Hydration Water Molecules Research Articles

Microgravimetric contributions of multivalent metal ions, water molecules and protons to the energy storage at the electrolyte/electrode interface of aqueous ion batteries

The aqueous battery employing multivalent metal ions as charge carriers is theoretically a high energy density electrochemical energy storage device due to the fact that it carries more charges. However, the storage mechanism of multivalent metal ions, especially Al3+ ions, in TiO2 materials has not been established.A new model associated with the contribution of multivalent metal ions, water molecules and protons in the charging/discharging process of the electrode is established in this work. The storage mechanism of aqueous Al3+ ion batteries is revealed from a new perspective by studying the relationship between the charge inside the TiO2 film and the mass at the interface. More importantly, a highlighted work is to reveal the changes in the amount of interfacial free water molecules and the hydration number of Al3+ ions due to the adsorption/desorption of hydrated ions during the charging/discharging process.The present work is of great significance for revealing the mechanism of multivalent cations on metal oxide surfaces in aqueous batteries, and meanwhile, the model established can also be extended to other fields such as supercapacitors, photovoltaics, and photocatalysis. The results obtained in this work may have an impact on the performance of energy storage batteries, especially the influence of the rearrangement of interface hydration ions and water molecules on the energy storage mechanism.

Unexpected excellent under-oil superhydrophilicity of poly(2-(dimethylamino)ethyl methacrylate) for water capture from oil and water-induced oil self-dewetting

The anti-oil-fouling property of cross-linked poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) was investigated by testing not only the under-oil superhydrophilicity but also the dewetting ability of oil that adheres on PDMAEMA coating surface. Different to other common hydrophilic polymers (e.g., polyacrylamide (PAM) and poly(methacrylic acid) (PMAA)), PDMAEMA coating shows the unique under-oil superhydrophilicity (wetting time <3 s for a 6 μL water drop), and even more interestingly, the oil that adheres on dry PDMAEMA coating surface can spontaneously dewet and detach off from the surface when immersed in water. The PDMAEMA coating shows lower surface energy (γs) (53.3 mN m−1) than PAM and PMAA (70.5 and 61.1 mN m−1, respectively) and moderate zeta potential (69 mV; 29 and −148 mV for PAM and PMAA, respectively), while it has much higher moisture absorption rate (∼16%) than PAM (∼6%) and PMAA (∼3%) coatings. The overall test results suggest that PDMAEMA coating has stronger hydration ability than PAM and PMAA coatings, and meanwhile, the hydrated water molecules in the coating can weaken the interaction of the PDMAEMA with oil, thus inducing the under-oil superhydrophilicity. Moreover, it is demonstrated that dry PDMAEMA coating can capture water drops from bulk oil, and thus the PDMAEMA coating can be utilized to prepare filtration membrane for the separation of surfactant-stabilized water-in-oil emulsions with high separation efficiency (>99.0%) and long-lasting stability.

The water structure around chloride ion investigated from D2O ↔ H2O substitution effect

The Raman spectra for (D2O + H2O) − NaCl solutions (nH2O/nD2O = 0/1, 1/4, 1/1, 4/1 and 1/0) were measured at temperatures from 293 to 573 K. We further discuss the water structure in the solutions based on the solute-correlated (SC) OD/OH stretch bands obtained from the Multivariate Curve Resolution method. The most prominent effect of H ↔ D substitution is raising the relative intensity of the high-wavenumber modes. The ∼ 2660 cm−1/∼3635 cm−1 mode can become the main peak when isotopic substitution ratio is 80 % at temperatures over 493 K. The spectral features suggest a flexible water structure in the Cl− hydration shell where most hydrating water molecules are engaged in hydrogen bonding configurations of single donor (SD) and single hydrogen-bond water (SHW). The increase of isotopic substitution ratio leads to the structural transition of SD → SHW → FW (free water) so that FW is greatly increased at isotopic substitution ratio of 80 % and temperatures over 493 K. Moreover, the estimated hydrogen bond number for hydrating water indicates a stronger D2O − Cl− interaction than H2O − Cl− interaction in the hydration shell.

