Enthalpy Stabilization Research Articles

Thermodynamics of the swelling work of wood and non-ionic polysaccharides: A revisit

Polysaccharides, even non-ionic ones, swell in water with potentially huge pressure, which can be sources of intended actuation or a cause of structural damage. The swelling pressure has been investigated since the 19th century, and thermodynamic considerations developed at the beginning of the 20th century. Such treatment is revisited with currently available data showing the swelling to be mostly enthalpy driven. The molecular origin of the heat of swelling is discussed, considering the specificity of polysaccharides or biopolymers with relatively stiff chain conformation that contradicts the compact packing of enthalpy stabilization. Part of the heat of swelling, and thus the potential work of swelling, would originate from the elastic energy stored in the rigid structure. This vision can be tested in the conception of actuation or control of swelling.

Enthalpy stabilization of superconductivity in an alloying S-P-H system: First-principles cluster expansion study under high pressure

The metallic ternary compound phase of S-P-H material is predicted to be the ground-state structure by first-principles cluster expansion. We find that the S0.75P0.25H3, S0.5P0.5H3, and S0.25P0.75H3 structures are thermodynamically stable. The characteristic of phonon dispersions shows that the S0.75P0.25H3 and S0.5P0.5H3 structures are dynamically stable at 200GPa. The solution of the Fermi surface supports the values of superconducting transition temperature (Tc). In addition, the electron localization function and the projected crystal orbital Hamilton populations reveal that the nature of chemical bonding shows the degree of covalent bonding of the S0.75P0.25H3 and S0.5P0.5H3 structures, supporting the Tc value. Our results show that the S0.5P0.5H3 structure is the highest Tc among the S-P-H system. The Tc of the S0.5P0.5H3 structure is estimated to be 89K at 200GPa.

Crystal structures of highly simplified BPTIs provide insights into hydration-driven increase of unfolding enthalpy

We report a thermodynamic and structural analysis of six extensively simplified bovine pancreatic trypsin inhibitor (BPTI) variants containing 19–24 alanines out of 58 residues. Differential scanning calorimetry indicated a two-state thermal unfolding, typical of a native protein with densely packed interior. Surprisingly, increasing the number of alanines induced enthalpy stabilization, which was however over-compensated by entropy destabilization. X-ray crystallography indicated that the alanine substitutions caused the recruitment of novel water molecules facilitating the formation of protein–water hydrogen bonds and improving the hydration shells around the alanine’s methyl groups, both of which presumably contributed to enthalpy stabilization. There was a strong correlation between the number of water molecules and the thermodynamic parameters. Overall, our results demonstrate that, in contrast to our initial expectation, a protein sequence in which over 40% of the residues are alanines can retain a densely packed structure and undergo thermal denaturation with a large enthalpy change, mainly contributed by hydration.

Thermodynamics of formation for the 18-crown-6-triglycine molecular complex in water-dimethylsulfoxide solvents

The effect of a water-dimethylsulfoxide (DMSO) solvent on the formation of a molecular complex of 18-crown-6 (18C6) with triglycine (diglycylglycine, 3Gly) is studied via calorimetric titration. It is found that switching from water to an H2O-DMSO mixture with DMSO mole fraction of 0.30 is accompanied by a monotonic increase in the stability of [3Gly18C6] complex, from logK ∘ = 1.10 to logK ∘ = 2.44, and an increase in the exothermicity of the reaction of its formation, from −5.9 to −16.9 kJ/mol. It is shown that the [3Gly18C6] complex exhibits enthalpy stabilization with negative values of enthalpy and entropy over the investigated range of H2O-DMSO solvents. Analysis of the reagents’ solvation characteristics reveals that the increase in the reaction’s exothermicity of transfer is due to differences in the solvation of [3Gly18C6] and 18C6 with a small solvation contribution from 3Gly. It is concluded that the change in the Gibbs energy of the reaction 3Glysolv + 18C6solv ↔ [3Gly18C6]solv is due to differences in the change in the solvation state of the complex and the peptide (Δtr G ∘([3Gly18C6])-Δtr G ∘(3Gly)).

