Spectroscopic study of association of a hemicyanine dye in mixed aqueous binary solvents

Electronic absorption and steady state emission properties of a hemicyanine dye [4-[4-(dimethylamino)styryl]-1-docosylpyridinium bromide], have been studied in several pure solvents and two mixed binary solvents (water+ethanol and water+acetonitrile). In pure solvents the band-width of the absorption spectrum correlates well with the Stoke's shift. In mixed aqueous solvents two different molecular forms of the solute, viz. the monomer and the dimer of the solute exists in equilibrium. Thermodynamic parameters (e.g. the Delta G degrees and Delta H degrees ) characterizing the equilibrium have been determined. While the value of Delta G degrees changes very slightly with the composition of the binary mixture, the value of Delta H degrees has been observed to depend significantly with solvent composition.     

Solvation of lithium chloride in aqueous and mixed solutions of an aprotic solvent

The dynamic hydration and solvation numbers of lithium chloride are estimated on the basis of experimental data on the limiting electrodialysis concentration of an electrolyte from aqueous and aqueousorganic solutions containing aprotic solvent N,N-dimethylacetamide. It is established that the dependence of the hydration numbers of the salt on the volume fraction of the aprotic solvent is of an extreme character, and its solvation number on N,N-dimethylacetamide does not depend on the composition of the mixed solution.     

Bulk and microscopic properties of copolymer networks in mixed aqueous solvents

Amphiphilic films and hydrogels have been prepared from ethanol/water solutions containing a hydrocarbon-grafted water-soluble cellulose ether. These materials are characterized by dispersed hydrophobic microdomains which form spontaneously in the solvent due to the inherent incompatibility of the side chains with water. At low applied shear stress, the microdomains behave as temporary linkages of finite lifetimes, imparting viscoelastic properties to the networks. The molecular weight between microdomains was found to be independent of the volume fraction of polymer in the gel, and the number of linkages per backbone ranges from 22.8 ± 1.3 to 26.2 ± 1.5 over the frequency range 30–50 rad/s. The behavior of the solutions and gels was characterized using fluorescence and dynamic rheological measurements. It was demonstrated that the microdomains are capable of sequestering water-insoluble solutes. © 1992 John Wiley & Sons, Inc.     

Spectral and thermodynamic characteristics of intermolecular hydrogen bonding in individual and mixed solvents

Various systems with intermolecular hydrogen bonding (IHB) have been studied by IR-spectroscopy. Calculation of integral intensities for absorption bands of valent vibrations of the proton donor OH-bonds, constants equilibrium and enthalpies for complex formation in individual solvents with various polarities and their mixtures has been performed. The effect of medium properties on the process of IHB formation in the presence of the active solvent in the mixture has been found to concern specific solvation of interacting components molecules in the ground state.     

Weak complexes of sulphur and selenium: a thermodynamic view on solvent effects in mixed solvents

The effect of mixed solvent on complex stability is discussed from a thermodynamic standpoint. The variation in the free energy of formation for a complex species in different solvent mixtures is an indication of solvent-solute and solvent-solvent interactions. The complexity of the systems in mixed solvents makes entropy data difficult to interpret. Evidence is given on solvent-solvent interactions.     