Study on ionic association behavior in sodium nitrate solution

Raman spectroscopy combined with component analysis and molecular dynamics simulation were used to study chemical species and their transformation in aqueous sodium solutions. Study shows that the characteristic vibrational frequency of nitrate ions (ν1-NO3-) blue-shifted from 1043.9 to 1046.9 cm−1, and the full width at half maximum increased from 6.8 to 10.8 cm−1 as the concentration increasing. When water/salt molar ratio (WSR) > 30, the relative concentration (RC) of free hydrated ions and solvent shared ion pair accounts for the vast majority, and there is almost no contact ion pair in solution. When WSR less than 30, due to the continuous reduction of the number of water molecules, the hydrated water molecules around the sodium ions and nitrate ions begin to decrease, and solvent shared ion pair or contact ion pair gradually forms. Sodium ions and nitrate ions mainly exist in a monodentate coordination. When WSR > 160, both the relative concentration of contact ion pair and complex structure is close to 0. This work proves that a lower RC of complex structure in solution, a smaller supersaturation of the solution is achieved, meaning aqueous sodium nitrate solution is easier to nucleate crystals.

Structure and dynamics of supercooled water in the hydration layer of poly(ethylene glycol).

The statics and dynamics of supercooled water in the hydration layer of poly(ethylene glycol) (PEG) were studied by a combination of quasi-elastic neutron scattering (QENS) and molecular dynamics (MD) simulations. Two samples, that is, hydrogenated PEG/deuterated water (h-PEG/D2O) and fully deuterated PEG/hydrogenated water (d-PEG/H2O) with the same molar ratio of ethylene glycol (EG) monomer to water, 1:1, are compared. The QENS data of h-PEG/D2O show the dynamics of PEG, and that of d-PEG/H2O reveals the motion of water. The temperature-dependent elastic scattering intensity of both samples has shown transitions at supercooled temperature, and these transition temperatures depend on the energy resolution of the instruments. Therefore, neither one is a phase transition, but undergoes dynamic process. The dynamic of water can be described as an Arrhenius to super-Arrhenius transition, and it reveals the hydrogen bonding network relaxation of hydration water around PEG at supercooled temperature. Since the PEG-water hydrogen bond structural relaxation time from MD is in good agreement with the average relaxation time from QENS (d-PEG/H2O), MD may further reveal the atomic pictures of the supercooled hydration water. It shows that hydration water molecules form a series of pools around the hydrophilic oxygen atom of PEG. At supercooled temperature, they have a more bond ordered structure than bulk water, proceed a trapping sites diffusion on the PEG surface, and facilitate the structural relaxation of PEG backbone.

Change in surface properties of hemoglobin induced by heme reaction with O2 and CO

The surface properties of hemoglobin bound to O2 (HbO2) or CO (HbCO) were investigated by ethanol precipitation, particle size analysis, and ζ potential measurements. We found that, compared with HbO2, HbCO is surrounded by more hydration water molecules, resulting in the greater physicochemical stability of HbCO in aqueous conditions. The intermolecular interactions of HbO2 and HbCO were studied by acquiring atomic force microscopy images under ambient air conditions. HbCO molecules easily aggregated on the hydrophilic mica substrate compared with HbO2 molecules during the dewetting process. We discuss these results in terms of a competing process between dispersion forces and adsorption on the hydrophilic mica substrate. The observed results suggest that the local structural differences between Fe–O2 and Fe–CO influence the surface structure of the protein, leading to the observed dissimilar physicochemical properties of HbO2 and HbCO.

Characterization of Hydration Water Bound to Choline Phosphate-Containing Polymers

Zwitterionic methacrylate polymers with either choline phosphate (CP) (poly(MCP)) or phosphorylcholine (PC) (poly(MPC)) side groups were analyzed to characterize the bound hydration water molecules as nonfreezing water (NFW), intermediate water (IW), or free water (FW). This characterization was carried out by differential scanning calorimetry (DSC) of polymer/water systems, and the enthalpy changes of cold crystallization and melting were determined. The electron pair orientation of CP is opposite to that of PC, and the former binds the alkyl terminal groups at the phosphate esters. The numbers of NFW and IW molecules per monomer unit of poly(MCP) with an isopropyl terminal group were estimated to be 10.7 and 11.3 mol/mol, respectively, which were slightly greater than those of the poly(MCP) bearing an ethyl terminal group. More NFW and IW molecules hydrated the phosphobetaine polyzwitterions, poly(MCP) and poly(MPC), compared with carboxybetaine and sulfobetaine polymers. Moreover, the hydration states of polyelectrolytes were compared with the zwitterionic polymers. Finally, we discuss the relationship between the amount of hydration water and bio-inert properties.

