Properties of n-butylpyridinium nitrate ionic liquid and its binary mixtures with water

The density and surface tension of the pure ionic liquid n-butylpyridinium nitrate ([BuPy]NO₃) were determined at temperature range from T = (293.15 to 338.15) K. The coefficient of thermal expansion, molecular volume and lattice energy of [BuPy]NO₃ were calculated from the experimental values of density. The surface entropy and enthalpy of [BuPy]NO₃ were investigated. The IL studied show much lower surface enthalpy and lattice energy in comparison with fused salts. The densities and surface tensions of binary mixtures of [BuPy]NO₃ with water have been measured within the whole composition range at T = 298.15 K and atmospheric pressure. Excess molar volumes V^E and surface tension deviations δγ were then deduced from the experimental results as well as partial molar volumes and excess partial molar volumes. Excess molar volumes have a negative deviation from ideal behavior and the surface tension deviations are negative over the whole compositions range. V^E and δγ were correlated with suitable equation respectively.     

Properties of L-ascorbic acid in water and binary aqueous mixtures of D-glucose and D-fructose at different temperatures

Using density and sound velocity partial molar volumes, partial molar adiabatic compressibilities, partial molar expansibilities and structure of L-ascorbic acid have been determined in water and aqueous mixtures of D-glucose and D-fructose at different concentrations and temperatures. Masson’s equation was used to analyze the measured data. The obtained parameters have been interpreted in terms of solute–solute and solute–solvent interactions. It is found that the L-ascorbic acid acts as structure breaker in water as well in binary studied mixtures.     

Changes of mesophase structure of BRIJ 96/water mixtures on addition of liquid paraffin

The influence of the addition of liquid paraffin (LP) on the structure of the lamellar (L ) and hexagonal (H₁) mesophases formed in mixtures of water (W) and BRIJ 96 (B) was studied. Mesophases were identified using polarization microscopy and small angle x-ray diffraction (SAXD). Repeat spacings were also determined with SAXD. Depending on theW/B ratio, addtion ofLP toL gives a large, almost linear one-dimensional swelling or an initial swelling followed by a gradual transition to H₁.L with a highLP-content gives a diffraction pattern showing only the first order diffraction maximum, possibly a result of undulations of the layers. The lamellar structure, however, was confirmed using freeze fracture electron microscopy (FFEM). Addition ofLP to H₁ gives an initial swelling followed by a transition to a transparent, highly viscous, isotropic phase, called the gel-phase (G). The diffraction pattern obtained fromG yields little information on its structure. A large swelling ofG withLP was observed. From the degree of swelling as a function of hydrocarbon content it was inferred that this phase consists of spherical aggregates forming a close-packed structure. Using FFEM, textures were visualized resembling those obtained from the isotropic mesophase (I₁) in water-surfactant mixtures. Finally, geometrical factors are discussed that may play a role in the formation of the gel-phase.     

Experimental determination and modeling of structure II hydrates in mixtures of methane+water+1,4-dioxane

Hydrate phase equilibrium conditions were measured with a Cailletet apparatus in the pressure range 2<14 MPa. The investigated 1,4-dioxane concentrations were 1, 2, 5, 7, 10, 20 and 30 mol% relative to water. The results show that adding 1,4-dioxane up to concentrations of 6 mol%, about the stoichiometric ratio of large sII cages to water (1/17), reduced the equilibrium pressure of hydrate formation. Adding 1,4-dioxane beyond 6 mol% caused a slow increase of the equilibrium pressures. The hydrate phase equilibria data were modeled as equilibrium between a liquid phase of water and 1,4-dioxane, with a small amount of methane, and a sII hydrate of 1,4-dioxane and methane. The chemical potential of the hydrate phase was described using the van der Waals and Platteeuw theory. Activity coefficients of the liquid phase were calculated by a van Laar relation, based on literature 1,4-dioxane+water VLE data. The predicted equilibrium pressures calculated were within 5% of the data up to a concentration of 20 mol% 1,4-dioxane relative to water.     

