Hydrogen bonding in polymer mixtures

The enthalpy–infrared frequency shift correlation for simple acids and bases is extended to study hydrogen bonding in polymer systems. The acidity of a polymer is calibrated by comparing the shifts in hydroxyl absorption frequency of the acidic polymer when mixed with a series of bases with the corresponding spectral shifts of known acids with the same bases. The basicity of a polymer is calibrated by measuring the hydroxyl frequency shifts of known acids when mixed with the basic polymer. For polymers containing carbonyl groups, the shift in carbonyl absorption is also a measure of basicity. The acidity and basicity constants obtained for polymers are in good agreement with the values for small-molecule analogs. The enthalpies of hydrogen bond formation in polymer mixtures are calculated from the acidity or basicity constants.     

Dielectric properties of glycerol/water mixtures at temperatures between 10 and 50 degrees C

At six temperatures T between 10 and 50 degrees C and at mole fractions x(g) of glycerol (0相似文献   

丙三醇/水混合溶剂中CTAC/NaSal胶束体系粘度的研究

本文研究了丙三醇/水混合溶剂中十六烷基三甲基氯化铵(CTAC)溶液的粘度性质,考察了水杨酸钠(NaSal)浓度、CTAC浓度以及丙三醇体积分数等因素对CTAC胶束溶液粘度的影响,并与乙二醇/水混合溶剂中胶束的粘度进行比较。结果表明:在丙三醇/水混合溶剂中,当CTAC浓度一定时,CTAC溶液的相对粘度随着NaSal浓度的增大而增大;当NaSal浓度一定时,CTAC溶液的相对粘度随着CTAC浓度的增大出现最大值;CTAC溶液的相对粘度随着丙三醇体积分数的增大而下降。这种变化趋势与乙二醇/水混合溶剂中的粘度性质相类似。     

Hydrogen bonding definitions and dynamics in liquid water

X-ray and neutron diffractions, vibrational spectroscopy, and x-ray Raman scattering and absorption experiments on water are often interpreted in terms of hydrogen bonding. To this end a number of geometric definitions of hydrogen bonding in water have been developed. While all definitions of hydrogen bonding are to some extent arbitrary, those involving one distance and one angle for a given water dimer are unnecessarily so. In this paper the authors develop a systematic procedure based on two-dimensional potentials of mean force for defining cutoffs for a given pair of distance and angular coordinates. They also develop an electronic structure-based definition of hydrogen bonding in liquid water, related to the electronic occupancy of the antibonding OH orbitals. This definition turns out to be reasonably compatible with one of the distance-angle geometric definitions. These two definitions lead to an estimate of the number of hydrogen bonds per molecule in liquid simple point charge/extended (SPC/E) water of between 3.2 and 3.4. They also used these and other hydrogen-bond definitions to examine the dynamics of local hydrogen-bond number fluctuations, finding an approximate long-time decay constant for SPC/E water of between 0.8 and 0.9 ps, which corresponds to the time scale for local structural relaxation.     

Enthalpy-entropy compensation in micellization of sodium dodecyl sulphate in water/methanol,water/ethylene glycol and water/glycerol binary mixtures

The enthalpy-entropy compensation in micellization of sodium dodecyl sulphate (SDS) in binary mixtures of water/methanol (MeOH), water/ethylene glycol (EG) and water/glycerol (GL) over a temperature range of 10–60°C was examined. When the cosolvent concentration was low, the critical micelle concentration (CMC) depended only on the total amount of the hydroxyl group added. When the cosolvent concentration was high, the increase in CMC followed the sequence: MeOH>EG>GL. Enthalpy and entropy changes were evaluated from which the compensation temperature was determined. Both enthalpy and entropy changes decreased on the addition of the cosolvents, indicating a lowering of solution hydrophobicity. The compensation temperature was found as a constant over the cosolvent concentration range, as a result, was not a good index for characterizing the solute/solvent interactions. The two reference temperatures at which the enthalpy-entropy change respectively became zero were strongly influenced by the cosolvent addition, therefore could serve as a proper index for solution hydrophobicity.     