Hydrogen-bond mediated and concentrate-dependent NaHCO3 crystal morphology in NaHCO3–Na2CO3 aqueous solution: Experiments and computer simulations

Adding Na2CO3 to the NaHCO3 cooling crystallizer, using the common ion effect to promote crystallization and improve product morphology, is a new process recently proposed in the literature. However, the mechanism of the impact of Na2CO3 on the crystal morphology is still indeterminate. In this work, the crystallization of NaHCO3 in water and Na2CO3–NaHCO3 aqueous solution was investigated by experiments and molecular dynamics simulations (MD). The crystallization results demonstrate that the morphology of NaHCO3 crystal changed gradually from needle-like to flake structure with the addition of Na2CO3. The simulation results indicate that the layer docking model and the modified attachment energy formula without considering the roughness of crystal surface can obtain the crystal morphology in agreement with the experimental results, but the lower molecules of the crystal layer have to be fixed during MD. Thermodynamic calculation of the NaHCO3 crystallization process verifies that the common ion effect from Na+ and the ionization equilibrium transformation from CO32– jointly promote the precipitation of NaHCO3 crystal. The radial distribution function analysis indicates that the oxygen atoms of Na2CO3 formed strong hydrogen bonds with the hydrogen atoms of the (0 1 1) face, which weakened the hydration of water molecules at the crystal surface, resulting in a significant change in the attachment energy of this crystal surface. In addition, Na+ and CO32– are more likely to accumulate on the (0 1 1) face, resulting in the fastest growth rate on this crystal surface, which eventually leads to a change in crystal morphology from needle-like to flake-like.

Mixed crystal of bis-(ammonium/oxonium) tetra-aqua-μ3-fluorido-dodeca-kis-(μ2-tri-fluoro-acetato)octa-hedro-hexa-ytterbiate(III) tetra-hydrate, [(NH4)1-x (H3O) x ]2[Yb6F8(O2CCF3)12(H2O)4]·4H2O (x = 1/4), containing a hexa-nuclear ytterbium(III) carboxyl-ate complex with face-capping fluoride ligands and comprising an unusual kind of substitutional disorder.

The reaction of ytterbium metal with ammonium tri-fluoro-acetate in liquid ammonia resulted in a green substance comprising a substantial amount of ytterbium(II) tri-fluoro-acetate that is a useful precursor for the oxidative synthesis of the new ytterbium(III) compound, [(NH4)1-x (H3O) x ]2[Yb6F8(O2CCF3)12(H2O)4]·4H2O (x = 1/4), in aqueous tri-fluoro-acetic acid. This mixed ammonium/oxonium crystalline solid is the first example of a substance containing an octa-hedro-hexa-nuclear ytterbium(III) complex with μ3-face-capping fluorido ligands. The main structural features of its [Yb6F8] core are non-bonding Yb⋯Yb distances and Yb-F bond lengths of 3.7576 (3)-3.9413 (5) and 2.2375 (17)-2.3509 (17) Å, respectively. Yb-O bond lengths involving the O atoms of O,O'-bridging carboxyl-ato ligands and vertex-substituting aqua ligands are in the ranges 2.23 (4)-2.329 (2) and 2.448 (2)-2.544 (3) Å, respectively. These bond lengths are in accordance with expectations, taking into account lanthanoid contraction. Inter-estingly, there is a significant ammonium versus oxonium ion site dependence, not only of the hydrate water mol-ecule positions within the solid's hydrogen-bonding framework, but also of the coordination sites of one carboxyl-ato and one aqua ligand in the hexa-nuclear complex.

Synthesis, Spectroscopic Studies for Five New Mg (II), Fe (III), Cu (II), Zn (II) and Se (IV) Ceftriaxone Antibiotic Drug Complexes and Their Possible Hepatoprotective and Antioxidant Capacities.

