Volume 92, Issue 10

Solvent basicity and polarity/polarizability parameters are analysed using molecular properties of solvents derived from computational chemistry. The results show that Kamlet and Taft’s measure of hydrogen bond basicity, β , is essentially identical to Gutmann’s donor number, a measure of Lewis basicity, both being determined by the charge on the most negative atom of the solvent molecule and the energy of the electron donor orbital. It is also found that, for both parameters, the calculated values for alcohols and N–H containing bases deviate systematically from those for aprotic solvents. This mirrors Kamlet and Taft’s earlier observation that different solvatochromic probes yield different β values in amphiprotic solvents. Reichardt’s E T (30) and Kamlet, Abboud and Taft’s π * both show direct dependences on the dipole moments and quadrupolar amplitudes of the solvent molecules and, surprisingly, an inverse dependence on the molecular polarizability. Additionally, E T (30) has a strong dependence on the charge on the most positive hydrogen atom of the solvent molecule, reflecting its sensitivity to hydrogen bonding. Unexpectedly, π * shows a dependence on the energy of the electron donor orbital. Kammet and Taaft’s hydrogen bond acidity parameter, α , is discussed in light of the results for π * and E T (30).

Solution chemistry is commonly regarded as the physical chemistry of reactions and chemical equilibria taking place in the bulk of a solvent, and between solutes in solution, and solids or gases in contact with the solution. Our knowledge about such reactions and equilibria in aqueous solution is very detailed such as their physico–chemical constants at varying temperature, pressure, ionic medium and strength. In this paper the solution chemistry in the surface region of aqueous solutions, down to ca. 10 Å below the water–air interface, will be discussed. In this region, the density and relative permittivity are significantly smaller than in the aqueous bulk strongly affecting the chemical behaviour of solutes. Surface sensitive X-ray spectroscopic methods have recently been applicable on liquids and solutions by use of liquid jets. This allows the investigation of the speciation of compounds present in the water–air interface and the surface region, a region hardly studied before. Speciation studies show overwhelmingly that neutral molecules are accumulated in the surface region, while charged species are depleted from it. It has been shown that the equilibria between aqueous bulk, surface region, solids and/or air are very fast allowing effective transport of chemicals over the aqueous surface region.

In many relevant situations, water is not in its bulk form but instead attached to some substrates or filling some cavities. We shall call water in the latter environment confined water as opposed to bulk water. It is known that the confined water is essential for the stability and the function of biological macromolecules. In this paper, we provide a review of the experimental and computational advances over the past decades concerning the understanding of the structure and dynamics of water confined in aqueous solutions of biological relevance. Examples involving water in solution of organic solutes (cryoprotectants such as dimethylsulfoxide (DMSO), sugars such as trehalose) are provided.

In-situ analytical methods are essential for the reliable observation of peptide reactions without perturbation of the system. In this work, a real-time in-situ NMR analysis was performed to gain insight into the initial stage of the aggregation of amyloid-beta (Aβ) 8–25 monomers, S 8 GY 10 EVHHQKLVFF 20 AEDVG 25 , in solution prior to the fibril formation. NMR chemical shift and intensity changes in combination with the CD spectra revealed no changes in Aβ secondary structure, but the presence of soluble, oligomeric intermediates followed by the appearance of insoluble and non-structured aggregates before β-fibril formation. Molecular views of intermediates and aggregation mechanisms were proposed in comparison with NMR spectral changes in wild-type Aβ 8–25 and its two mutants, A21G and E22G. The mutation of just one amino acid modified the aggregation properties of Aβ 8–25; it slowed or accelerated the fibril formation by controlling the progress of conversion from monomer to aggregate via a soluble, small oligomer.

