Solute-solvent interactions in imidazolium camphorsulfonate ionic liquids

We directly observe the interaction between 1-butyl-3-methylimidazolium (bmim) or 1-butyl-2,3-dimethylimidazolium (bm(2)im) and the solute, ethyl acrylate (EA), which is the popular dienophile in the Diels-Alder reaction and an H-bonding acceptor, by using specially designed electrospray mass spectrometry. In imidazolium ionic liquids, cation-anion interactions are controlled by selecting the appropriate anion, and the naked C(2)-H of imidazolium, which loosely interacts with its counterion, can readily interact with an H-bonding acceptable solute. The ion-counterion (solvent-solvent) interaction affects the ion-solute (solvent-solute) interaction. This relation is one of the key criteria for selecting the cation-anion combination in tailoring ILs.     

Long-range orientation correlation in liquids

Strong short-range intermolecular interactions result in position and orientation correlations between nearest neighbour molecules in isotropic liquids, and it is generally assumed that such correlations extend at most a few molecular diameters. This assumption is contradicted by results from second harmonic light scattering experiments presented here, which reveal long-range orientation correlations in several isotropic liquids including water. These experiments measure the polarization dependence of the scattered light, and the observations are interpreted in terms of transverse and longitudinal polar collective modes, as well as simple explicit models. The results revise our understanding of the structure of molecular liquids and provide a test of computational simulations.     

The effect of soft cores and anisotropic forces on vibrational energy relaxation in liquids

Starting from the isolated-binary-collision hypothesis, simple explicit expressions for the vibrational energy relaxation time in liquids are obtained. We stress the role of soft repulsive forces and of anisotropic interactions. In both cases, deviations from a model developed by Delalande and Gale, assuming an attractive hard-sphere potential, are obtained and discussed. The specific case of liquid HCl and DCl is treated as an application.     

Solute-solvent interaction involving MX2Ln type complexes in mixed solvent media. I. Outer-sphere interactions of the NiCl2py4 complex in pyridine + diluent solutions

The standard enthalpies and entropies of transfer from pyridine to pyridine+diluent mixtures of varying composition have been determined for the NiCl₂py₄ complex at 25°C using chloroform, 1,1,2,2-tetrachloroethane, aniline, benzene, and chlorobenzene as the diluents. Large negative enthalpies of transfer to solvent mixtures involving the protic diluents contrast with the small effects observed for the aprotic diluents. For the latter, the negative entropies of transfer agree with the assumption that pyridine molecules remain attached to the complex when the diluent is added to pyridine, indicating outer-sphere interaction of the amine with the complex. Solvent effects observed in the near infrared ligand-field spectrum of NiCl₂py₄ support the assumption of specific outer-sphere interactions with protic diluents. Additional information about these interactions is provided by the CTCT(t _2g^*) metal-to-ligand band in the UV.Deceased.     

Intermolecular vibrational relaxation in liquids

The influence of intermolecular vibrational relaxation on dipole moment correlation functions, as obtained from IR band shapes, is discussed. It is explicitly shown that vibrational relaxation due to intermolecular interactions depends on the reorientational behaviour of the molecules in the liquid.Therefore, an a priori separation of the dipole moment correlation function into independent reorientational and vibrational factors is not generally possible. The implications for various procedures used to “correct” Raman and IR band shapes for vibrational relaxation are discussed.The expression derived for the intermolecular vibrational relaxation is used to calculate theoretically the effect of transition dipole-transition dipole coupling on dipole moment correlation functions.Experimental data obtained from isotopic dilution measurements support the interpretation of the isotopic dilution effect in terms of the transition dipole-transition dipole coupling.     

Non-DLVO silica interaction forces in NMP-water mixtures. I. A symmetric system

Despite the success of DLVO theory, there exist numerous examples of interactions that do not follow its predictions. One prominent example is the interaction between hydrophilic surfaces in mixtures of water with another polar, associating solvent. Interactions of such surfaces are still poorly understood yet play a key role in a wide variety of processes in nature, biology, and industry. The interaction forces between a silica sphere and a glass plate in N-methyl-2-pyrrolidone (NMP)-water binary mixtures were measured using the AFM technique. The interactions in pure NMP and pure water agreed qualitatively with DLVO theory. In contrast, the addition of NMP to water drastically altered the interactions, which no longer followed DLVO predictions. An unusually strong, long-range (50-80 nm), multistepped attractive force was observed on the approach of hydrophilic surfaces in the NMP concentration range of 30-50 vol %, where the adhesive pull-off force was also maximized. The maximum attractive force was observed at an NMP concentration near 30 vol %, consistent with the formation of a strong hydrogen-bonded complex between NMP and water near the solid surface. The analysis of force profiles, zeta potentials, solution viscosity, and contact angles suggests that attraction arises from the bridging of surface-adsorbed macrocluster layers known to form on hydrophilic surfaces in mixtures of associating liquids.     