Magnesium, copper, zinc, iron and selenium complexes of ceftriaxone were prepared in a 1:1 ligand to metal ratio to investigate the ligational character of the antibiotic ceftriaxone drug (CFX). The complexes were found to have coordinated and hydrated water molecules, except for the Se (IV) complex, which had only hydrated water molecules. The modes of chelation were explained depending on IR, 1HNMR and UV–Vis spectroscopies. The electronic absorption spectra and the magnetic moment values indicated that Mg (II), Cu (II), Zn (II), Fe (III) and Se (VI) complexes form a six-coordinate shape with a distorted octahedral geometry. Ceftriaxone has four donation sites through nitrogen from NH2 amino, oxygen from triazine, β-lactam carbonyl and carboxylate with the molecular formulas [Mg(CFX)(H2O)2]·4H2O, [Cu(CFX)(H2O)2]·3H2O, [Fe(CFX)(H2O)(Cl)]·5H2O, [Zn(CFX)(H2O)2]·6H2O and [Se(CFX)(Cl)2]·4H2O and acts as a tetradentate ligand towards the five metal ions. The morphological surface and particle size of ceftriaxone metal complexes were determined using SEM, TEM and X-ray diffraction. The thermal behaviors of the complexes were studied by the TGA(DTG) technique. This study investigated the effect of CFX and CFX metal complexes on oxidative stress and severe tissue injury in the hepatic tissues of male rats. Fifty-six male rats were tested: the first group received normal saline (1 mg/kg), the second group received CFX orally at a dose of 180 mg/kg, and the other treated groups received other CFX metal complexes at the same dose as the CFX-treated group. For antibacterial activity, CFX/Zn complex was highly effective against Streptococcus pneumoniae, while CFX/Se was highly effective against Staphylococcus aureus and Escherichia coli. In conclusion, successive exposure to CFX elevated hepatic reactive oxygen species (ROS) levels and lipid peroxidation final marker (MDA) and decreased antioxidant enzyme levels. CFX metal complex administration prevented liver injury, mainly suppressing excessive ROS generation and enhancing antioxidant defense enzymes and in male rats.

A temperature‐dependent hydrating structure around SO42− investigated from Raman spectroscopy

AbstractThe Raman spectra of H2O/D2O−Na2SO4 solutions are measured at temperatures (T) up to 573 K. The hydrating structure of SO42− is investigated based on the solute‐correlated (SC) OD/OH stretch bands obtained by the multivariate curve resolution method. Remaining a nearly constant wavenumber of the main peak with increasing temperature, the SC OD/OH bands are narrowed first due to the reduced relative intensity of the shoulder at 2395/~3245 cm−1 when T &lt; ~473 K/~513 K but then because the OD/OH bands get sunken around 2610/3580 cm−1 above ~473/~513 K (letting the ~2660/~3635 cm−1 mode be more prominent and even a separate peak). These spectral features incline to support a temperature‐dependent water structure in the SO42− hydration shell balanced by the SO42−···H2O(D2O) and O–H(D)···O hydrogen bonds (those with D are stronger) as well as the electrostatics. Most hydrating water molecules are engaged in hydrogen bonding configurations of single donor (SD) and single hydrogen‐bond water (SHW). As temperature rises, the average number of hydrogen bonds per hydrating water molecule decreases below ~373 K because of the SD → SHW transition and increases afterward owning to the rapid increase of the ion pairs, and SO42− changes its role from structure breaker to structure maker above ~453 K. The temperature‐dependent solubility behavior of Na2SO4 in H2O can be rationalized by the ion pairing effect and temperature‐dependent structure details in SO42− hydration shell.

Excitation of neutral red dye in aqueous media: comparative theoretical analysis of neutral and cationic forms.