Hydrogen bonding topology influences gelating properties of malonamides

Preparation, structural characterisation and topology of hydrogen bonding networks of bis(phenylglycinol)malonamide, as well as its Cα mono- and dialkyl-substituted derivatives are described. Their hydrogen bonding motifs are described in view of their gelling properties. Topology of hydrogen bonding typical of malonamide gelators is compared with those of well-examined oxalamide gelators.     

Hydrogen bonding in phenol, water, and phenol-water clusters

Structure, stability, and hydrogen-bonding interaction in phenol, water, and phenol-water clusters have been investigated using ab initio and density functional theoretical (DFT) methods and using various topological features of electron density. Calculated interaction energies at MP2/6-31G level for clusters with similar hydrogen-bonding pattern reveal that intermolecular interaction in phenol clusters is slightly stronger than in water clusters. However, fusion of phenol and water clusters leads to stability that is akin to that of H(2)O clusters. The presence of hydrogen bond critical points (HBCP) and the values of rho(r(c)) and nabla(2)rho(r(c)) at the HBCPs provide an insight into the nature of closed shell interaction in hydrogen-bonded clusters. It is shown that the calculated values of total rho(r(c)) and nabla(2)rho(r(c)) of all the clusters vary linearly with the interaction energy.     

Hydrogen production by glycerol reforming in supercritical water over Ni/MgO-ZrO2 catalyst

Nano ZrO₂ and MgO-ZrO₂ were prepared by a self-assembly route and were employed as the support for Ni catalysts used in hydrogen production from glycerol reforming in supercritical water (SCW). The reforming experiments were conducted in a tubular fixed-bed flow reactor over a temperature range of 600–800 °C. The influences of process variables such as temperature, contact time, and water to glycerol ratio on hydrogen yield were investigated and the catalysts were charactered by ICP, BET, XRD and SEM. The results showed that high hydrogen yield was obtained from glycerol by reforming in supercritical water over the Ni/MgO-ZrO₂ catalysts in a short contact time. The MgO in the catalyst showed significant promotion effect for hydrogen production likely due to the formation of the alkaline active site. Even when the glycerol feed concentration was up to 45 wt%, glycerol was completely gasified and transfered to the gas products containing hydrogen, carbon dioxide, and methane along with small amounts of carbon monoxide. At a diluted feed concentration of 5 wt%, near theoretical yield of 7 mole of H₂/mol of glycerol could be obtained.     

Preferential hydration of lysozyme in water/glycerol mixtures: a small-angle neutron scattering study

In solution small-angle neutron scattering has been used to study the solvation properties of lysozyme dissolved in water/glycerol mixtures. To detect the characteristics of the protein-solvent interface, 35 different experimental conditions (i.e., protein concentration, water/glycerol fraction in the solvent, content of deuterated compounds) have been considered and a suitable software has been developed to fit simultaneously the whole set of scattering data. The average composition of the solvent in the close vicinity of the protein surface at each experimental condition has been derived. In all the investigated conditions, glycerol resulted especially excluded from the protein surface, confirming that lysozyme is preferentially hydrated. By considering a thermodynamic hydration model based on an equilibrium exchange between water and glycerol from the solvation layer to the bulk, the preferential binding coefficient and the excess solvation number have been estimated. Results were compared with data previously derived for ribonuclease A in the same mixed solvent: even if the investigated solvent compositions were very different, the agreement between data is noticeable, suggesting that a unique mechanism presides over the preferential hydration process. Moreover, the curve describing the excess solvation number as a function of the solvent composition shows the occurrence of a region of maximal hydration, which probably accounts for the changes in protein stability detected in the presence of cosolvents.     

Hydrogen bonding and anaesthesia

General anaesthetics act by perturbing intermolecular associations without breaking or forming covalent bonds. These associations might be due to a variety of van der Waals interactions or hydrogen bonding. Neurotransmitters all contain OH or NH groups, which are prone to form hydrogen bonds with those of the neurotransmitter receptors. These could be perturbed by anaesthetics.

