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.

What do far-infrared spectra of solitary water in "water-in-solvent" systems reveal about water's solvation and dynamics?

Classical molecular dynamics simulations of water in ionic and dipolar solvents were used to interpret the far-infrared (FIR) rotation/libration spectra of "solitary water" in terms of water's rotational dynamics and interactions with solvents. Seven solvents represented by nonpolarizable all-atom force fields and a series of idealized variable-charge solvents were used to span the range of solvent polarities (hydrogen bonding) studied experimentally. Simulated spectra capture the solvent dependence observed, as well as the relationship between the frequencies of water libration (νL) and OH stretching bands (νOH). In more strongly interacting solvents, simulated νL are ∼20% higher than those of experiment. In all solvents, the simulated spectra are composites of rotational motions about the two axes perpendicular to water's dipole moment, and the different frequencies of these two motions are responsible for the breadth of the libration band and the bimodal shape observed in halide ionic liquids. Simulations overestimate the separation of these two components in most solvents. The character of water rotational motions changes markedly with solvent polarity, from quasi-free rotation in nonpolar and weakly polar solvents to highly constrained libration in strongly hydrogen bonding environments. The changeover to librational motions dominating the spectrum occurs between solvents such as benzene (νL ∼ 250 cm-1) and acetonitrile (νL ∼ 400 cm-1). For solvents in the latter category, the mean frequency of the experimental FIR band provides a direct measure of mean-squared torques and, therefore, force constants associated with interactions constraining water's librational motion.

Rapid water dynamics structures the OH-stretching spectra of solitary water in ionic liquids and dipolar solvents.

In a recent study [J. Phys. Chem. B 126, 4584-4598 (2022)], we have used infrared spectroscopy to investigate the solvation and dynamics of solitary water in ionic liquids and dipolar solvents. Complex shapes observed for water OH-stretching bands, common to all high-polarity solvents, were assigned to water in several solvation states. In the present study, classical molecular dynamics simulations of a single water molecule in four ionic liquids and three dipolar solvents were used to test and refine this interpretation. Consistent with past assignments, simulations show solitary water usually donates two hydrogen bonds to distinct solvent molecules. Such symmetrically solvated water produces the primary pair of peaks identified in the OH spectra of water in nearly all solvents. We had further proposed that additional features flanking this main peak are due to asymmetric solvation states, states in which only one OH group makes a hydrogen bond to solvent. Such states were found in significant concentrations in all of the systems simulated. Simulations of the OH stretching spectra using a semiclassical description and the vibrational map developed by Auer and Skinner [J. Chem. Phys. 128, 224511-224512 (2008)] provided semi-quantitative agreement with experiment. Analysis of species-specific spectra confirmed assignment of the additional features in the experimental spectra to asymmetrically solvated water. The simulations also showed that rapid water motions cause a marked motional narrowing compared with the inhomogeneous limit. This narrowing is largely responsible for making the additional features due to minority solvation states manifest in the spectra.

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 (xw ∼ 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.

Solvation of Methyl Lactate in Water: Molecular Dynamics Studies.

Methyl lactate (ML), a chiral α-hydroxy ester, has been probed widely to understand the competition between two types of intramolecular H-bonds in solvents of different polarities. Recent experimental and high-level quantum chemical studies have revealed the predominant existence of ML-water insertion complexes over addition complexes in aqueous solution. Although the stability of monohydrate insertion conformer was studied accurately, ab initio quantum chemical calculations failed to predict the most stable dihydrate conformer in analogy with the experimental spectroscopic search. Atomistic molecular dynamics simulations of aqueous solution of methyl lactate predict that the population and lifetime of different H-bonded ML-water addition complexes are dictated by their interaction energies. Although the population of dihydrate insertion complexes is higher than that of the monohydrate complexes, the lifetime of the former is smaller than the latter, which is in good agreement with the experimental result. The nature of intramolecular H-bonds within a methyl lactate molecule in aqueous solution is opposite to that in the gas phase due to the solvation process in water by intermolecular H-bonding interactions.