The main goal of this work was the theoretical interpretation of the absorption spectra of neutral red in an aqueous solution (both neutral NR0 and protonated NR+ forms). To achieve this problem, TD-DFT/DFT calculations with different hybrid functionals, the IEFPCM solvent model, and the 6-31 + + G(d,p) basis set were used. MN12SX functional provided the best agreement with the experiment for both dye forms. While the absorption band of the cationic form of the dye in the visible region of the spectrum is due to one transition S0 → S1 (HOMO-LUMO), for its neutral form, there are two transitions S0 → S1 (HOMO → LUMO) and S0 → S2 (HOMO-1 → LUMO), with the latter having a higher intensity. The protonation of the dye chromophore introduces significant changes in HOMO shape. At the same time, LUMOs are almost the same for the protonated and neutral forms of the NR. During the transition from NR0 to the S1 state, its dipole moment increases more significantly than during the transition to the S2 state. Calculations confirmed the assumption of Singh et al. about the existence of two closely spaced excited states of NR0, the first of which has a larger dipole moment. However, the hypothesis of these authors about the corresponding intramolecular charge transfer, as well as the huge value of the dipole moment of this excited state (~ 20 D) declared by them, was not confirmed by present calculations. It was shown that the photoinduced charge redistribution in both the neutral and cationic forms of the dye is local, and the corresponding dipole moment is ~ 10 D. This agrees with other early theoretical work by Aaron et al. The influence on the NR+ absorption spectrum of hydrating water molecules was also analyzed. It was found that the interplay of electrostatic and site-specific contributions leads to the fact that NR solvatochromism does not have a pronounced dependence on the polarity of the solvent.

Excitations of safranin and phenosafranin in aqueous solution: Comparative theoretical analysis

In the present work, the excitations of safranin (SAF) and phenosafranin (PheSAF) in an aqueous solution are analyzed by DFT/TD-DFT, considering the vibronic transitions and hydration. The frontier orbitals, transitions between which cause the absorption and emission of dyes in the visible region of the spectrum, almost do not cover benzene rings and methyl groups of molecules. As a consequence, benzene rings, aromatic hydrogen atoms, as well as SAF methyl groups are not covered by the change in electron density. These features explain the same excitation energies of SAF and PheSAF observed experimentally. At the same time, during relaxation to an excited equilibrium state, the changes in the vast majority of atomic charges again turn out to be different for SAF and PheSAF, which causes a slight difference between the emission energies of dyes. Excitations of both dyes do not lead to twisting of their benzene rings relative to the chromophores - the dihedrals between them are strictly φ = 90°. The thermal twisting of these dihedrals is practically free up to Δφ∼±30°, and then it is sharply hampered. The heights of potential barriers separating their energy minima are very high (∼400 kT). The increase in the dipole moments upon excitation is due to the greatest extent to the significant increase in the electron densities at the N10 atoms of both dyes. All five H-bonds of each of the dyes with water are strengthened upon excitation. The strongest and most enhanced due to excitation are the H-bonds of waters with the N10 atoms. There is a strong photoinduced polarization of water molecules bound to the N10 atoms of the dyes. H-bonds with water of amino groups are stronger in PheSAF, and with the N10 atom, in SAF. The influence of hydrating water molecules leads to significant bathochromic shifts of the maxima of the calculated spectra (23 nm for SAF and 22 nm for PheSAF) relative to the purely implicit specification of the solvent.

Characterization of hydrogen bond network of waters around polyethylene glycol by broadband dielectric spectroscopy

Polyethylene glycol (PEG) is a well-known water retention agent in biomedical products, the hydration efficiency of which is affected by its molecular weight. Using a broadband dielectric spectroscopy (100 MHz–18 THz), the hydration state of PEG aqueous solutions with various molecular weights was quantitatively evaluated. As the molecular weight increases, the restriction strength of hydration water increases in potency, while the number of hydration water molecules decreases. Owing to the opposite changes in hydration number and restriction strength, the measured collective hydrogen bond (HB) strength shows negligible molecular weight dependence. PEG with larger Mw produces a more heterogenous HB network. The internal folding and twining caused by the growth of the PEG chain obstruct the proper exposure of hydrophilic part of the monomer producing less hydration waters. The evaluation result supports an application of PEGs with low molecular weight in contact lens package solutions.

Current status of neutron crystallography in structural biology.

