrate hydrates that satisfy the ice rules, have the lowest potential energy configuration for the protons, and give a net zero dipole moment. Possible proton coordinates in the unit cell were chosen by analyzing the symmetry of protons on the hexagonal or pentagonal faces in the hydrate cages and generating all possible proton distributions which satisfy the ice rules. We found that in the sI and sII unit cells, proton distributions with small net dipole moments have fairly narrow potential energy spreads of about 1 kJ∕mol. The total Coulomb potential on a test unit charge placed in the cage center for the minimum energy∕minimum dipole unit cell configurations was calculated. In the sI small cages, the Coulomb potential energy spread in each class of cage is less than 0.1 kJ∕mol, while the potential energy spread increases to values up to 6 kJ∕mol in sH and 15 kJ∕mol in the sII cages. The guest environments inside the cages can therefore be substantially different in the sII case. Cartesian coordinates for oxygen and hydrogen atoms in the sI, sII, and sH unit cells are reported for reference.

Synthesis and characterization of a structure H hydrate formed with carbon dioxide and 3,3-dimethyl-2-butanone.

Experiments were carried out to synthesize and characterize a structure H clathrate hydrate containing CO(2) and 3,3-dimethyl-2-butanone (pinacolone) by means of phase equilibrium and powder X-ray diffraction measurements. Molecular dynamics simulations of this structure H hydrate were performed to understand the nature of guest-host molecular interactions.

Molecular storage of ozone in a clathrate hydrate: an attempt at preserving ozone at high concentrations.

This paper reports an experimental study of the formation of a mixed O(3)+ O(2)+ CO(2) hydrate and its frozen storage under atmospheric pressure, which aimed to establish a hydrate-based technology for preserving ozone (O(3)), a chemically unstable substance, for various industrial, medical and consumer uses. By improving the experimental technique that we recently devised for forming an O(3)+ O(2)+ CO(2) hydrate, we succeeded in significantly increasing the fraction of ozone contained in the hydrate. For a hydrate formed at a system pressure of 3.0 MPa, the mass fraction of ozone was initially about 0.9%; and even after a 20-day storage at -25°C and atmospheric pressure, it was still about 0.6%. These results support the prospect of establishing an economical, safe, and easy-to-handle ozone-preservation technology of practical use.