Initial Molecular Photocurrent: Nanostructure and Motion of Weakly Bound Charge-Separated State in Organic Photovoltaic Interface

To elucidate mechanism of the efficient photocarrier generation by the photoactive bulkheterojunction layers of the organic photovoltaic (OPV) devices, we have employed the time-resolved electron paramagnetic resonance method on solid blends composed of [6,6]-C61-butyric acid methyl ester (PCBM) and of poly(3-hexylthiophene-2,5-diyl) (P3HT) with different regioregularities. The photoinduced charge-separated (CS) states have been detected at the boundary regions between the P3HT and PCBM domains at T = 77 K. We have characterized molecular geometries, electronic couplings, and molecular motions of the long-range CS states. From the CS structure, it is indicated that the pentagonal or hexagonal aromatic rings of the buckyball in PCBM directly face the aromatic plane of the π-stacked P3HT surfaces. It has been concluded that the distant CS states are produced via fast hole-delocalization process from the contact charge-transfer (CT) states. Such hole dynamics is explained by a coupling of the hole to librations of chains in the conjugated polymer. It has been concluded that both the enthalpy stabilization and the enhancement of the entropy occur through the orbital delocalization by the electron–phonon coupling, overcoming the initial CT binding to generate the molecular photocurrent.

Thermodynamic and structural analysis of highly stabilized BPTIs by single and double mutations

Enhancing protein conformational stability is an important aspect of protein engineering and biotechnology. However, protein stabilization is difficult to rationalize as it often results from the small cumulative and intertwined effects of multiple mutations. Here, we analyzed the mechanisms behind a remarkable 13 degrees stabilization produced by a single A14G and a double A14GA38V mutation in BPTI-[5,55], a natively folded bovine pancreatic trypsin inhibitor variant. Differential scanning calorimetry analysis of three BPTI-[5,55] variants (A14G, A38V, and A14GA38V) indicated that the A14G mutation stabilized the structure enthalpically, whereas the A38V stabilization was entropy driven. We also determined the structure of the A14GA38V mutant at 1.09 A resolution, whereas the A38V variant did not crystallize, and we previously reported the A14G variant's structure (2ZJX). The overall structures of the A14G and A14GA38V variants were very similar to that of wild-type BPTI, but small local structure perturbations around residues 14 and 38 strongly suggested potential factors contributing to the enthalpy stabilization. First, the A14G mutation displaced the local backbone structures around residues 14 and 38 by up to 0.7 A, presumably increasing local van der Waals interactions. Next, this displacement produced steric clashes between neighboring residue's side-chains in all but the variants containing the A14G mutation. Noteworthy, these clashes are not predicted from the wild type BPTI structure. These observations provide one of the first unambiguous analyses of how a subtle interplay between the sidechain and backbone structures can have a major effect on protein stability.

Toward an Understanding of the Acceleration of Diels−Alder Reactions by a Pseudo-intramolecular Process Achieved by Molecular Recognition. A DFT Study

The pseudo-intramolecular Diels-Alder (DA) reaction between a 2-substituted furan (1) and a N-maleimide derivative (2) has been analyzed using DFT methods. Formation of two hydrogen bonds between the appendages on furan and maleimide derivatives favors thermodynamically the formation of a molecular complex (MC1) through an efficient molecular recognition process. The large enthalpy stabilization associated with the molecular recognition overcomes the unfavorable activation entropy associated with the bimolecular process. As a consequence, the subsequent DA reaction is clearly accelerated through a pseudo-intramolecular process.

