The role of urea in formation of the sodium acetate trihydrate (SAT)-urea eutectic liquid: a neutron diffraction and isotopic substitution study.

Neutron diffraction with isotopic substitution has been used to investigate the structure of the liquid sodium acetate trihydrate-urea eutectic (mole fraction (χurea) of 0.60) at 50 °C. Urea competes with acetate anions and water molecules in the solvation of sodium ions, displacing water and, simultaneously, stabilising the liberated 'excess' water through hydrogen bonding between water and urea molecules in the eutectic liquid. This provides a direct insight into the role of urea as both denaturant and hydrogen-bond network former in generating eutectic liquids.

Hydration of sulfobetaine dizwitterions as a function of alkyl spacer length.

The solvation and structure of bolaform dizwitterions containing two sulfobetaine moieties in concentrated aqueous solution were determined using neutron diffraction with isotopic substitution (NDIS) combined with modelling of the measured structure factors using Empirical Potential Structure Refinement (EPSR). Strongly directional local hydration was observed in the polar regimes of the dizwitterions with 48-52 water molecules shared between dizwitterion molecules in a first shell water network around each zwitterion pair. Overall, the double zwitterions were highly hydrated, providing experimental evidence in support of the potential formation of protein-resistant hydration layers at zwitterion-water interfaces.

Water-in-CO2 Microemulsions Stabilized by an Efficient Catanionic Surfactant.

To facilitate potential applications of water-in-supercritical CO2 microemulsions (W/CO2 µEs) efficient and environmentally responsible surfactants are required with low levels of fluorination. As well as being able to stabilize water-CO2 interfaces, these surfactants must also be economical, prevent bioaccumulation and strong adhesion, deactivation of enzymes, and be tolerant to high salt environments. Recently, an ion paired catanionic surfactant with environmentally acceptable fluorinated C6 tails was found to be very effective at stabilizing W/CO2 µEs with high water-to-surfactant molar ratios (W0) up to ∼50 (Sagisaka, M.; et al. Langmuir 2019, 35, 3445-3454). As the cationic and anionic constituent surfactants alone did not stabilize W/CO2 µEs, this was the first demonstration of surfactant synergistic effects in W/CO2 microemulsions. The aim of this new study is to understand the origin of these intriguing effects by detailed investigations of nanostructure in W/CO2 microemulsions using high-pressure small-angle neutron scattering (HP-SANS). These HP-SANS experiments have been used to determine the headgroup interfacial area and volume, aggregation number, and effective packing parameter (EPP). These SANS data suggest the effectiveness of this surfactant originates from increased EPP and decreased hydrophilic/CO2-philic balance, related to a reduced effective headgroup ionicity. This surfactant bears separate C6F13 tails and oppositely charged headgroups, and was found to have a EPP value similar to that of a double C4F9-tail anionic surfactant (4FG(EO)2), which was previously reported to be one of most efficient stabilizers for W/CO2 µEs (maximum W0 = 60-80). Catanionic surfactants based on this new design will be key for generating superefficient W/CO2 µEs with high stability and water solubilization.

Multicharge zwitterionic molecules: Hydration, kosmotropicity and anti-fouling potential.

This manuscript presents a comparative study of the physico-chemical behaviour of sulfobetaine-type single and double zwitterions and zwitterionic salts, and structurally similar mono- and di-cationic tetraalkylammonium salts in aqueous solutions. The study includes experimental determination of the density and viscosity of highly diluted aqueous solutions with derivation of the Jones-Dole viscosity B-coefficient, partial molal volumes at infinite dilution, and hydration numbers. The study also examines the effects of addition of the salts on the surface tension of cationic and anionic surfactants, upper critical solution temperature of a non-ionic surfactant, solubility of amino acids, and stability of a protein. The experimental investigation was performed taking a broad bottom-up approach with the aim to elucidate the effect of molecular architecture and charge (two versus four) on the degree of surface hydration of a molecule, kosmotropicity, and interactions with charged and hydrophilic/hydrophobic surfaces - all-important characteristics which define ability of a functional group to resist protein attachment. The novel multicharged zwitterionic materials have exhibited superior qualities, thus paving the way to development of a new platform in design of hydrophilic and anti-fouling surfaces by employing the four-charge bearing molecular motifs.

Water-in-CO2 Microemulsions Stabilized by Fluorinated Cation-Anion Surfactant Pairs.