Solvation of ions in liquid solvents and proteins

Despite their well-known drawbacks, the approaches of continuum electrostatics are widely used at the analysis of the energies of solvation and reorganization. We propose a method to check the applicability of these approaches in the determination of the solvation energy, which is based on measuring the difference of redox potentials ΔE of two consecutive redox reactions, e.g. for the pairs Co(Cp) ₂ ⁺ /Co(Cp)₂/Co(Cp) ₂ ^? (here, Cp is cyclopentadienyl). In this difference, the solvophobic effects and the liquid junction potential between the working and reference electrodes, which is impossible to measure, cancel out. From the difference of ΔE in two different solvents, the sum of the electrostatic components of the cation-and anion-transfer energies is determined. It is shown that, for large low-charged ions in aprotic media, the continuum electrostatics proves to be true in a wide range of dielectric permittivities including those typical for proteins. The Stokes shift of fluorescence spectra for proflavine (PF) showed that the water reorganization energy and, hence, the energy of the static dielectric response are anomalously high. To study this effect on the solvation energy, we determined the redox potentials of the Co(Cp) ₂ ⁺ /Co(Cp)₂ pair in a number of water-organic media. The organic cosolvent breaks the water structure and reduces the reorganization energy. Accordingly, the redox potential turns more positive. This allowed us to determine the energy of transfer of Co(Cp) ₂ ⁺ ions (and, hence, of other ions) nonviolated by the water structure specifics. The experimental energies of the acetate transfer exceed those calculated by an order of magnitude. This demonstrates the incorrectness of the widely used semicontinuum calculations of the pK of ionogenic groups of proteins. A new algorithm, which permits overcoming this discrepancy, is proposed, namely, the short-range interactions are taken into account based on the experimental energies of the transfer to a model DMF solvent, while the transfer energy from this solvent to the protein is calculated electrostatically. The energy of the ion charging in a protein consists of two physically different components, namely, the charging energy in the pre-existing field of protein dipoles and charges and the energy of the dielectric response of the medium. The former energy is determined by the electronic polarization of the protein (its optical dielectric permittivity), while the latter is determined by all kinds of polarization (static permittivity). Taking into account all the aforementioned peculiarities leads to reasonable agreement with the experiment when estimating the pK of certain groups in α-chymotrypsin. These calculations as well as experimental data (both our and taken form the literature (molecular dynamics)) point to the enhanced dielectric permittivity of the outer layers of proteins.     

Solvation peculiarities of nicotinamide in aqueous ethanol

The solvation state of biologically active provitamin PP, viz., 3-pyridinecarboxamide, was studied in a water-ethanol solvent by ¹H and ¹³C NMR and IR spectroscopy. The resolvation of the N heteroatom was found to occur at an EtOH content of 0.05?C0.25 molar fraction. The degree of conjugation of the carbamide group with the heterocycle changes with an increase in the ethanol concentration. The structures of aqueous and ethanol nicotinamide solvates were optimized by the B3LYP/6-311G(d,p) method. The ¹³C chemical shifts (GIAO) and the IR spectra were calculated.     

Limiting conductances of electrolytes and the walden product in mixed solvents in a phenomenological approach

The applicability of the Quint-Viallard conductivity equation to the representation of electrical conductivities in mixed solvents is examined. The concept of the modified Walden product is introduced, and the benefits compared with the ordinary Walden product are discussed. The universal curve of limiting conductances for all electrolytes (or for all ions) in a given pair of solvents is introduced and examined in a number of mixtures which include methanol, ethanol, 1-propanol, tert-butyl alcohol, 1,4-dioxane, N, N-dimethylformamide, sulfolane, tetrahydrofuran, and ethylene carbonate with water. Also examined are nonaqueous mixtures of acetone-ethanol, acetone-1-propanol, dimethyl sulfoxide-propylene carbonate, acetonitrile-methanol, acetonitrile-carbon tetrachloride, and acetonitrile-propylene carbonate. Many electrolytes were involved in the evaluation of the universal curves, but the majority are alkali-metal halides, tetraalkylammonium halides, tetraalkylammonium tetraphenylborides, and potassium xanthates (inorganic and organic acids are treated separately). If in a given mixed solvent system the limiting conductance of electrolyte is unknown, the universal curve permits estimating its value and gives an indication about the quality of performed conductivity measurements. The existence of universal curves of limiting conductances indicates that the properties of electrolytes in pure solvents are, to a great extent, preserved also in the mixture of solvents due to the simple dilution effect.     

Surface charge properties of Fe2O3 in aqueous and alcoholic mixed solvents

Iron oxide (Fe2O3) was identified and characterized by surface area, X-ray diffractometry, and FTIR analyses. Surface charge densities, point of zero charge (PZC), and surface ionization constants were determined from the potentiometric titration data in various aqueous and aqueous organic mixed solvents in the temperature range 293-313 K. The surface charge densities were observed to decrease with the increase in temperature and concentration of metal ions in both the aqueous and aqueous organic mixed solvents. The absolute values of the surface charge density were found to change in the order aqueous > aqueous/methanol > aqueous/ethanol. Further, the PZC of the iron oxide was observed to shift to the higher pH values in the order ethanol > methanol > aqueous solution, which indicated a decrease in the acidity of the surface -OH groups. The pKa1 and pKa2 values of iron oxide were also determined and then used for determination of the surface potential (psi0) of the solid in aqueous and aqueous organic mixed solvents. The surface potential-surface charge curves generally supplemented the results derived from psi0-pH curves.     