Entecavir, triphosphorylated in liver cells, is an antiviral reagent against Hepatitis B virus (HBV). The reagent inhibits reverse transcription of RNA inside the virus capsid. In the present study, free energy profile of an Entecavir triphosphate (ETVTP) molecule has been calculated when it passes through pores of the capsid along two- and three-fold rotational symmetry axes in order to investigate permeation pathway of the reagent to the inside of the capsid. The calculations have been done based on thermodynamic integration (TI) method combined with all-atomistic molecular dynamic (MD) calculations. A free energy minimum of −19 kJ/mol was found at the entrance of the pore from the outside along the three-fold symmetry axis. This stabilization is from the interaction of negatively charged ETVTP with positively charged capsid methionine residues. This excess free energy concentrates of the reagent at the entrance of the pore by a factor of about 2000. A free energy barrier of approximately 13 kJ/mol was also found near the exit of the pore to the inside of the capsid due to narrow space of the pore surrounded by hydrophobic wall made by proline residues and negatively charged wall by aspartic acid residues. There, ETVTP is partially dehydrated in order to pass through the narrow space, which causes the great free energy loss. Further, the negatively charged residues produce repulsive forces on the ETVTP molecule. In contrast, in the case of the pore along the two-fold symmetry axis, the calculated free energy profile showed shallower free energy minimum, −4 kJ/mol at the entrance in spite of the similarly high barrier, 7 kJ/mol, near the exit of the pore.

The use of dielectric relaxation spectroscopy (DRS) for studying electrolyte solutions is reviewed, focussing on the authors’ investigations over the last three decades. It is shown that this often-overlooked technique provides powerful insights into the nature of ion-ion and ion-solvent interactions. DRS is revealed to be particularly useful for detection of weak ion association and, due to its unique ability to detect solvent-separated species, the quantitation of ion pairing. It is demonstrated that DRS correctly determines chemical speciation for ion-paired systems where major spectroscopic techniques (NMR, Raman, UV-vis) fail. DRS also provides important insights into ion solvation. In aqueous solutions, it has been used to build up a coherent set of ‘effective’ hydration numbers for ions based on the dynamics of proximate water molecules, and has a unique ability to detect ‘slow’ water resulting from hydrophilic and hydrophobic hydration of solutes. DRS has been especially useful for characterising the behaviour of ionic liquids (ILs), e.g. showing they possess rather low dielectric constants and, surprisingly, contain no significant concentrations of ion pairs. Neat ILs and their mixtures with molecular solvents are shown by ultra-broadband DRS to exhibit extremely complicated behaviour especially at frequencies in the THz region.

Characterization of structural heterogeneity of liquid solutions and the pursuit of its nature have been challenging tasks to solution chemists. In the last decade, an emerging method called excess spectroscopy has found applications in this area. The method, combining the merits of molecular spectroscopy and excess thermodynamic functions, shows the ability to enhance the apparent resolution of spectra, provides abundant information concerning solution structures and intermolecular interactions. In this review, the thinking and mathematics of the method, as well as its developments, are presented first. Then, research progress related to the exploration of the method is thoroughly reviewed. The materials are classified into two parts, small-molecular solutions and ionic liquid solutions. Finally, potential challenges and the perspective for further development of the method are discussed.

The structure of aq. sodium acetate solution (CH 3 COONa, NaOAc) was studied by X-ray scattering and density function theory (DFT). For the first hydrated layer of Na + , coordination number ( CN ) between Na + and O(W, I) decreases from 5.02 ± 0.85 at 0.976 mol/L to 3.62 ± 1.21 at 4.453 mol/L. The hydration of carbonyl oxygen (OC) and hydroxyl oxygen (OOC) of CH 3 COO − were investigated separately and the OC shows a stronger hydration bonds comparing with OOC. With concentrations increasing, the hydration shell structures of CH 3 COO − are not affected by the presence of large number of ions, each CH 3 COO − group binds about 6.23 ± 2.01 to 7.35 ± 1.73 water molecules, which indicates a relatively strong interaction between CH 3 COO − and water molecules. The larger uncertainty of the CN of Na + and OC(OOC) reflects the relative looseness of Na-OC and Na-OOC ion pairs in aq. NaOAc solutions, even at the highest concentration (4.453 mol/L), suggesting the lack of contact ion pair (CIP) formation. In aq. NaOAc solutions, the so called “structure breaking” property of Na + and CH 3 COO − become effective only for the second hydration sphere of bulk water. The DFT calculations of CH 3 COONa (H 2 O) n=5–7 clusters suggest that the solvent-shared ion pair (SIP) structures appear at n = 6 and become dominant at n = 7, which is well consistent with the result from X-ray scattering.

