Do Electrostatics Control the Diffusive Dynamics of Solitary Water? NMR and MD Studies of Water Translation and Rotation in Dipolar and Ionic Solvents

NMR-based measurements of the diffusion coefficients and rotation times of solitary water and benzene at 300 K are reported in a diverse collection of 13 conventional organic solvents and 10 imidazolium ionic liquids. Proton chemical shifts of water are found to be correlated to water OH-stretching frequencies, confirming the importance of electrostatic interactions in these shifts. However, the influence of magnetic interactions in aromatic solvents renders chemical shifts a less reliable indicator of electrostatics. Diffusion coefficients (DB) and rotational correlation times (τB) of benzene in the solvents examined are accurately described as functions of viscosity (η) by DB ∝ η-0.81 and τB ∝ η0.64. Literature values of DB and τB in alkane and normal alcohols, which were not included among the solvents studied here, are systematically faster than predicted by these correlations, indicating that factors beyond solvent viscosity play a role in determining the friction on benzene. In contrast to benzene, water diffusion and rotation are poorly described in terms of viscosity alone, even in the dipolar and ionic solvents measured here. The present data and the substantial literature data already available on dilute water diffusion show a systematic dependence of DW on solvent polarity among isoviscous solvents. The aspect of solvent polarity most relevant to water dynamics is the ability of a solvent to accept hydrogen bonds from water, as conveniently quantified by the frequency of water's OH stretching band, ΔνOH. The friction on translation, ζtr = kBT/DW, and rotation, ζrot = kBTτW, are both well correlated by functions of the form ζ(η, ΔνOH) = a1ηa2 exp (a3ΔνOH), where the ai are adjustable parameters. Molecular dynamics simulations reveal a strong coupling between electrostatic and nonelectrostatic water-solvent interactions, which makes it impossible to dissect the friction on water into additive dielectric and hydrodynamic components. Simulations also provide a tentative explanation for the unusual form of the correlating function ζ(η, ΔνOH), at least in the case of ζrot.

OH Stretching and Libration Bands of Solitary Water in Ionic Liquids and Dipolar Solvents Share a Single Dependence on Solvent Polarity

Ionic liquids are an emerging class of materials which are finding application in a variety of technologically important areas. Because of their hydrophilic character, at least a small concentration of water is often present when ionic liquids are used in practical applications. This study employs infrared spectroscopy in the OH stretching and libration regions together with DFT calculations to better characterize the state of dilute water in ionic liquids. Water mole fractions (x_w ∼ 0.1) are chosen such that nearly all water occurs in monomeric form and spectra probe the solvation structure and dynamics of solitary water molecules. New data are reported for a series of 1-ethyl-3-methylimidazolium liquids [Im₂₁][X] with X^- = (C₂F₅)₃F₃P^-, (CF₃SO₂)₂N^-, BF₄^-, B(CN)₄^-, CF₃SO₃^-, C₂H₅SO₄^-, NO₃^-, SCN^-, and CH₃CO₂^-, as well as for the two 1-hexyl-3-methylimidazolium liquids [Im₆₁][Cl] and [Im₆₁][I]. For comparison, spectra are also recorded in a variety of dipolar solvents, and much of the available literature data are summarized, providing a comprehensive perspective on monomeric water in homogeneous solution. Most prior studies of dilute water in ionic liquids interpreted OH stretching spectra only in terms of water being specifically bonded to two anions in A^-···H-O-H···A^- type solvates. The more detailed analysis presented here indicates the additional presence of asymmetrically solvated water, which in some cases includes both singly solvated (A^-···H-O-H) and more subtle forms of asymmetric solvation. The same pattern of solvation also pertains to dipolar solvents capable of accepting hydrogen bonds from water. No clear distinction is found between OH spectra in high-polarity conventional solvents and ionic liquids. In all solvents, OH frequencies are strongly correlated to measures of solvent basicity or hydrogen bond accepting ability. Far-infrared spectra of the water libration band also show common trends in ionic and dipolar solvents. Despite the different character of the libration and OH modes, the frequencies of these vibrations show virtually the same solvent dependence (apart from sign) except in weakly polar or nonpolar solvents.

Unraveling the Sorption Mechanism of CO2 in a Molecular Crystal without Intrinsic Porosity

The facile uptake of CO₂ gas in a nonporous molecular crystal constituted by long molecules with carbazole and ethynylphenyl moieties was reported in experiments recently. Herein, the mechanism of gas uptake by this crystal is elucidated using atomistic molecular simulations. The uptake of CO₂ is shown to be facilitated by (i) the capacity of the crystal to expand in volume because of weak intermolecular interactions, (ii) the parallel orientation of the long molecules in the crystal, and (iii) the ability of the molecule to marginally bend, yet not lose crystallinity because of the anchoring of the terminal carbazole groups. The retention of crystallinity upon sorption and desorption cycles is also demonstrated. At high enough pressures, near-neighbor CO₂ molecules sorbed in the crystal are found to be oriented parallel to each other.

Effect of spacer chain length on the liquid structure of aqueous dicationic ionic liquid solutions: molecular dynamics studies

Cations in an aqueous [C₁₆(MIm)₂][2Br] solution aggregate spontaneously to form micelles, with a hydrophobic core composed of spacer alkyl chains and a hydrophilic surface composed of imidazolium head groups.

Segregation of ions at the interface: molecular dynamics studies of the bulk and liquid–vapor interface structure of equimolar binary mixtures of ionic liquids

Ions in an equimolar binary mixture of [emim][TfO] and [omim][TfO] segregate at the liquid–vapor interface to form a hydrophobic surface composed of octyl chains.

Ionic liquids at nonane-water interfaces: molecular dynamics studies

The structures of ternary systems with water, nonane, and an ionic liquid, with the ionic liquid placed between water and nonane, have been studied using atomistic molecular dynamics simulations. Three different ionic liquids with 1-n-butyl-3-methylimidazolium cation and bromide, tetrafluoroborate, and trifluoromethanesulfonate anions have been studied. The ionic liquids disperse into the aqueous phase quickly and are solubilized in water within 15 ns to form two equivalent nonane-aqueous ionic liquid interfaces. The interfacial region is enriched with ionic liquids due to the amphiphilicity of the cations. The presence of ionic liquids at the interface reduces the interfacial tension between the nonane and water, thus facilitating the mixing of aqueous and nonane phases. The reduction in the interfacial tension is found to be inversely related to the solubility of the corresponding ionic liquid in water. The butyl chains of the cations and the trifluoromethanesulfonate anions present in the interfacial region are found to be preferentially oriented parallel to the interface normal.

Design and in vitro evaluation of polyvinyl chloride microcapsules containing sulphamethoxazole

Polyvinyl chloride microcapsules containing sulphamethoxazole have been prepared by phase separation coacervation in non-aqueous solvents. Phase separation of the polyvinyl chloride in the solution of chloroform was achieved with n-hexane. Scanning electron micrographs revealed uniform encapsulation of the sulphamethoxazole particles. In vitro dissolution studies were conducted under changing pH conditions. The reproducibility of the in vitro drug release was highly significant. The mechanism of drug release was suggested as an integrated process of diffusion controlled dissolution. Controlled drug release was obtained over a prolonged period of 8 h.