Electrostatics of proteins in dielectric solvent continua. I. Newton's third law marries qE forces

The authors reformulate and revise an electrostatic theory treating proteins surrounded by dielectric solvent continua [B. Egwolf and P. Tavan, J. Chem. Phys. 118, 2039 (2003)] to make the resulting reaction field (RF) forces compatible with Newton's third law. Such a compatibility is required for their use in molecular dynamics (MD) simulations, in which the proteins are modeled by all-atom molecular mechanics force fields. According to the original theory the RF forces, which are due to the electric field generated by the solvent polarization and act on the partial charges of a protein, i.e., the so-called qE forces, can be quite accurately computed from Gaussian RF dipoles localized at the protein atoms. Using a slightly different approximation scheme also the RF energies of given protein configurations are obtained. However, because the qE forces do not account for the dielectric boundary pressure exerted by the solvent continuum on the protein, they do not obey the principle that actio equals reactio as required by Newton's third law. Therefore, their use in MD simulations is severely hampered. An analysis of the original theory has led the authors now to a reformulation removing the main difficulties. By considering the RF energy, which represents the dominant electrostatic contribution to the free energy of solvation for a given protein configuration, they show that its negative configurational gradient yields mean RF forces obeying the reactio principle. Because the evaluation of these mean forces is computationally much more demanding than that of the qE forces, they derive a suggestion how the qE forces can be modified to obey Newton's third law. Various properties of the thus established theory, particularly issues of accuracy and of computational efficiency, are discussed. A sample application to a MD simulation of a peptide in solution is described in the following paper [M. Stork and P. Tavan, J. Chem. Phys., 126, 165106 (2007).     

Theoretical study on cation-anion interaction and vibrational spectra of 1-allyl-3-methylimidazolium-based ionic liquids

In order to deepen the understanding of the cation-anion interaction in ionic liquids, the structures of cation, anions, and cation-anion ion-pairs of 1-allyl-3-methylimidazolium-based ionic liquids are optimized using density functional theory (DFT), and their most stable geometries are discussed. The structural parameters, hydrogen bonds and interaction energies of 1-allyl-3-methylimidazolium dicyanamide ([Amim]DCA), 1-allyl-3-methylimidazolium chloride ([Amim]Cl), 1-allyl-3-methylimidazolium formate ([Amim]FmO) and 1-allyl-3-methylimidazolium acetate ([Amim]AcO) ion pairs are studied. The vibrational frequencies of [Amim]DCA and [Amim]Cl have been calculated and scaled values have been compared with experimental FT-IR and FT-Raman spectra. The complete assignments were performed on the basis of the potential energy distribution (PED) of the vibrational modes.     

Solute-solvent complex kinetics and thermodynamics probed by 2D-IR vibrational echo chemical exchange spectroscopy

The formation and dissociation kinetics of a series of triethylsilanol/solvent weakly hydrogen bonding complexes with enthalpies of formation ranging from -1.4 to -3.3 kcal/mol are measured with ultrafast two-dimensional infrared (2D IR) chemical exchange spectroscopy in liquid solutions at room temperature. The correlation between the complex enthalpies of formation and dissociation rate constants can be expressed with an equation similar to the Arrhenius equation. The experimental results are in accord with previous observations on eight phenol/solvent complexes with enthalpies of formation from -0.6 to -2.5 kcal/mol. It was found that the inverse of the solute-solvent complex dissociation rate constant is linearly related to exp(-DeltaH0/RT) where DeltaH0 is the complex enthalpy of formation. It is shown here, that the triethylsilanol-solvent complexes obey the same relationship with the identical proportionality constant, that is, all 13 points, five silanol complexes and eight phenol complexes, fall on the same line. In addition, features of 2D IR chemical exchange spectra at long reaction times (spectral diffusion complete) are explicated using the triethylsilanol systems. It is shown that the off-diagonal chemical exchange peaks have shapes that are a combination (outer product) of the absorption line shapes of the species that give rise to the diagonal peaks.     

NMR solvent shifts and structure elucidation in coumarins

The use of the solvent shift techniques enables the position of methyl, methoxyl and aromatic protons in coumarins to be determined and should prove a valuable aid to structural elucidation in this class of compounds.     

Solvent-polymer interaction: I. Molecular transport of some selected organic liquids in polymer membrane

Methods and microtechniques for determining solubility, diffusivity, thermodynamic properties, and kinetic parameters of 12 selected organic liquid solvents in polyurethane membrane by thermogravimetry (TG) are described. TG provides a simple, sensitive, rapid, and accurate microtechnique for measuring a minute change in weight (or mass) of a substance as a function of time at isothermal condition or as a function of temperature at dynamic manner. Thus from a single isothermal TG-desorption experiment, solubility and diffusivity of solvent molecules in polyurethane membrane were obtained simultaneously. Furthermore, by a dynamic TG-desorption run, kinetic parameter such as activation energy of desorption of solvent molecules from polyurethane membrane was determined. In addition, much other useful information such as equilibrium sorption constant, the changes in standard enthalpy and standard entropy of sorption, permeability, the activation energy of diffusion and so forth for solvents in polyurethane membrane are also evaluated and discussed. Finally, the correlation between the microscopic molecular structure and macroscopic properties of solvent molecules in polymer membrane is interpreted in terms of linear free energy relationships.     