Hydrogen atoms and hydration water molecules in proteins are essential for many biochemical processes, especially enzyme catalysis. Neutron crystallography enables direct observation of hydrogen atoms, and reveals molecular recognition through hydrogen bonding and catalytic reactions involving proton-coupled electron transfer. The use of neutron crystallography is still limited for proteins, but its popularity is increasing owing to an increase in the number of diffractometers for structural biology at neutron facilities and advances in sample preparation. According to the characteristics of the neutrons, monochromatic or quasi-Laue methods and the time-of-flight method are used in nuclear reactors and pulsed spallation sources, respectively, to collect diffraction data. Growing large crystals is an inevitable problem in neutron crystallography for structural biology, but sample deuteration, especially protein perdeuteration, is effective in reducing background levels, which shortens data collection time and decreases the crystal size required. This review also introduces our recent neutron structure analyses of copper amine oxidase and copper-containing nitrite reductase. The neutron structure of copper amine oxidase gives detailed information on the protonation state of dissociable groups, such as the quinone cofactor, which are critical for catalytic reactions. Electron transfer via a hydrogen-bond jump and a hydroxide ion ligation in copper-containing nitrite reductase are clarified, and these observations are consistent with the results from the quantum chemical calculations. This review article is an extended version of the Japanese article, Elucidation of Enzymatic Reaction Mechanism by Neutron Crystallography, published in SEIBUTSU-BUTSURI Vol. 61, p.216–222 (2021).

Bridging Structure, Dynamics, and Thermodynamics: An Example Study on Aqueous Potassium Halides.

Aqueous salt systems are ubiquitous in all areas of life. The ions in these solutions impose important structural and dynamic perturbations to water. In this study, we employ a combined neutron scattering, nuclear magnetic resonance, and computational modeling approach to deconstruct ion-specific perturbations to water structure and dynamics and shed light on the molecular origins of bulk thermodynamic properties of the solutions. Our approach uses the atomistic scale resolution offered to us by neutron scattering and computational modeling to investigate how the properties of particular short-ranged microenvironments within aqueous systems can be related to bulk properties of the system. We find that by considering only the water molecules in the first hydration shell of the ions that the enthalpy of hydration can be determined. We also quantify the range over which ions perturb water structure by calculating the average enthalpic interaction between a central halide anion and the surrounding water molecules as a function of distance and find that the favorable anion-water enthalpic interactions only extend to ∼4 Å. We further validate this by showing that ions induce structure in their solvating water molecules by examining the distribution of dipole angles in the first hydration shell of the ions but that this perturbation does not extend into the bulk water. We then use these structural findings to justify mathematical models that allow us to examine perturbations to rotational and diffusive dynamics in the first hydration shell around the potassium halide ions from NMR measurements. This shows that as one moves down the halide series from fluorine to iodine, and ionic charge density is therefore reduced, that the enthalpy of hydration becomes less negative. The first hydration shell also becomes less well structured, and rotational and diffusive motions of the hydrating water molecules are increased. This reduction in structure and increase in dynamics are likely the origin of the previously observed increased entropy of hydration as one moves down the halide series. These results also suggest that simple monovalent potassium halide ions induce mostly local perturbations to water structure and dynamics.

Microhydration of sorbitan monostearate (Span 60) investigated using hybrid QM/MM calculations in the gas phase

The binding interactions and structural and electronic properties involved in microhydration of Span 60•(H2O)n (n = 1–10) complexes were investigated using hybrid QM/MM calculations in the gas phase. The Span 60 head group and its hydrating water molecules were treated using the QM method, while the Span 60 tail group was treated with the MM method. For the monohydration complex, it was revealed that the hydration energy and binding force are strongly dependent on the hydration sites (O1–O6) of the Span 60 head group. The ether oxygen (O4) exhibited the highest hydration energy and the strongest hydrogen bonds. An ester oxygen atom (O2) showed a lower binding force due to its delocalized lone pair electrons. For polyhydration complexes, it was found that the average hydration energy of each polyhydration complex is close to that of the monohydrated form because of steric hindrance and electrostatic repulsions between water molecules. Interaction between the ether oxygen (O4) and water molecules is still prominent for all polyhydration complexes. The charge transfer phenomenon is noticeably altered when a few water molecules bind with a particular oxygen atom of the Span 60 head group, leading to an increase in the polarity of this group. Hydrogen bond lengths tend to decrease slightly and the conformation change of the Span 60 head group is significantly affected as the number of hydrated water molecules increases.