Ion association of persulphates in water and water+acetone mixtures

Conductometric studies have been made for the persulphates of sodium, potassium, and ammonium in water and water+acetone (X_acetone=0.016, 0.033, 0.072, 0.117 and 0.171) mixtures at 298.15, 303.15, 308.15 and 313.15 K, using Shedlovsky and Fuoss–Kraus extrapolation techniques. The limiting molar conductance (Λ_o), association constant (K_A), Walden product (Λ_oη_o) and the thermodynamic functions for the process of ion-pair formation have been evaluated and discussed.     

On the collective network of ionic liquid/water mixtures. I. Orientational structure

In this work, the collective structure of aqueous solutions of ionic liquids was studied by means of molecular dynamics simulations. Various concentrations of 1-butyl-3-methyl-imidazolium tetrafluoroborate and TIP3P water were simulated at the very same size of the simulation box. For the analysis, the ternary system cation/anion/water was subdivided into binary networks. The local structure of each of these six networks is investigated by atom-atom radial distribution functions as well as by the so-called g coefficients, which reveal the mutual orientation of the network constituting partners. Furthermore, the collective structure of the whole samples was characterized by the contribution of each species to the static dielectric constant epsilon(omega=0) and to the Kirkwood G(K) factor. The combination of the analysis tools mentioned above provides knowledge about the cross-linking of the ionic species with the dipolar water. Thereby, the interplay between charge-charge and hydrogen bond networks is analyzed in detail.     

Studies of crotyl anion reactions and some regularities of copolymerization in hexamethylphosphortriamide

Cis-trans isomerization of butenes in free-anion systems proceeds through the α-butene intermediate. Direct cis- and trans-transformation of crotyl anions does not occur. Reactions of crotyl anion and its ion pairs with proton-donor compounds in hexamethylphosphortriamide (HMPTA) are accompanied by the formation of an equilibrium mixture of butenes containing 97 per cent β-butenes. The addition of these active species to alkylene oxides and sulphides, cyclohexadiene-1,3, styrene and 2,3-dimethylbutadiene occurs with the participation of the terminal carbon atom of the crotyl active centre, resulting in the predominant formation of 1,4-units. It has been shown that the reactivity ratios r₁ and r₂ for the pairs butadiene-styrene, butadiene-2,3-dimethylbutadiene and butadiene-cyclohexadiene-1,3 become very close in HMPTA.Studies of copolymer microstructure reveal an increase of butadiene 1,4-units content paralleling a rise of second monomer content.     

Conformation and solvation structure for an isolated n-octadecane chain in water, methanol, and their mixtures

Configurational-bias Monte Carlo simulations in the isobaric-isothermal ensemble (T = 323 K and p = 10 atm) were carried out to probe structural properties of an isolated n-octadecane chain solvated in water, methanol, water-rich, or methanol-rich mixtures and, for comparison, of an isolated chain in the gas phase and for neat liquid n-octadecane. The united-atom version of the TraPPE (transferable potentials for phase equilibria) force field was used to represent n-octadecane and methanol and the TIP-4P model was used for water. In all six environments, broad conformational distributions are observed and the n-octadecane chains are found to predominantly adopt extended, but not all-trans conformations. In addition, a small fraction of more collapsed conformations in which the chain ends approach each other is observed for aqueous hydration, the water-rich solvent mixture and the gas phase, but the simulation data do not support a simple two-state picture with folded and unfolded basins of attraction. For chains in these three "poor" solvent environments, the dihedral angles near the center of the chain show an enhancement of the gauche population. The ensemble of water-solvated chains with end-to-end contacts is preferentially found in a U-shaped conformation rather than a more globular state. An analysis of the local solvation structures in the water-methanol mixtures shows, as expected, an enrichment of the methyl group of methanol near the methylene and methyl segments of the n-octadecane chain. Interestingly, these local bead fractions are enhanced by factors of 2.5 and 1.5 for methyl and methylene segments reflecting the more hydrophobic nature of the former segments.     