Aromatic rings in amino acids can act as weak hydrogen bond acceptors. On the other hand the acidic hydrogen in halothane type anaesthetics are weak proton donors. These two facts together lead to a probable mechanism of action for all general anaesthetics.     

Hydrogen bonding interaction between N-methylformamide and alcohols

The hydrogen bonding interactions between N-methylformamide and primary, secondary, and tertiary alcohols have been studied using the FTIR spectroscopic method. The most likely association complex between alcohol and N-methylformamide is the 1:1 stoichiometric complex formed between the hydroxyl group of alcohol and the carbonyl group of N-methylformamide. The formation constant of the 1:1 complexes has been calculated using the Nash method. It appears that the primary alcohols have larger formation constant compared with the secondary and tertiary alcohols. The results showed that the proton-donating ability of the alcohols decreased in the order: primary>secondary>tertiary, and that the association constant increased with the increase in carbon chain of the alkyl group of alcohols.     

Hydrogen bonding in liquid water probed by resonant Auger-electron spectroscopy

We have measured resonant and off-resonant Auger-electron spectra of liquid water. Continuumlike transitions near and above the O1s vertical ionization energy are identified by the characteristic normal Auger-electron spectra. On the contrary, well-resolved spectator shifts of the main Auger-electron peak are observed at the liquid-water O1s absorption main edge and near the absorption pre-edge. The shifts of 1.4 and 1.9 eV arise from the localized nature of the excitation. Excited-state localization/delocalization is also discussed for the analogous vacuum ultraviolet (VUV) transitions, and we point out the similarities between x-ray and VUV absorption spectra of liquid water.     

Hydrogen bonding in protonated water clusters: an atoms-in-molecules perspective

This article highlights the results of a detailed study of hydrogen bonding in the first and the second solvation shells of Eigen (H3O+) and Zundel (H5O2+) cations solvated by water in a stepwise manner. It is evident from the results that an electron density analysis clearly distinguishes the first and the second solvation shell and helps in quantifying the strength of hydrogen bonding in these clusters.     

Hydrogen bonding and mechanical properties of thin films of polyether-based polyurethane-silica nanocomposites

Two series of thin films of polyether-based polyurethane-silica nanocomposites having hard segment content of 51% and 34% and different concentrations of SiO₂ nanoparticles (0, 0.5, 1.0 and 3.0 vol.%) have been prepared. Infrared linear dichroic (LDIR) ratio, mechanical and differential scanning calorimetry (DSC) measurements were performed in order to determine the influence of hydrogen bonding on their mechanical and thermal properties. The degree of phase separation (DPS) and orientational functions in dependence on strain were calculated from the polarized IR spectra. The presence of silica nanoparticles gives rise to significant differences in the mechanical (stress-strain) properties of the nanocomposites with regard to the pure polymer. The nanocomposite thin films with lower hard segment content (HSC) displayed decreased stiffness and tensile and increased elongation at break in comparison to the nanocomposites with higher HSC. There was no distinctive influence of nanoparticles on the glass transition temperatures of soft segments. Nanosilica significantly affected the melting behavior of the hard phase only in samples with higher HSC.     

The solvent shells of cluster ions produced by direct electric field extraction from glycerol/water mixtures

Electric field extraction of gaseous negative ions directly from water/glycerol solutions by use of a track membrane technique was investigated. The distributions of numbers of solvent molecules in the extracted cluster ions for different compounds were obtained. It is shown that the extraction mechanism is a direct field-stimulated evaporation of cluster ions from liquid, with a subsequent loss of several solvent molecules in the vacuum. For relatively simple ions a good correspondence of results was obtained with a continuous medium model. It was found that the number of solvent molecules in a cluster shell, for more complicated ions such as amino acids, is significantly greater than that for halide ions or ions of simple organic acids. An increase in the number of solvent molecules in the case of amino acid negative ions is rationalized in terms of the existence of several charged groups, each of which gives an additional contribution to the cluster shell.     

