Assessing the Dispersive and Electrostatic Components of the Cohesive Energy of Ionic Liquids Using Molecular Dynamics Simulations and Molar Refraction Data

Molecular dynamics simulation and experimental verification of the effects of vinyl silicone oil viscosity on the mechanical properties of silicone rubber foam

The molecular motion trajectories of silicone rubber foam (SRF) at various vinyl silicone oil viscosities were studied via molecular dynamics (MD) simulation from the perspective of all atomic molecules. The influence of different viscosities of vinyl silicone oil on interaction, compatibility, and aggregation degree of molecules was determined based on the mean square displacement, diffusion coefficient, binding energy, solubility parameter, radial distribution function, and radius of gyration. The mechanical properties of the SRF were also experimentally verified. Results revealed that as the viscosity of vinyl silicone oil increased, the mean square displacement, fractional free volume, diffusion coefficient, and solubility parameter of the system decreased, whereas its larger radius of gyration increased. Moreover, the radial distribution function showed a weaker relative interaction between molecular chains. The calculated binding energy demonstrated that the system had better compatibility at a viscosity of 0.45 Pa s. This study provided a deeper insight into the relation between the viscosity of vinyl silicone oil and mechanical properties of the SRF. As the viscosity of vinyl silicone oil increased, the changing trend in MD simulation results of elastic modulus, shear modulus, bulk modulus, and Poisson's ratio was consistent with the experimental results. The MD simulations can promote theoretical predictions and scientific basis for the design of the SRF with desired performances.

Unravelling free volume in branched-cation ionic liquids based on silicon

The branching of ionic liquid cation sidechains utilizing silicon as the backbone was explored and it was found that this structural feature leads to fluids with remarkably low density and viscosity. The relatively low liquid densities suggest a large free volume in these liquids. Argon solubility was measured using a precise saturation method to probe the relative free volumes. Argon molar solubilities were slightly higher in ionic liquids with alkylsilane and siloxane groups within the cation, compared to carbon-based branched groups. The anion size, however, showed by far the dominant effect on argon solubility. Thermodynamic solvation parameters were derived from the solubility data and the argon solvation environment was modelled utilizing the polarizable CL&Pol force field. Semiquantitative analysis was in agreement with trends established from the experimental data. The results of this investigation demonstrate design principles for targeted ionic liquids when optimisation for the free volume is required, and demonstrate the utility of argon as a simple, noninteracting probe. As more ionic liquids find their way into industrial processes of scale, these findings are important for their utilisation in the capture of any gaseous solute, gas separation, or in processes involving the transformation of gases or small molecules.

The branching of ionic liquid cation sidechains utilizing silicon as the backbone was explored and it was found that this structural feature leads to fluids with remarkably low density and viscosity.

Insight into the thermal stability of DNA in hydrated ionic liquids from multi-wavelength UV resonance Raman experiments

The utility of ionic liquids (ILs) as alternative solvents for stabilizing and preserving the native structure of DNA over the long term may be envisaged for biotechnological and biomedical applications in the near future. The delicate balance between the stabilizing and destabilizing effects of IL-mediated interactions with the structure of DNA is complex and is still not well understood. This work reports a fundamental study dealing with the effect exerted by cations and anions in imidazolium-based ILs on the thermal structural stability of large nucleic acid molecules. Multi-wavelength UV resonance Raman spectroscopy is used for selectively detecting heat-induced structural transitions of DNA localized on specific base tracts. Our study reveals the establishment of preferential interactions between the imidazolium cations of ILs and the guanine bases in the DNA groove that lead to more effective stacking between the guanine bases even at high temperatures. Interestingly, we observe that this trend for ILs sharing the same chloride anion is further enhanced as the alkyl chain on the imidazolium cation gets shorter. The results from the present investigation lead to a more comprehensive view of the IL-mediated interactions with A-T and G-C base pairs during thermal unfolding.