Self-Assembly of Cations in Aqueous Solutions of Hydroxyl-Functionalized Ionic Liquids: Molecular Dynamics Studies.

The effect of presence of a hydroxyl-functionalized alkyl chain of varying carbon number on the self-assembly of cations in aqueous solutions of 1-(n-hydroxyalkyl)-3-decylimidazolium bromide (where the alkyl groups are ethyl, butyl, heptyl, and decyl) has been studied using atomistic molecular dynamics simulations. Spontaneous self-assembly of cations to form aggregates with hydrophobic core and hydrophilic surface is observed. The shape of the aggregates changes from quasispherical in the case of cations with hydroxyheptyl or smaller substituent chain, to a thin film like intercalated aggregate in the case of cations with hydroxydecyl chain. Cations with hydroxydecyl substituent chain exhibit long-range spatial correlations, and the anions are associated with cations to a greater extent due to the higher surface charge density of the aggregate. The ordered film like aggregate is stabilized by the dispersion interactions between the intercalated substituent chains and the intermolecular hydrogen bonds formed between the alkoxy oxygen atoms and the hydrogen atoms of the imidazolium ring. The cations form less compact aggregates with lower aggregation number than their nonhydroxyl analogues in the corresponding aqueous solutions. The intracationic and aggregate structures are governed by the length of the hydroxyalkyl chain.

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

The structures of three different equimolar binary ionic liquid mixtures and their liquid-vapor interface have been studied using atomistic molecular dynamics simulations. Two of these binary mixtures were composed of a common cation 1-n-butyl-3-methylimidazolium and varying anions (chloride and hexafluorophosphate in one of the mixtures and chloride and trifluoromethanesulfonate in the other) and the third binary mixture was composed of a common anion, trifluoromethanesulfonate and two imidazolium cations with ethyl and octyl side chains. Binary mixtures with common cations are found to be homogeneous. The anions are preferentially located near the ring hydrogen atoms due to H-bonding interactions. Segregation of ions is observed at the interface with an enrichment of the liquid-vapor interface layer by longer alkyl chains and bigger anions with a distributed charge. The surface composition is drastically different from that of the bulk composition, with the longer alkyl tail groups and bigger anions populating the outermost layer of the interface. The longer alkyl chains of the cations and trifluoromethanesulfonate anions with a smaller charge density show orientational ordering at the liquid-vapor interface.

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

The liquid structure of aqueous solutions of five different imidazolium based gemini dicationic ionic liquids 1,n-bis(3-methylimidazolium-1-yl) alkane bromide (n being the length of the spacer alkyl chain), with propyl, pentyl, octyl, decyl and hexadecyl spacer chains has been studied using atomistic molecular dynamics simulations. While solutions with propyl and pentyl spacers are homogeneous, those with octyl and decyl spacers show spatial heterogeneity. Microscopic inhomogeneity in the bulk solution phase increases with an increase in the length of the spacer chain leading to polydisperse aggregates in the solution with a hexadecyl spacer. Organization of the cations at the solution-vapor interface also depends upon the length of the spacer chain with the most organized interfacial layer observed in the solution with a hexadecyl spacer chain.

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.

Effect of cation asymmetry on the aggregation in aqueous 1-alkyl,3-decylimidazolium bromide solutions: molecular dynamics studies.

Self-assembly of cations in aqueous solutions of 1-alkyl,3-decylimidazolium bromide (with four different alkyl chains, methyl, butyl, heptyl, and decyl chain,) have been studied using atomistic molecular dynamics simulations. Polydisperse aggregates of cations are formed in the solution with alkyl tails in the core and the polar head groups present at the surface of the aggregates. The shape of the aggregates is dictated by the length of the alkyl chain. Aggregation numbers increase steadily with the increasing alkyl chain length. The greater asymmetry in the two-substituent chain length leads to a different surface structure compared to that of the cations with alkyl chains of similar length.