Interfacial properties of mixtures of lecithin with a block copolymer surfactant at the water/air and water/oil interfaces

Surface pressure-area isotherms have been determined for both a pure lecithin (L, -dipalmitoyl phosphatidyl choline) and an impure lecithin (soya bean lecithin) at the water/air and water/oil interfaces. Equations of state have been applied and an equation of Gaines was found to be particularly successful in describing the isotherms. Mixed monolayers with an ABA nonionic block copolymer surfactant (A is poly(12-hydroxystearic) acid and B is poly(ethylene oxide)) were also investigated. The additivity rule was obeyed only at high surface pressures; inefficient packing was observed at low surface pressures. The polymer may promote a horizontal headgroup orientation in the lecithin, which gives rise to this effect. The presence of electrolyte up to very high concentrations in the aqueous phase (8.75 mol dm^–3 NH₄NO₃) was shown to expand the lecithin monolayer.Glossary of symbols W/A Water-air interface - W/O Water-oil interface - E/A Electrolyte-air interface - L-C Liquid-condensed - A _c Area per molecule obtained by conventional extrapolation of the -A isotherm at close-packing - A _e Experimentally determined area per molecule - A _t Theoretically predicted area per molecule - A _v Area per molecule obtained by vertical extrapolation of the -A isotherm at close-packing - A ₀ Head group area term - f _i Activity coefficient of water in surface region - i Constant - x _i Mol fraction of componenti - Z Compressibility factor=A/kT - Interfacial tension - Surface pressure - _i Partial molar area of component i     

Hydrogen bonding of water, hydrogen sulphide and related acceptors with electron donors

Properties of hydrogen bonds formed by 1:1 interaction of H₂O with oxygen, nitrogen, sulphur and other electron donors have been evaluated by extended Hückel and CNDO methods and the results are discussed in relation to the experimental data. A detailed analysis of the variation of the dissociation energies and charge densities with bond distances is presented for the amine-water system. 1:2 complexes of water with donors are found to contain weaker hydrogen bonds than 1:1 complexes. Results of molecular orbital calculations on the hydrogen bonding of H₂S and CH₃SH with some donors are presented. The theoretical value of hydrogen bond dissociation energy varies linearly with the overlap population, and stretching force constant of the hydrogen bond as well as with the experimental O—H frequency shift.     

Hydrogen bonding between formic acid and water: complete stabilization of the intrinsically unstable conformer

We studied hydrogen bonding between formic acid (FA) and water in solid argon and identified the first water complex with the higher-energy conformer cis-FA. In sharp contrast to cis-FA monomer, cis-FA interacting with water is very stable at low temperatures, which was explained by strong O-H...O hydrogen bonding. These benchmark results show that hydrogen bonding can terminate proton tunneling reactions and efficiently stabilize intrinsically unstable conformational structures in complex asymmetrical hydrogen-bonded networks. This general effect occurs when the energy difference between conformers is smaller than the hydrogen bond interaction energy, which opens perspectives in chemistry on intrinsically unstable conformers.     

Microcalorimetric study of thermal unfolding of lysozyme in water/glycerol mixtures: an analysis by solvent exchange model

Folded protein stabilization or destabilization induced by cosolvent in mixed aqueous solutions has been studied by differential scanning microcalorimetry and related to difference in preferential solvation of native and denatured states. In particular, the thermal denaturation of a model system formed by lysozyme dissolved in water in the presence of the stabilizing cosolvent glycerol has been considered. Transition temperatures and enthalpies, heat capacity, and standard free energy changes have been determined when applying a two-state denaturation model to microcalorimetric data. Thermodynamic parameters show an unexpected, not linear, trend as a function of solvent composition; in particular, the lysozyme thermodynamic stability shows a maximum centered at water molar fraction of about 0.6. Using a thermodynamic hydration model based on the exchange equilibrium between glycerol and water molecules from the protein solvation layer to the bulk, the contribution of protein-solvent interactions to the unfolding free energy and the changes of this contribution with solvent composition have been derived. The preferential solvation data indicate that lysozyme unfolding involves an increase in the solvation surface, with a small reduction of the protein-preferential hydration. Moreover, the derived changes in the excess solvation numbers at denaturation show that only few solvent molecules are responsible for the variation of lysozyme stability in relation to the solvent composition.