Mixing divalent ionic liquids: effects of charge and side-chains

We have prepared novel divalent ionic liquids (ILs) based on the bis(trifluoromethylsulfonyl)imide anion where two charged imidazolium groups in the cations are either directly bound to each other or linked by a single atom. We assessed the influence of the side-chain functionality and divalency on their physical properties and on the thermodynamics of mixing. The results indicate that shortening the spacer of a divalent IL reduces its thermal stability and increases its viscosity. Mixtures of divalent and monovalent ILs show small but significant deviations from ideality upon mixing. These deviations appear to depend primarily on the (mis)match of the nature and length of the cation side-chain. The non-ideality imposed by mixing ILs with different side-chains appears to be enhanced by the increase in formal charge of the cations in the mixture.

Elucidating the effect of the ionic liquid type and alkyl chain length on the stability of ionic liquid-iron porphyrin complexes

The present study is motivated by the long-term objective of understanding how ionic liquids are biodegraded by cytochrome P450, which contains iron porphyrin (FeP) serving as the catalytic center. To this end, the current study is designed to elucidate the impact of types and conformations of ionic liquids on the binding energy with FeP, the key interactions that stabilize the ionic liquid-FeP complex, and how the electron uptake ability of FeP is altered in the presence of ionic liquids. Four classes of ionic liquids are considered: 1-alkyl-3-methylimidazolium, 1-alkyl-pyridinium, 1-alkylsulfonium, and N-methyl-N-alkylpyrrolidinium. The influence of linear alkyl chains of ethyl, butyl, hexyl, octyl, and decyl is examined on the favorable binding modes with FeP, considering two widely different conformations: tail up and tail down with respect to FeP. Electronic structure calculations are performed at the M06 level of theory with the 6-31G(d,p) basis set for C, H, and N atoms, while the Lanl2DZ basis set is employed for Fe. Donor-acceptor interactions contributing to the binding of ionic liquids to FeP are unraveled through the natural bond orbital analysis. The results from this study indicate that the binding energies are dependent not only on the class of ionic liquids but also on the conformations presented to FeP. The propensity of FeP to acquire an electron is significantly enhanced in the presence of ionic liquid cations, irrespective of the type and the alkyl chain length.

Structural investigations of molecular solutes within nanostructured ionic liquids

Ionic liquids (ILs) containing sufficiently long alkyl chains form amphiphilic nanostructures with well-defined polar and non-polar domains. Here we have explored the robustness of these amphiphilic nanostructures to added solutes and gained insight into how the nature of the solute and IL ions affect the partitioning of these solutes within the nanostructured domains of ILs. To achieve this, small angle X-ray scattering (SAXS) investigations were performed and discussed for mixtures of 9 different molecular compounds with 6 different ILs containing imidazolium cations. The amphiphilic nanostructure of ILs persisted to high solute concentrations, over 50 mol% of added solute for most 1-butyl-3-methylimidazolium ILs and above 80 mol% for most 1-decyl-3-methylimidazolium ILs. Solute partitioning within these domains was found to be controlled by the inherent polarity and size of the solute, as well as specific interactions between the solute and IL ions, with SAXS results corroborated with IR spectroscopy and molecular dynamics simulations. Molecular dynamics simulations also revealed the ability to induce π⁺-π⁺ stacking between imidazolium cations through the use of these added molecular compounds. Collectively, these results provide scope for the selection of IL ions to rationally influence and control the partitioning behaviour of given solutes within the amphiphilic nanostructure of ILs.