Raman spectroscopy study for the systems (LiCl-H2O and LiCl-MgCl2-H2O): Excess spectra and hydration shell spectra

The micro-structure of hydration shell of solute in water is significant for understanding the properties of aqueous solutions. Raman spectroscopy has been employed for studying the hydration shell structure of the solute for decades, however, Raman imaging data is still seriously overlapped, making it challenging to obtain information on the spectrum of hydrated water molecules. In this paper, Raman spectroscopy was employed to study the O-H vibration peaks of LiCl aqueous solution and LiCl-MgCl2-H2O mixed aqueous solution. The changes of stretching vibration peak of 2800 ∼ 3800 cm-1O-H and hydrogen bond network structure in aqueous solution were analyzed at room temperature and ion association. With the increase of magnesium salt ratio, the damage of solute to the bulk water gradually decreases in the mixed solution, which indicated that LiCl has a more significant influence on the bulk water molecules. It is mainly due to the intense hydration of Li+, which can not only affect the water molecules in the first hydration shell but also affect the water molecules in the second hydration shell. The number of water molecules in the first hydration shell were obtained by extracting the spectra of different solute first hydration shells from the solution spectra. Those spectra of the hydration shell were employed to study the micro-structures of the first hydration shells of anions, and the aggregation behavior of ions in the the mixed solution.

Characterizing Mass-Transfer mechanism during gas hydrate formation from water droplets

Understanding the transport properties of water and guest molecules in dense hydrate structures is pivotal in controlling their formation and stability; this is nevertheless hindered by challenges in characterizing the time-varying diffusion and permeation process. This paper reports experimental results on the mass-transfer characteristics of water and guest molecules across hydrate shells during their formation from water droplets. The time-dependent 3D morphologies and parameters of hydrate shells were in-situ obtained via X-ray computed tomography to reveal the transport mechanism in hydrates. It was found that in the mass-transfer-limited stage, hydrate growth occurred primarily at the hydrate-gas interface, indicating a more mobile characteristic of water molecules in those hydrate shells than gas. The resulting outward permeation of water led to the growth of hydrate protrusions on the outer surface of the hydrate shell, suggesting a transition of the water transport schema through the hydrate shell from diffusion to permeation. Consequently, a diffusion-based shell growth model was developed to quantify the diffusivity of gas and water molecules in hydrates. Combining the measurements in the temperature range of 275.15–283.15 K, the effective diffusion coefficient of gas through the hydrates was estimated to be in the range of 1.34 × 10−14 − 1.90 × 10−13 m2/s, with that of interstitial water in the range of 2.87 × 10−12 − 3.83 × 10−11 m2/s. These results are of fundamental value in developing an improved understanding of the kinetics of hydrate formation from gas-water interfaces, which is essential in the optimization of hydrate-based techniques.

Single-Crystal-to-Single-Crystal Cluster Transformation in a Microporous Molybdoarsenate(V)-Metalorganic Framework.

The hybrid compound [Cu(cyclam)(H2O)2]0.5[{Cu(cyclam)}1.5{B-H2As2Mo6O26(H2O)}]·9H2O (1) (cyclam = 1,4,8,11-tetraazacyclotetradecane) was synthesized in aqueous solution by reacting the {Cu(cyclam)}2+ complex with a mixture of heptamolybdate and an arsenate(V) source. Crystal packing of 1 exhibits a supramolecular open-framework built of discrete covalent molybdoarsenate/metalorganic units and additional [Cu(cyclam)(H2O)2]2+ cations, the stacking of which generates squarelike channels parallel to the z axis with an approximate cross section of 10 × 11 Å2 where all the hydration water molecules are hosted. Thermal evacuation of solvent molecules yields a new anhydrous crystalline phase, but compound 1 does not preserve its single-crystalline nature upon heating. However, when crystals are dehydrated under vacuum, they undergo a structural transformation that proceeds via a single-crystal-to-single-crystal pathway, leading to the anhydrous phase [{Cu(cyclam)}2(A-H2As2Mo6O26)] (2). Total dehydration results in important modifications within the inorganic cluster skeleton which reveals an unprecedented solid-state B to A isomerization of the polyoxoanion. This transition also involves changes in the CuII bonding scheme that lead to covalent cluster/metalorganic layers by retaining the open-framework nature of 1. Compound 2 adsorbs ambient moisture upon air exposure, but it does not revert back to 1, and the hydrated phase [{Cu(cyclam)}2(A-H2As2Mo6O26)]·6H2O (2h) is obtained instead. Structural variations between 1 and 2 are reflected in electron paramagnetic resonance spectroscopy measurements, and the permanent microporosity of 2 provides interesting functionalities to the system such as the selective adsorption of gaseous CO2 over N2.