Anodic stripping voltammetry of tellurium(IV) in aqueous solution and mixed solvents

The anodic stripping determination of Te(IV) using a rotating glassy carbon disc electrode is described. 0.1 ppm tellurium can be determined in 1 M HCl. Interfering effects of the metals forming intermetallic compounds with tellurium on the electrode are eliminated by solvent extraction. Te(IV) is determined either after its extraction — as an ion associate or as the chelate with diphenylthiocarbazone — in the organic phase which is adjusted for the stripping determination by the addition of methanolic solution of LiCl or after the separation of all interfering elements by an extraction procedure involving the extraction of Te(IV) into methylisobutylketone and back extraction of Te(IV) with water followed by the separation of the remaining interfering elements into diisopropylether. The proposed method enables the determination of 10 μg Te in the presence of 25 mg Pb and Sb and 0.5 mg Ag, Cu, Bi, As, Sn, Cd, Zn with reproducible results and error ±5%.     

Solvation in aqueous N,N-dimethylformamide

From cryoscopic and calorimetric measurements on aqueous N,N-dimethylformamide, we have found that N,N-dimethylformamide + water forms an associated solution. The hydrate, evident at low temperatures, is always present at 303.15 and 343.15 K at which temperatures the calorimetric measurements were made. The simplified model of an ideal-associated solution with two hydrates, explains satisfactorily the excess enthalpies.     

The effect of aqueous organic solvents on the dissociation constants and thermodynamic properties of alkanolamines

The dissociation constants of protonated monoethanolamine and N-methyldiethanolamine have been determined in methanol–water, ethanol–water, and t-butanol–water solvents. The alcohol mole fractions were ranging from 0.2 to 0.95 and the temperatures from 283 to 323 K, 283 to 333 K, and at 298.15 K, respective to the different solvents. The experimental results are reported with the standard state thermodynamic properties. The basic strength of the protonated alkanolamine decreases with decreasing dielectric constant and increasing temperature of the solvent.     

Solvation thermodynamic mapping of molecular surfaces in AmberTools: GIST

The expulsion of water from surfaces upon molecular recognition and nonspecific association makes a major contribution to the free energy changes of these processes. In order to facilitate the characterization of water structure and thermodynamics on surfaces, we have incorporated Grid Inhomogeneous Solvation Theory (GIST) into the CPPTRAJ toolset of AmberTools. GIST is a grid‐based implementation of Inhomogeneous Fluid Solvation Theory, which analyzes the output from molecular dynamics simulations to map out solvation thermodynamic and structural properties on a high‐resolution, three‐dimensional grid. The CPPTRAJ implementation, called GIST‐cpptraj, has a simple, easy‐to‐use command line interface, and is open source and freely distributed. We have also developed a set of open‐source tools, called GISTPP, which facilitate the analysis of GIST output grids. Tutorials for both GIST‐cpptraj and GISTPP can be found at ambermd.org. © 2016 Wiley Periodicals, Inc.     

The thermodynamic characteristics of formation of nanoparticles from supercritical solvents

A thermodynamic model for calculating equilibria and the phase states of fluid-solid systems over wide temperature, pressure, and composition ranges was suggested. Calculations were performed to determine the parameter regions where a substance was isolated in the form of a solid depending on the chemical composition of the initial mixture and the amounts of and ratio between the solvent and antisolvent in it. The calculations were performed for a model system containing ethanol, carbon dioxide, and phenanthrene. The rapid expansion of this supercritical fluid was simulated to model the conditions of the formation of a solid (phenanthrene) and nanoparticles based on it. It was shown that the selection of the state parameters of the supercritical mixture determined the optimum conditions of solid state formation as the fluid expanded.     