Current Status of AMOEBA-IL: A Multipolar/Polarizable Force Field for Ionic Liquids

Computational simulations of ionic liquid solutions have become a useful tool to investigate various physical, chemical and catalytic properties of systems involving these solvents. Classical molecular dynamics and hybrid quantum mechanical/molecular mechanical (QM/MM) calculations of IL systems have provided significant insights at the atomic level. Here, we present a review of the development and application of the multipolar and polarizable force field AMOEBA for ionic liquid systems, termed AMOEBA–IL. The parametrization approach for AMOEBA–IL relies on the reproduction of total quantum mechanical (QM) intermolecular interaction energies and QM energy decomposition analysis. This approach has been used to develop parameters for imidazolium– and pyrrolidinium–based ILs coupled with various inorganic anions. AMOEBA–IL has been used to investigate and predict the properties of a variety of systems including neat ILs and IL mixtures, water exchange reactions on lanthanide ions in IL mixtures, IL–based liquid–liquid extraction, and effects of ILs on an aniline protection reaction.

Anomalous and Not-So-Common Behavior in Common Ionic Liquids and Ionic Liquid-Containing Systems

This work highlights unexpected, not so well known responses of ionic liquids and ionic liquid-containing systems, which are reported in a collective manner, as a short review. Examples include: (i) Minima in the temperature dependence of the isobaric thermal expansion coefficient of some ILs; (ii) Viscosity Minima in binary mixtures of IL + Molecular solvents; (iii) Anomalies in the surface tension within a family of ILs; (iv) The constancy among IL substitution of C_p/V_m at and around room temperature; (v) ILs as glass forming liquids; (vi) Alternate odd-even side alkyl chain length effects; (vii) Absolute negative pressures in ILs and IL-containing systems; (viii) Reversed-charged ionic liquid pairs; (ix) LCST immiscibility behavior in IL + solvent systems.

Experimental Data of Fluid Phase Equilibria- Correlation and Prediction Models: A Review

The examples of phase equilibria in binary systems, solid/liquid (SLE), liquid/liquid (LLE), vapor/liquid (VLE), as well as liquid/liquid equilibria in ternary systems mainly containing ionic liquids (ILs), or the infragrance materials, or pharmaceuticals with molecular organic solvents, such as an alcohol, or water, or hydrocarbons, are presented. The most popular correlation methods of the experimental phase equilibrium data are presented, related to the excess Gibbs free energy models such as Wilson, universal-quasichemical, UNIQUAC and non-random two-liquid model, NRTL as well as several popular theories for the modeling of the phase equilibria and excess molar enthalpy, HE in binary or ternary mixtures are presented: the group contribution method (Mod. UNIFAC) and modified UNIFAC model for pharmaceuticals and lattice theory based on non-random hydrogen bonding (NRHB). The SLE, LLE, or VLE and HE of these systems may be described by the Perturbed-Chain Polar Statistical Associating Fluid Theory (PC-SAFT), or a Conductor-like Screening Model for Real Solvents (COSMO-RS). The examples of the application of ILs as extractants for the separation of aromatic hydrocarbons from alkanes, sulfur compounds from alkanes, alkenes from alkanes, ethylbenzene from styrene, butan-1-ol from water phase, or 2-phenylethanol (PEA) from water are discussed on the basis of previously published data. The first information about the selectivity of extrahent for separation can be obtained from the measurements of the limiting activity coefficient measurements by the gas–liquid chromatography technique. This review outlines the main research work carried out over the last few years on direct measurements of phase equilibria, or HE and limiting activity coefficients, the possibility of thermodynamic modeling with emphasis on recent research achievements and potential for future research.

Insight into conformationally-dependent binding of 1-n-alkyl-3-methylimidazolium cations to porphyrin molecules using quantum mechanical calculations

The first step in the biodegradation of imidazolium-based ionic liquids involves the insertion of the –OH group into the alkyl side chain, and it is believed to be triggered by cytochrome P450. In this work, we investigate the effect of conformations on binding energies of ionic liquid cations to the catalytic center of P450.