Strong slowing down of water reorientation in mixtures of water and tetramethylurea

We use mid-infrared pump-probe spectroscopy to study the ultrafast dynamics of HDO molecules in mixtures of tetramethylurea (TMU) and water. The composition of the studied solutions ranges from pure water to an equimolar mixture of water and TMU. We find that the vibrational relaxation of the OD-stretching vibration of HDO proceeds via an intermediate level in which the molecule is more strongly hydrogen bonded than in the ground state. As the TMU concentration is increased, the lifetime of the excited state and of the intermediate increase from 1.8 to 5.2 ps and from 0.7 to 2.2 ps, respectively. The orientational relaxation data indicate that the solutions contain two types of water molecules: bulk-like molecules that have the same reorientation time constant as in the pure liquid (taurot = 2.5 ps) and molecules that are strongly immobilized (taurot > 10 ps). The immobilized water molecules turn out to be involved in the solvation of the methyl groups of the tetramethylurea molecule. The fraction of immobilized water molecules grows with increasing TMU concentration, reaching a limiting value of 60% at very high concentrations.     

Effect of temperature and concentration on the structure of tert-butyl alcohol/water mixtures: near-infrared spectroscopic study

The effect of temperature and concentration on the structure of tert-butyl alcohol/water binary mixtures in the alcohol-rich region (X(H2O) < 0.3) has been studied by using Fourier transform near-infrared (FT-NIR) spectroscopy. The obtained results demonstrate that the addition of a small amount of water to tert-butyl alcohol (2-methyl-2-propanol, abbreviated as TBA) leads to minor changes in the structure of neat TBA and suggest that molecules of TBA in the mixture are in the same environment as those in pure TBA. The bands of water are red-shifted in the mixture relative to bulk water, implying that the molecules of water in TBA are involved in stronger hydrogen bonding. The present experimental data give no evidence for the existence of nonbonded water in the mixture. Even at a very low content of water, the main NIR bands of water (nu(2) + nu(3) and nu(1) + nu(3)) have two components showing markedly different behavior upon an increase in temperature. From the power spectra, it is seen that the extent of intensity changes due to the free OH groups of TBA is smaller in the mixture relative to pure TBA. All of these results support the model of chain-end bonding of water molecules to TBA associates. An increase in X(H2O) reduces the population of nonbonded OH groups of TBA, yet both processes do not appear at the same rate. The amount of bonded OH groups of water increases faster than that of the nonbonded ones. It seems that the water-water interaction becomes more important as X(H2O) increases. At high alcohol content, the position of the CH alkyl stretching bands is constant, evidencing a negligible role of the hydrophobic hydration in the mixture.     

Diffusive dynamics of water in tert-butyl alcohol/water mixtures

Quasielastic neutron scattering has been used to investigate the dynamical behavior of H(2)O in water/tert-butyl alcohol solutions. The measurements were made at fixed temperature (293 K) as a function of tert-butyl alcohol molar fraction, x, in the range 0-0.042. The data have been compared to those of pure water in the temperature range 269-293 K. The effect of tert-butyl alcohol addition on water dynamics is equivalent to that obtained by lowering the temperature of pure water by an amount proportional to the alcohol concentration. The temperature dependence of the diffusivity parameters in pure water and their concentration dependence in tert-butyl alcohol/water solutions can be rescaled to a common curve attributing to each solution a concentration-dependent "structural temperature" lower than the actual thermodynamic one. These results can be understood in terms of Stillinger's picture of water structuring and of other more recent theoretical pictures that emphasize the influence of the geometrical properties of hydrogen bond networks on water mobility.     