Solvation of ions. XXVII. Comparison of methods to calculate single ion free energies of transfer in mixed solvents

Free energies of transfer of ions from water to mixtures of water with acetonitrile (AN), with dimethylformamide (DMF), with dimethylsulfoxide (DMSO), and with ethylene glycol have been determined using both the tetraphenylarsonium tetraphenylboride [TATB] and the negligible liquid junction potential [E _j ] assumptions. By making use of ΔG _tr (Ag⁺)[TATB]=12 kJ-mol^?1 for transfer from DMSO to AN and by assuming negligible liquid junction potential in the cell $${\text{Ag|AgNO}}_{\text{3}} {\text{(0}}{\text{.01}}M{\text{),S}}\parallel {\text{Et}}_{\text{4}} {\text{NPic(0}}{\text{.1}}M{\text{),AN}}\parallel {\text{AgNO}}_{\text{3}} {\text{(0}}{\text{.01}}M{\text{),AN|Ag}}$$ single ion free energies of transfer of silver ion ΔG _tr (Ag⁺)[E _j ] from DMSO to 35 pure and mixed solvents show a standard deviation of only 2 kJ-mol^?1 when compared with ΔG _tr (Ag⁺) calculated from the TATB assumption that ΔG _tr (Ph ₄ As⁺)=ΔG _tr (Ph ₄ B^?). The ferrocene assumption [Fc] also gives acceptable agreement with ΔG _tr (Ag⁺)[TATB] provided that the solvents are not highly aqueous. Other cells with other junctions give less acceptable agreement between the E _j and TATB assumptions. It is essential that the salt bridge is always tetraethylammonium picrate in AN, if the E _j assumption is assumed. Because of the ease of making potentiometric measurements compared with the difficulty of measurements required for the TATB assumption, the negligible liquid junction potential method in the cell shown is recommended for estimating transfer free energies of single ions. The ferrocene assumption is acceptable only for non-structured aprotic solvents.     

Polymer solubility in mixed solvents

Modeling results indicate that polymer chains in mixtures of a good with a bad solvent exhibit preferential adsorption of the good solvent. That phenomenon is found to be strongly dependent on molecular weight and it increases with a decrease in chain length. These results have important consequences on polymer solubility. Thus, a low molecular weight chain in a solvent mixture behaves as if it were dissolved in the pure good solvent component, whereas the solubility of a longer chain is controlled by the average mixture composition. As a result, quenching a polydisperse system below the cloud point may induce molecular weight segregation between the two phases: the longer chains, which precipitate out first, tend to populate the polymer rich phase whereas the shorter chains, having greater solubility, remain in the solvent phase. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2782–2787, 1999     

Kinetic aspects and swelling changes of magnesium and cerium titano-antimonates in aqueous and mixed solvents

Magnesium titano-antimonate (MgTi₅Sb₂O₁₆·12.5H₂O) and cerium titano-antimonate (Ce₂.₇Ti₅Sb₂O₁₉·15.0H₂O) were synthesized as new cation exchangers using the in situ precipitation technique. Physico-chemical investigations showed different behaviors for the obtained materials. The materials have significant stability at high acid concentration and temperature. The ion exchange capacity for Cs⁺ in the presence of different alcoholic solvents was found to increase and generally obey the order C₂H₅OH > CH₃OH > H₂O. Diffusion coefficients (D_i) and thermodynamic parameters of Cs⁺ exchange in both magnesium and cerium titano-antimonates in aqueous and alcoholic solutions were calculated. The swelling ratios of the materials were predicted by applying modified calculations at constant values of D_i. The results showed insignificant swelling behavior in the presence of organic solvents.     

Solvation and structure of LiAlH(4) in ethereal solvents

The nature of the solute species present in ethereal solutions of LiAlH(4) is of crucial importance for understanding the mechanisms for the reduction of ketones and other functional groups by LiAlH(4). We have employed a combination of theoretical and experimental techniques to investigate the structure of LiAlH(4) in ethereal solutions. Using complexation agents, we measured the IR spectra of LiAlH(4) and AlH(4)(-) in tetrahydrofuran (THF). Hybrid quantum-classical (QM-MM) simulations have also been carried out to compute the IR spectra of associated and dissociated LiAlH(4) species and the free-energy profile for the dissociation process in solution. Our experimental estimate of the dissociation constant in THF is 0.021 +/- 0.002, while the predicted computational value corresponding to a model dimethyl ether solvent is 0.001. The free-energy profile shows only one minimum corresponding to a contact ion pair at a Li-Al separation distance of 3.0 A. These results are consistent with the fact that LiAlH(4) is essentially associated in ethereal solutions forming contact ion pairs.