Chromatographic chemical characterization of solvent refined coal I and II liquids for toxicological testing

Summary High resolution gas chromatography and gas chromatography/mass spectrometry were employed to determine the components in samples of solvent refined coal (SRC) II naphtha, middle distillate, heavy distillate, and fuel oil blend and SRC-I light oil, wash solvent, and process solvent in concentrations higher than approximately 0.5%. Quantitation was based on the addition of n-alkane internal standards and peak area response of a flame ionization detector with the use of fused silica and glass capillary columns coated with SE-52 and SF-96. The major differences in the SRC-I and II materials were the molecular weight ranges and sizes of the chemical components found in the products, an effect of the differences in the nominal boiling ranges of the materials. Alkanes, alkenes, hydroxylated aromatics, hydrogenated and polycyclic aromatic hydrocarbon ring systems, and alkylated derivatives of the above were found in greatest concentration in the unfractionated materials. Sulfur and nitrogen heteroatomic species were in greates concentration in the higher nominal boiling range materials. A major purpose of these investigations was to develop a method to obtain data on the composition of these complex sample matrices for use in designing studies for toxicological evaluation of these materials.     

Boundary layers and surface forces in pure nonaqueous liquids

Further progress has taken place in the experimental studies and molecular simulations of the structure and properties of boundary layers and thin interlayers of pure nonaqueous liquids. Recent findings in the field, including a deviation of density and shear response, layering static and dynamic structuring upon interlayer thinning are discussed in the context of the surface forces and the disjoining pressure contribution to the liquid film stability.     

Ionic liquids as a novel solvent for lanthanide extraction.

Octyl(phenyl)-N,N-diisobutylcarbamoylmethyl phosphine oxide (CMPO) dissolved in an ionic liquids, 1-butyl-3-methyl-imidazolium hexafluorophosphate, greatly enhances extractability and selectivity of lanthanide cations compared to that dissolved in conventional organic solvents; further, the recovery of lanthanides extracted into ionic liquids can be accomplished using several stripping solutions containing complexing agents. The possibility of utilizing ionic liquids as novel separation media in an industrial liquid-liquid extraction process was demonstrated.     

Solute-solvent contact by intermolecular cross relaxation. I. The nature of the water-hydrophobic interface

Intermolecular cross-relaxation rates between solute and solvent were measured by {1H} 19F nuclear magnetic resonance experiments in aqueous molecular solutions of ammonium perfluoro-octanoate and sodium trifluoroacetate. The experiments performed at three different magnetic fields provide frequency-dependent cross-relaxation rates which demonstrate clearly the lack of extreme narrowing for nuclear spin relaxation by diffusionally modulated intermolecular interactions. Supplemented by suitable intramolecular cross-relaxation, longitudinal relaxation, and self-diffusion data, the obtained cross-relaxation rates are evaluated within the framework of recent relaxation models and provide information about the hydrophobic hydration. In particular, water dynamics around the trifluoromethyl group in ammonium perfluoro-octanoate are more retarded than that in the smaller trifluoroacetate.     

Solute-solvent interactions in surfactant adsorption: Part I. The influence of the polar head

This study deals with the variation of the interfacial tension of mercury aqueous solutions with potential and concentrations for solutions of butanol. butyric acid, butyramide and butyl monoether glycol. The results obtained have been analysed using an isotherm proposed by Bennes leading to values for the energy of adsorption corrected for the influence of the solute-solvent interactions in solution.A correlation has been revealed been to exist between the corrected energy of adsorption, the hydrophobicity coefficient and the dipole moment of the molecule which are all parameters related to the nature of the hydrophilic grouping.These result serve to show how the adsorption on mercury can be used to approach the problem of interactions between molecules in surfactant solutions.     

The role of attractive forces in viscous liquids

We present evidence from computer simulation that the slowdown of relaxation of a standard Lennard-Jones glass-forming liquid and that of its reduction to a model with truncated pair potentials without attractive tails are quantitatively and qualitatively different in the viscous regime. The pair structure of the two models is however very similar. This finding, which appears to contradict the common view that the physics of dense liquids is dominated by the steep repulsive forces between atoms, is characterized in detail, and its consequences are explored. Beyond the role of attractive forces themselves, a key aspect in explaining the differences in the dynamical behavior of the two models is the truncation of the interaction potentials beyond a cutoff at typical interatomic distance. This leads us to question the ability of the jamming scenario to describe the physics of glass-forming liquids and polymers.