Calorimetric Determination of the Enthalpies of Formation of Hydrotalcite-Like Solids and Their Use in the Geochemical Modeling of Metals in Natural Waters

AbstractInterest in hydrotalcite-like compounds has grown due to their role in controlling the mobility of aqueous metals in the environment as well as their use as catalysts, catalyst precursors and specialty chemicals. Although these materials have been studied in a number of contexts, little is known of their thermodynamic properties. High-temperature oxide melt solution calorimetry was used to measure the standard enthalpy of formation for compounds M(II)1−xAlx(OH)2(CO3)x/2·mH2O (0.2 < x < 0.4, M(II) = Mg, Co, Ni and Zn). The enthalpy of formation of these compounds from the relevant single cation phases was also determined. The formation of HTLCs results in a 5–20 kJ/mol enthalpy stabilization from the single cation hydroxides and carbonates and water. The data are correlated to two variables: the ratio of divalent to trivalent cation in the solid (M(II)/Al) and the identity of the divalent cation. It was observed that the M(II)/Al ratio exerts a minor influence on the enthalpy of formation from single-cation phases, while greater differences in stabilization resulted from changes in the chemical nature of the divalent cation. However, the data do not support any statistically significant correlation between the composition of HTLCs and their heats of formation. Equilibrium geochemical calculations based upon the thermodynamic data illustrate the effect of HTLCs on the speciation of metals in natural waters. These calculations show that, in many cases, HTLCs form even in waters that are undersaturated with respect to the individual divalent metal hydroxides and carbonates. Phase diagrams and stability diagrams involving Ni-bearing HTLCs and the single-cation components are presented. The Ni(II) concentration as a function of pH as well as the stability diagram for the equilibrium among minerals in the CaO-NiO-Al2O3-SiO2-CO2-H2O system at 298 K are plotted.

Design of the High-<tex>$rm T_rm c$</tex>Superconducting TF Coil for the Tight Aspect Ratio Tokamak Power Reactor (VECTOR)

The tight aspect ratio tokamak power reactor (VECTOR) is proposed as a compact fusion reactor in a cost-effective way at JAERI. In the coil system, a center solenoid is discarded from the inner side of the plasma to realize low aspect configuration and the TF coil inner leg side is a solid-like integrated center post structure. The toroidal field (TF) coil consists of 12 D-shaped coil units whose inner bores are 12 m in height and 6 m in width. The magnetic field at the plasma center is designed to be 5 T. The maximum field in winding of the TF coil is required to be 18.3 T because of low aspect ratio. The magnetic stored energy is 10.7 GJ much less than the 40 GJ for ITER. For such high field generation and high performance coil realization, a high T/sub c/ superconductor is applied to the TF coil. The coil operating current is 178 kA at 18.3 T. The structure of the TF coil unit is composed of 5 radial plates and 2 support plates unified through bolts without coil case. The radial plates are designed with "beak type" structure that its inner leg part is bent to attain high current density in winding. The conductor that is composed of high T/sub c/ superconductor strands impregnated with lead, is inserted into grooves of the radial plates. The lead plays an important role on solidification of the strands cabled and on enthalpy stabilization with its high heat capacity. For the high T/sub c/ material, Bi2212 is chosen. The operating temperature of the conductor is 20 K. The conductor is cooled indirectly from cooling plates provided between the radial plates. The conductor has the temperature margin of 3 K proofed against disturbance of 1 J/cc at 20 K.

Extraction of tetravalent neptunium isoxazolonates as their TOPO adducts

SummaryThe extraction behaviour of Np(IV) by 3-phenyl-4-benzoyl-5-isoxazolone (HPBI) in the presence of tri-n-octyl phosphine oxide (TOPO) was investigated from nitric acid medium. The speciation studies, carried out by the slope analysis method, yielded binary species of the type Np(PBI)4when only HPBI was used as the extractant and ternary species of the type Np(PBI)4·TOPO when both HPBI and TOPO were used as the extractants. The two-phase extraction constant for the binary species (logkex) was calculated to be 10.11±0.03, while that for the ternary species was found to be 17.24±0.05. Thermodynamic parameters for the binary and ternary extraction systems were also determined. Both the binary and ternary extraction systems were found to be highly spontaneous processes with predominant enthalpy stabilization. The very high negative entropy factor suggested contribution of unfavourable steric factors.