Kosmotrope character of maltose in water mixtures

It is well known that water plays a fundamental role for living beings, because the nature of water transformations provides for the ability to preserve biostructures. Solute can be classified as “kosmotropes” or “chaotropes” depending on the interaction strength with water. In the case of solutes destroying the natural hydrogen bonded network of water, called “kosmotropes” or “structure-makers”, the denaturation processes can be inhibited.

The aim of this work is to investigate the vibrational behaviour of maltose/H₂O mixtures in order to characterise the changes induced by the sugar on the H₂O hydrogen-bonded network. The obtained findings point out that maltose has a destructuring effect on the water tetrahedral network and emphasise its kosmotrope character.     

An investigation of water dynamics in binary mixtures of water and dimethyl sulfoxide

The motion of water molecules in mixtures of water and d6-dimethyl sulfoxide (DMSO) has been explored through molecular dynamics (MD) simulations using the SPC/E water model (J. Chem. Phys. 1987, 91, 6269) and the P2 DMSO model (J. Chem. Phys. 1993, 98, 8160). We evaluate the self-intermediate scattering functions, FS(Q,t), which are related by a Fourier transform to the incoherent structure factors, S(Q,omega), measured in quasielastic neutron scattering (QNS) experiments. We compare our results to recent QNS experiments on these mixtures reported by Bordallo et al. (J. Chem. Phys. 2004, 121, 12457). In addition to comparing the MD data to the experimental signals, which correspond to a convolution of S(Q,omega) with a resolution function, we examine the rotational and translational components of FS(Q,t) and investigate to what extent simulation results for the single-molecule dynamics follow the dynamical models that are used in the analysis of the experimental data. We find that the agreement between the experimental signal and the MD data is quite good and that the portion of FS(Q,t) due to translational dynamics is well represented by the jump-diffusion model. The model parameters and their composition dependence are in reasonable agreement with experiment, exhibiting similar trends in water mobility with composition. Specifically, we find that water motion is less hindered in water-rich and water-poor mixtures than it is near equimolar composition. We find that the extent of coupling between rotational and translational motion contributing to FS(Q,t) increases as the equimolar composition of the mixture is approached. Thus, the decoupling approximation, which is used to extract information on rotational relaxation from QNS spectra at higher momentum transfer (Q) values, becomes less accurate than that in water-rich or DMSO-rich mixtures. We also find that rotational relaxation deviates quite strongly from the isotropic rotational diffusion model. We explore this issue further by investigating the behavior of orientational time correlations for different unit vectors and corresponding to Legendre polynomials of orders 1-4. We find that the rotational time correlations of water molecules behave in a way that is more consistent with the extended jump rotation model recently proposed by Laage and Hynes (Science 2006, 311, 832).     

Effect of molecular structure on wetting behavior of water+amphiphile mixtures: a density functional approach

A density functional approach is applied to investigate the effect of molecular structure on wetting behavior of water+amphiphile mixtures. The interaction-site model is employed to describe isomeric amphiphile structures. The hydrogen bonding between water and amphiphile is mimicked by energy enhancement according to specific molecular orientation. The calculations show that these systems exhibit Cahn-type criticality-related wetting transitions and pronounced adsorption behavior difference between isomeric systems. Excellent qualitative agreements with experiments are achieved.     

Picosecond dynamics and microheterogenity of water+dioxane mixtures

Dielectric spectra from 200 MHz up to 3 THz were determined to study the fast dynamics of dilute water+1,4-dioxane. epsilon(nu) could be fitted by a collision induced oscillator at high frequencies plus two Debye relaxations in the microwave region. Isotope substitution was used to assign water and dioxane modes. The presence of the cooperative hydrogen-bond network relaxation down to a water mole fraction of 0.005 suggests a microheterogeneous structure of the mixtures even at low water content. The collision mode of dioxane at approximately 2 THz grows upon water addition, revealing the presence of H2O molecules in dioxane-rich domains.