Surface structure evolution in a homologous series of ionic liquids

Interfaces of room temperature ionic liquids (RTILs) are important for both applications and basic science and are therefore intensely studied. However, the evolution of their interface structure with the cation's alkyl chain length [Formula: see text] from Coulomb to van der Waals interaction domination has not yet been studied for even a single broad homologous RTIL series. We present here such a study of the liquid-air interface for [Formula: see text], using angstrom-resolution X-ray methods. For [Formula: see text], a typical "simple liquid" monotonic surface-normal electron density profile [Formula: see text] is obtained, like those of water and organic solvents. For [Formula: see text], increasingly more pronounced nanoscale self-segregation of the molecules' charged moieties and apolar chains yields surface layering with alternating regions of headgroups and chains. The layering decays into the bulk over a few, to a few tens, of nanometers. The layering periods and decay lengths, their linear [Formula: see text] dependence, and slopes are discussed within two models, one with partial-chain interdigitation and the other with liquid-like chains. No surface-parallel long-range order is found within the surface layer. For [Formula: see text], a different surface phase is observed above melting. Our results also impact general liquid-phase issues like supramolecular self-aggregation and bulk-surface structure relations.

Quantifying intermolecular interactions of ionic liquids using cohesive energy densities

For ionic liquids (ILs), both the large number of possible cation + anion combinations and their ionic nature provide a unique challenge for understanding intermolecular interactions. Cohesive energy density, ced, is used to quantify the strength of intermolecular interactions for molecular liquids, and is determined using the enthalpy of vaporization. A critical analysis of the experimental challenges and data to obtain ced for ILs is provided. For ILs there are two methods to judge the strength of intermolecular interactions, due to the presence of multiple constituents in the vapour phase of ILs. Firstly, ced_IP, where the ionic vapour constituent is neutral ion pairs, the major constituent of the IL vapour. Secondly, ced_C+A, where the ionic vapour constituents are isolated ions. A ced_IP dataset is presented for 64 ILs. For the first time an experimental ced_C+A, a measure of the strength of the total intermolecular interaction for an IL, is presented. ced_C+A is significantly larger for ILs than ced for most molecular liquids, reflecting the need to break all of the relatively strong electrostatic interactions present in ILs. However, the van der Waals interactions contribute significantly to IL volatility due to the very strong electrostatic interaction in the neutral ion pair ionic vapour. An excellent linear correlation is found between ced_IP and the inverse of the molecular volume. A good linear correlation is found between IL ced_IP and IL Gordon parameter (which are dependent primarily on surface tension). ced values obtained through indirect methods gave similar magnitude values to ced_IP. These findings show that ced_IP is very important for understanding IL intermolecular interactions, in spite of ced_IP not being a measure of the total intermolecular interactions of an IL. In the outlook section, remaining challenges for understanding IL intermolecular interactions are outlined.

Design and New Energy Application of Ionic Liquids

New electrochemical application using room-temperature ionic liquids (ILs) are introduced, such as lithium secondary batteries, electrochemical double layer capacitors, and novel types of electrical devices for sustainable and renewal energy society. ILs have so many combinations, owing to many cation/anion species. In this chapter, we introduce properties from fundamental (general and special physicochemical properties) to electrochemical applications of ILs. We also discuss importance of molecular design and application target of ILs.

Linking the structures, free volumes, and properties of ionic liquid mixtures

The formation of ionic liquid (IL) mixtures has been proposed as an approach to rationally fine-tune the physicochemical properties of ILs for a variety of applications. However, the effects of forming such mixtures on the resultant properties of the liquids are only beginning to be understood. Towards a more complete understanding of both the thermodynamics of mixing ILs and the effect of mixing these liquids on their structures and physicochemical properties, the spatial arrangement and free volume of IL mixtures containing the common [C₄C₁im]⁺ cation and different anions have been systematically explored using small angle X-ray scattering (SAXS), positron annihilation lifetime spectroscopy (PALS) and ¹²⁹Xe NMR techniques. Anion size has the greatest effect on the spatial arrangement of the ILs and their mixtures in terms of the size of the non-polar domains and inter-ion distances. It was found that differences in coulombic attraction between oppositely charged ions arising from the distribution of charge density amongst the atoms of the anion also significantly influences these inter-ion distances. PALS and ¹²⁹Xe NMR results pertaining to the free volume of these mixtures were found to strongly correlate with each other despite the vastly different timescales of these techniques. Furthermore, the excess free volumes calculated from each of these measurements were in excellent agreement with the excess volumes of mixing measured for the IL mixtures investigated. The correspondence of these techniques indicates that the static and dynamic free volume of these liquid mixtures are strongly linked. Consequently, fluxional processes such as hydrogen bonding do not significantly contribute to the free volumes of these liquids compared to the spatial arrangement of ions arising from their size, shape and coulombic attraction. Given the relationship between free volume and transport properties such as viscosity and conductivity, these results provide a link between the structures of IL mixtures, the thermodynamics of mixing and their physicochemical properties.