Group theoretical arguments are used to find the subgroup corresponding to symmetry reduction along a particular irreducible representation of a group. The results are used to guide the search for stationary points on the potential energy surface of hydrated copper(I) ion at the HF/6-31G∗, HF/6-31+G∗, HF/6-311+G∗, MP2/6-31G∗, MP2/6-31+G∗, MP2/6-311+G∗, B3LYP/6-31G∗, B3LYP/6-31+G∗, and B3LYP/6-311+G∗ levels. The better levels give the most stable coordination number of two. The effect of desymmetrization on the Cu-O distances and stretching frequencies has been examined.

Adsorption is one of the most widely used processes in physicochemical operations. To design an adsorbent for a specific adsorbate, it is important to understand the interactions between adsorbents and adsorbates, which are very important for both adsorption capacity and selectivity. Electrostatic interactions, hydrogen bonding, hydrophobic interactions, complexation, and precipitation are comprehensively discussed. Adjusting solution pH and ionic strength is an effective method to improve the adsorption, especially when electrostatic and hydrophobic interactions are main interactions. With the increase in ionic strength, the hydrophobic interactions between adsorbents and adsorbates increase, while the electrostatic interactions decrease.

A compilation of available experimental data for acetone-water mixtures with the reciprocal salt system Na + , K + || Cl − , SO 4 2− is presented. Significant inconsistencies among experimental data are pointed out. New freezing point measurements are reported for the binary acetone-water system at 12 different compositions. UNIQUAC parameters are determined on the basis of the available data from literature. Modeling results are presented. Vapor-liquid, liquid-liquid, and solid-liquid equilibria together with thermal properties are reproduced well by the model using only 14 parameters. The major drawback of the model is that the calculated liquid-liquid equilibrium regions of systems with KCl and NaCl are larger than the experimentally determined regions. The model is valid in the temperature range from −16 to 100 °C.

Densities of the sodium arsenate aqueous solution with the molality varied from (0.04165 to 0.37306) mol · kg −1 were determined experimentally at temperature intervals of 5 K from 283.15 K to 363.15 K and ambient pressure using a precise Anton Paar Digital vibrating-tube densimeter. The apparent molar volumes ( V ϕ ), thermal expansion coefficient ( α ) and partial molar volume ( V ¯ B ) $({\bar V_{\rm{B}}})$ were obtained based on the results of density measurement. The 3D diagram of apparent molar volume against temperature and molality as well as the diagram of thermal expansion coefficient and partial molar volume against molality were plotted, respectively. On the basis of the Pitzer ion-interaction equation of apparent molar volume model, the Pitzer single-salt parameters ( ( β M,X ( 0 ) v , β M,X ( 1 ) v ,   β M,X ( 2 ) v  and  C M,X v , M X = N a 3 A s O 4 ) $(\beta _{{\rm{M,X}}}^{(0)v},\beta _{{\rm{M,X}}}^{(1)v},{\rm{ }}\beta _{{\rm{M,X}}}^{(2)v}{\rm{ and }}C_{{\rm{M,X}}}^v,MX = N{a_3}As{O_4})$ and their temperature-dependent correlation F ( i , p , T ) =  a 1  +  a 2 ln( T /298.15) +  a 3 ( T  – 298.15) +  a 4 /(620 –  T ) +  a 5 /( T  – 227) (where T is temperature in Kelvin, a i is the correlation coefficient) for Na 3 AsO 4 were obtained on account of the least-squares method. Predictive apparent molar volumes agree well with the experimental values, and those results indicate that the single-salt parameters and their relational coefficients of temperature-dependence for Na 3 AsO 4 obtained are reliable.