Stability in the ventilated, cabled superconductor proposed for the Navy SMES cable test apparatus

Improved superconductors for SMES devices will be developed and tested in the Navy SMES cable development program. To test these improved SMES cables, Westinghouse Science and Technology Center is constructing a background field coil for the SMES cable test apparatus that will provide a magnetic field of up to 4 T on a 1.8 meter diameter test specimen. The background coil superconductor design operating point is determined by stability considerations and the superconductor critical surface. Stability ventilated, cabled superconductor proposed background field magnet is reviewed in terms of cryostability, current redistribution, and enthalpy stabilization of surrounding liquid helium.

Thermochemistry of glasses along joins of pyroxene stoichiometry in the system Ca2Si2O6-Mg2Si2O6-Al4O6

Enthalpies of solution in 2PbO · B2O3 at 974 K have been measured for glasses along the joins Ca2Si2O6 (Wo)-Mg2Si2O6 (En) and Mg2Si2O6-MgAl2SiO6 (MgTs). Heats of mixing are symmetric and negative for Wo-En with WH = −31.0 ± 3.6 kJ mol−. Negative heats of mixing were also found for the En-MgTs glasses (WH = −33.4 ± 3.7 kJ mol−).Enthalpies of vitrification of pyroxenes and pyroxenoids generally increase with decreasing alumina content and with decreasing basicity of the divalent cation.Heats of mixing along several glassy joins show systematic trends. When only non-tetrahedral cations mix (outside the aluminosilicate framework), small exothermic heats of mixing are seen. When both nontetrahedral and framework cations mix (on separate sublattices, presumably), the enthalpies of mixing are substantially more negative. Maximum enthalpy stabilization near compositions with Al/Si ≈ 1 is suggested.

Metallic materials for superconductor stabilization with very high specific heat and good thermal conductivity

In a search for materials useful for enthalpy stabilization of technical superconductors we have performed specific-heat measurements on PrCu 2, PrB 6, NdSn 3 and Nd 0.9Pr 0.1Sn 3 between 3 and 10 K and in magnetic fields up to 7 T. Due to different types of phase transitions these compounds exhibit large peaks in the heat-capacity curves. In these materials the high specific heat values are not substantially reduced the the external magnetic fields, which makes them attractive for technical application.

Enthalpy improved dielectric insulation for NbTi

Two new dielectric insulation systems applicable to NbTi are described. Both systems incorporate significant enthalpy stabilization in that the volumetric specific heats of the dielectrics are equivalent to, or much larger than, that of Pb below 10K. For one of the systems, the thermal conductivity at 5K approaches that of commercial Cu. Both coating systems can be applied to NbTi without the use of a fugitive binder: One system can be hot-extruded directly on the wire, and the other system can be dip-coated with a room-temperature cure. Both coating systems withstand demanding bend tests. For 0.13-mm thick coatings, the thermal time constants at 5K are 0.24 μs and 2.5 ms; the corresponding enthalpies per unit volume at 6K (relative to 4.2K) are 50.5 and 54.6 mJ/cm3At 8K; the enthalpies are 179 and 311 mJ/cm3respectively. The thermal properties of both systems are unaffected by intense magnetic fields.

Transient heat transfer and stability of superconducting composites

The rate of change of temperature with Joule heating of a sample in liquid helium is dependent on the specific heat and thermal conductivity of the sample and the heat transfer coefficient to liquid helium. By measuring the temperature of a sample of manganin wire in liquid helium during the first few milliseconds of Joule heating we have been able to measure very high heat transfer coefficients at heat flux densities as high as 3 W cm −2. The results have been used to describe a form of enthalpy stabilization in minimum propagating current coils.

Enthalpy stabilized superconducting magnets

An essential requirement for complete stabilization of a superconducting magnet is adequate circulation of liquid helium in the winding, resulting in a low average current density, in partially stabilized magnets, the size of the cooling channels is reduced, but access for liquid helium is still considered necessary. With enthalpy stabilization, however, the need for helium penetration into the winding is eliminated, resulting in better packing of the conductors and a higher average current density. A small enthalpy stabilized magnet has been tested that was fully impregnated with epoxy resin and operated at a current density above 104A/cm2. The basis of the design of such magnets is discussed, with particular reference to the choice of conductor size and its effect on average current density in the magnet. Possible applications of this technique are also discussed.