Comparative study of the intermolecular dynamics of imidazolium-based ionic liquids with linear and branched alkyl chains: OHD-RIKES measurements

This article describes a comparative study of the low-frequency (0-450 cm^-1) Kerr spectra of the branched 1-(iso-alkyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([(N - 2)mC_N-1C₁im][NTf₂] with N = 3-7) ionic liquids (ILs) and that of the linear 1-(n-alkyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([C_NC₁im][NTf₂] with N = 2-7) ILs. The spectra were obtained by use of femtosecond optical heterodyne-detected Raman-induced Kerr effect spectroscopy (OHD-RIKES). The intermolecular spectrum of a branched IL is similar to that of a linear IL that is of the same alkyl chain length rather than of the same number of carbon atoms in the alkyl chain. This similarity and the lack of a correlation of the first spectral moments and widths of the intermolecular spectra with chain length is mainly attributed to the increase in the dispersion contribution to the total molar cohesive energy being compensated by stretching of the ionic network due to the increasing size of the nonpolar domains, which is dependent only on the length of the alkyl chain.

Quantum Chemical Methods for the Prediction of Energetic, Physical, and Spectroscopic Properties of Ionic Liquids

The accurate prediction of physicochemical properties of condensed systems is a longstanding goal of theoretical (quantum) chemistry. Ionic liquids comprising entirely of ions provide a unique challenge in this respect due to the diverse chemical nature of available ions and the complex interplay of intermolecular interactions among them, thus resulting in the wide variability of physicochemical properties, such as thermodynamic, transport, and spectroscopic properties. It is well understood that intermolecular forces are directly linked to physicochemical properties of condensed systems, and therefore, an understanding of this relationship would greatly aid in the design and synthesis of functionalized materials with tailored properties for an application at hand. This review aims to give an overview of how electronic structure properties obtained from quantum chemical methods such as interaction/binding energy and its fundamental components, dipole moment, polarizability, and orbital energies, can help shed light on the energetic, physical, and spectroscopic properties of semi-Coulomb systems such as ionic liquids. Particular emphasis is given to the prediction of their thermodynamic, transport, spectroscopic, and solubilizing properties.

Corresponding-states behavior of an ionic model fluid with variable dispersion interactions

Guggenheim's corresponding-states approach for simple fluids leads to a remarkably universal representation of their thermophysical properties. For more complex fluids, such as polar or ionic ones, deviations from this type of behavior are to be expected, thereby supplying us with valuable information about the thermodynamic consequences of the interaction details in fluids. Here, the gradual transition of a simple fluid to an ionic one is studied by varying the relative strength of the dispersion interactions compared to the electrostatic interactions among the charged particles. In addition to the effects on the reduced surface tension that were reported earlier [F. Leroy and V. C. Weiss, J. Chem. Phys. 134, 094703 (2011)], we address the shape of the coexistence curve and focus on properties that are related to and derived from the vapor pressure. These quantities include the enthalpy and entropy of vaporization, the boiling point, and the critical compressibility factor Zc. For all of these properties, the crossover from simple to characteristically ionic fluid is seen once the dispersive attraction drops below 20%-40% of the electrostatic attraction (as measured for two particles at contact). Below this threshold, ionic fluids display characteristically low values of Zc as well as large Guggenheim and Guldberg ratios for the reduced enthalpy of vaporization and the reduced boiling point, respectively. The coexistence curves are wider and more skewed than those for simple fluids. The results for the ionic model fluid with variable dispersion interactions improve our understanding of the behavior of real ionic fluids, such as inorganic molten salts and room temperature ionic liquids, by gauging the importance of different types of interactions for thermodynamic properties.