Various toxic metal ions were successfully removed from solid matrix into supercritical CO 2 (scCO 2 ) by open-chain crown ether bridged diphosphates at 313.15 K and 20 MPa, these diphosphates with different ester side chains and different length of ethylene oxide bridge group are highly soluble in supercritical CO 2 . The extraction efficiency ( E %) of heavy metals is between 55 and 89%. Mulliken charge distribution of ligand’s P=O coordination group was calculated to indicate the stability of metal complex. The ligand structure effects and the rationale for different selectivity were discussed. In addition, binding property of these diphosphates towards the alkaline earth metals was further studied following the same extraction procedures. Alkaline earth metal ions Ca 2+ , Sr 2+ and Ba 2+ were extracted with E % at 49–74%, 50–73% and 16–64%, respectively. DFT calculations were performed to investigate the interaction energy of the complexes and the correlation with the E % was discussed.

It has been recognised that ionic liquids (ILs) with long alkyl-chains have a segregated structure due to the inhomogeneous distribution of polar parts and non-polar parts. This inhomogeneity of ILs brings about unique solvation phenomena of solute molecules dissolved in ILs. We have investigated various solvation-state selective phenomena by using laser spectroscopic techniques such as solvation state selective vibrational spectroscopy, translational and rotational dynamics of small molecules in ILs, and solvation state selective fundamental chemical reactions. In this paper, we have reviewed an intramolecular electron transfer (ET) reaction in the Marcus inverted region of N , N -dimethyl- p -nitroaniline and an intramolecular proton transfer (IPT) reaction in 4′- N , N -diethylamino-3-hydroxyflavone as examples of chemical reactions affected by unique solvation in ILs.

Certain complexing agents (such as D-gluconate, D-isosaccharinate, etc.) as well as actinides and lanthanides are simultaneously present in cementitious radioactive waste repositories and (in the presence of water) are capable of forming complex compounds. Such processes may immobilize radionuclides and are of importance in the thermodynamic modelling of the aqueous chemistry of waste repositories. Nd(III) is considered to be a suitable model for trivalent lanthanides and actinides, due to the similarity of their ionic radii. In the current work, solid complexes isolated from aqueous solution containing Nd(III), Ca(II) and D-gluconate (Gluc − ) were investigated. In an aqueous solution containing Nd(III) and Gluc − , the formation of a precipitate was observed at pH ≥ 8. This precipitate was found to redissolve around pH ~ 11, but reprecipitated when Ca(II) ions were added to the solution. In order to gain an insight in binary and ternary aqueous systems, in the present work we report the structure of these solid complexes obtained from XRD, FT-IR, Raman, SEM-EDAX and UV-DRS measurements. The structure of these solids, where possible, was compared with those identified in solution. The compositions of these complexes are suggested to be NdGlucH −1 (OH) · 2H 2 O and CaNdGlucH −1 (OH) 3  · 2H 2 O, respectively. In these, the chemical environment of the Nd(III) was found to be the same as that in the NdGlucH −1 (OH) 0 (aq) solution species.

Phenolic compounds are important basic materials for the organic chemical industry, such as pesticides, medicines and preservatives. Phenolic compounds can be obtained from biomass, coal and petroleum via pyrolysis and liquefaction, but they are mixtures in oil. The traditional methods to separate phenols from oil using alkaline washing are not environmentally benign. To solve the problems, deep eutectic solvents (DESs) and ionic liquids (ILs) have been developed to separate phenols from oil, which shows high efficiency and environmental friendliness. In this article, we summarized the properties of DESs and ILs and the applications of DESs and ILs in the separation of phenols and oil. There are two ways in which DESs and ILs are used in these applications: (1) DESs formed in situ using different hydrogen bonding acceptors including quaternary ammonium salts, zwitterions, imidazoles and amides; (2) DESs and ILs used as extractants. The effect of water on the separation, mass transfer dynamics in the separation process, removal of neutral oil entrained in DESs, phase diagrams of phenol + oil + extractant during extraction, are also discussed. In the last, we analyze general trends for the separation and evaluate the problematic or challenging aspects in the separation of phenols from oil mixtures.