Viscosity minima in binary mixtures of ionic liquids + molecular solvents

The viscosity (η) of four binary mixtures (ionic liquids plus molecular solvents, ILs+MSs) was measured in the 283.15 < T/K < 363.15 temperature range. Different IL/MS combinations were selected in such a way that the corresponding η(T) functions exhibit crossover temperatures at which both pure components present identical viscosity values. Consequently, most of the obtained mixture isotherms, η(x), exhibit clear viscosity minima in the studied T-x range. The results are interpreted using auxiliary molecular dynamics (MD) simulation data in order to correlate the observed η(T,x) trends with the interactions in each mixture, including the balance between electrostatic forces and hydrogen bonding.

Molecular dynamics study of the effect of alkyl chain length on melting points of [CnMIM][PF6] ionic liquids

Based on molecular dynamics simulations, the melting points Tm of a series of 1-alkyl-3-methylimidazolium hexafluorophosphate ionic liquids [CnMIM][PF6] with n = 2, 4, 10, 12, and 14 were studied using the free energy-based pseudosupercritical path (PSCP) method. The experimental trend that the Tm decreases with increasing alkyl chain length for ILs with short alkyl chains and increases for the ones with long alkyl chains was correctly captured. Further analysis revealed that the different trends are the results of the balance between fusion enthalpy and fusion entropy. For the ILs with short alkyl chains (ethyl and butyl groups), fusion entropy plays the dominant role so that [C4MIM][PF6], which has a larger fusion entropy due to its higher liquid phase entropy has the lower melting temperature. As for the ILs with long alkyl chains, due to the enhanced van der Waals interactions brought about by the long non-polar alkyl chains, enthalpy becomes the deciding factor and the melting points increase when the alkyl chain goes from C10 to C14. While the melting points for [C2MIM][PF6] and [C4MIM][PF6] were quantitatively predicted and the trends for the long chain ILs were captured correctly, the absolute melting points for [C10MIM][PF6], [C12MIM][PF6] and [C14MIM][PF6] were systematically overestimated in the simulations. Three possible reasons for the overestimation were studied but all ruled out. Further simulation or experimental studies are needed to explain the difference.

Importance of dispersion forces for prediction of thermodynamic and transport properties of some common ionic liquids

An extensive study of interaction energies in ion pairs of pyrrolidinium and imidazolium ionic liquids is presented. The Cnmpyr and Cnmim cations with varying alkyl chains from Methyl, Ethyl, n-Propyl to n-Butyl were combined with a wide range of routinely used IL anions such as chloride, bromide, mesylate (CH3SO3 or Mes), tosylate (CH3PhSO3 or Tos), bis(trifluoromethanesulfonyl)amide (NTf2), dicyanamide (N(CN)2 or dca), tetrafluoroborate (BF4) and hexafluorophosphate (PF6). A number of energetically favourable conformations were studied for each cation-anion combination. The interaction energy and its dispersion component of the single ion pairs were calculated using a sophisticated state-of-the-art approach: a second-order of Symmetry Adapted Perturbation Theory (SAPT). A comparison of energetics depending on the cation-anion type, as well as the mode of interaction was performed. Dispersion forces were confirmed to be of importance for the overall stabilisation of ionic liquids contributing from 28 kJ mol(-1) in pyrrolidinium ion pairs to 59 kJ mol(-1) in imidazolium ion pairs. The previously proposed ratio of total interaction energy to dispersion components and melting points was assessed for this set of ionic liquids and was found to correlate with their melting points for the anionic series, producing separate trends for the Cnmim and Cmpyr series of cations. Chlorides, bromides and tetrafluoroborates formed close-to-ideal correlations when both types of cations, Cnmim and Cnmpyr, were combined in the same trend. Correlation of the dispersion component of the interaction energy with transport properties such as conductivity and viscosity was also considered. For imidazolium-based ionic liquids strong linear correlations were obtained, whereas pyrrolidinium ionic liquids appeared to be insensitive to this correlation.

Radiation induced physicochemical changes in FAP (fluoro alkyl phosphate) based imidazolium ionic liquids and their mechanistic pathways: influence of hydroxyl group functionalization of the cation

Future applications of ionic liquids (ILs) in a variety of areas, especially those having high radiation fields such as the nuclear fuel cycle and in space technology, are under serious consideration nowadays. For such applications to be possible, however, radiation stability of the ILs is an important issue that needs to be addressed. We envisaged that the ultra-hydrophobic, bulky and hydrolytically stable FAP (tris(perfluoroalkyl)trifluorophosphate) anion might shield the radiolytically vulnerable imidazolium cations from degradation and our result shows that these anions indeed enhance their radiolytic stability. However, introduction of a hydroxyl group into the alkyl side chain of the imidazolium moiety resulted in significant changes in the physical properties of the IL with respect to onset temperatures, conductivity and the electrochemical window. Furthermore, a nonlinear trend in absorbance with an increase in radiation dose accompanied by NMR (nuclear magnetic resonance) and mass spectrometry studies clearly demonstrated that the presence of the hydroxyl group promotes various degradation channels. Interestingly, a perturbation of the hydrogen bond between the hydroxyl group (present in the side chain of the cation) and the fluorine atom of the anion (OHF) was evident in the case of irradiated hydroxyl functionalized FAP ILs. Besides, the hydrogen gas yields of the ILs were determined and found to be comparable to those of a radiolytically stable aromatic compound, benzene. Finally, through transient spectroscopic studies we could delineate the mechanism of the radiation induced changes in the physicochemical properties of the non-hydroxyl and hydroxyl containing FAP ILs. We have clearly demonstrated that a simple functionalization of the molecular structure of the FAP based imidazolium ILs might cause marked differences in the reactivity, reaction center and the nature of the radiolytic products, which eventually lead to significant changes in their physicochemical properties.

Impact of self-aggregation on the formation of ionic-liquid-based aqueous biphasic systems

This work reports on the systematic investigation of the influence of the cation alkyl side-chain length of 1-alkyl-3-methylimidazolium chloride ionic liquids ([C(n)C(1)im]Cl, with n = 1-14), as well as the substitution of the most acidic hydrogen in the imidazolium core by a methyl group, in the formation of aqueous biphasic systems. Ternary phase diagrams, tie-lines, tie-line slopes, tie-line lengths, and critical points for the several systems (ionic liquid + water + K(3)PO(4)) were determined and reported at 298 K and atmospheric pressure. It is shown that the increase of the cation alkyl chain length enhances the formation of aqueous biphasic systems if alkyl chain lengths until the hexyl are considered. The results for longer alkyl side chains show, nevertheless, that the phenomenon is more complex than previously admitted and that the capacity of the ionic liquid to self-aggregate also governs its ability to phase separate. The effect of the alkyl side-chain length on the phase-forming ability of the studied systems was quantitatively evaluated based on their salting-out coefficients derived from a Setschenow-type behavior. The aptitude of each ionic liquid for liquid-liquid demixing as a function of the cation alkyl side-chain length clearly follows three different patterns. The results obtained for the trisubstituted cation indicate that the hydrogen-bonding interactions between the ionic liquid cation and water are not a relevant issue in the formation of aqueous two-phase systems. In general, for the [C(n)C(1)im]Cl series, a multifaceted ratio between entropic contributions and the ability of each ionic liquid to self-aggregate in aqueous media control the phase behavior.