High-water-content water-in-supercritical CO2 (W/CO2) microemulsions are considered to be green, universal solvents, having both polar and nonpolar domains. Unfortunately, these systems generally require environmentally unacceptable stabilizers like long and/or multifluorocarbon-tail surfactants. Here, a series of catanionic surfactants having more environmentally friendly fluorinated C4-C6 tails have been studied in terms of interfacial properties, aggregation behavior, and solubilizing power in water and/or CO2. Surface tensions and critical micelle concentrations of these catanionic surfactants are, respectively, lowered by ∼9 mN/m and 100 times than those of the constituent single fluorocarbon-tail surfactants. Disklike micelles in water were observed above the respective critical micelle concentrations, implying the catanionic surfactants have a high critical packing parameter, which should be suitable for the formation of reverse micelles. Based on visual observation of phase behavior and Fourier transform infrared spectroscopic and small-angle neutron scattering studies, one of the three catanionic surfactants tested was found to form transparent single-phase W/CO2 microemulsions with a water-to-surfactant molar ratio of up to ∼50. This is the first successful demonstration of the formation of W/CO2 microemulsions by synergistic ion-pairing of anionic and cationic single-tail surfactants. This indicates that catanionic surfactants offer a promising approach to generate high-water-content W/CO2 microemulsions.

Solubility-Modifying Power of Zwitterionic Salts.

The separation of small, hydrophilic molecules from aqueous solutions on one side and the dissolution of hydrophobic organic molecules in water on the other are nowadays among the most difficult challenges in chemical and bio-technology. Even though these two tasks are seemingly of opposite nature, it is demonstrated herein that both processes can be facilitated by addition of zwitterionic salts, a new class of compounds that can act either as a solubility enhancer or a phase-separation promoter depending on the structure of the solute dissolved in an aqueous solution. At a more fundamental level, this study investigates the salting in/out propensity of the ions and supports the importance of both salt-solute interaction and their concentration.

LCST Phase Behavior and Complexation with Water of an Ionic Liquid Incorporating the 5-Phenyltetrazolate Anion.

The use of 5-phenyltetrazole, a bioisostere of benzoic acid, as an anion source to prepare an ionic liquid is described for the first time. Tetrabutylphosphonium 5-phenyltetrazolate undergoes phase separation from water with lower critical solution temperature phase behavior, in contrast to completely water miscible tetrabutylphosphonium benzoate, and also exhibits strong complexation with water with both eutectic and peritectic behavior that has not previously been observed in ionic liquid+water systems. The anhydrous and trihydrate salts were isolated and characterized by single crystal X-ray diffraction.

Phase behaviour, interactions, and structural studies of (amines+ionic liquids) binary mixtures.

We present a study on the phase equilibrium behaviour of binary mixtures containing two 1-alkyl-3-methylimidazolium bis{(trifluoromethyl)sulfonyl}imide-based ionic liquids, [C(n)mim] [NTf(2)] (n=2 and 4), mixed with diethylamine or triethylamine as a function of temperature and composition using different experimental techniques. Based on this work, two systems showing an LCST and one system with a possible hourglass shape are measured. Their phase behaviours are then correlated and predicted by using Flory-Huggins equations and the UNIQUAC method implemented in Aspen. The potential of the COSMO-RS methodology to predict the phase equilibria was also tested for the binary systems studied. However, this methodology is unable to predict the trends obtained experimentally, limiting its use for systems involving amines in ionic liquids. The liquid-state structure of the binary mixture ([C(2)mim] [NTf(2)]+diethylamine) is also investigated by molecular dynamics simulation and neutron diffraction. Finally, the absorption of gaseous ethane by the ([C(2)mim][NTf(2)]+diethylamine) binary mixture is determined and compared with that observed in the pure solvents.

Solubility of alkanes, alkanols and their fluorinated counterparts in tetraalkylphosphonium ionic liquids.

We investigated the mutual solubility of mixtures of phosphonium-based ionic liquids with alkanes, alkanols, fluorinated alkanes and fluorinated alkanols. The solubilities of other solute molecules like water, formamide, 1,4-dioxane, benzene, and dimethylsulfoxide were also tested. Whenever possible, the corresponding temperature-composition (T-x) phase diagrams at atmospheric pressure were built from cloud-point temperature determinations. The influence of the size of the solute was tested with binary mixtures of trihexyl(tetradecyl)phosphonium acetate, [P(6 6 6 14)][Ac], with hexane, decane or tetradecane. The influence of the anion of the ionic liquid, namely acetate, [Ac], bis-[(trifluoromethyl)sulfonyl]imide, [Ntf(2)], trifluoromethanesulfonate, [Otf], and dicyanamide, [dca], on the solubility of the ionic liquids in hexane was also studied. For the ionic liquid [P(6 6 6 14)][Ntf(2)] the liquid-liquid phase diagrams were determined with different solutes-alkanes, perfluoroalkanes, partially fluorinated alkanes, and partially fluorinated alkanols-with the aim of analysing the solute-solvent interactions. A comparison of the phase behaviour of solutions containing phosphonium-based ionic liquids and 1-alkyl-3-methylimidazolium based ionic liquids, including a discussion of their different morphologies at a structural level, is also provided. It was found that fluorination of the aliphatic chains of organic compounds can be used as an effective way to control the solubility limits of these compounds in phosphonium- or imidazolium-based ionic liquids, both in terms of concentration and temperature.

New catanionic surfactants based on 1-alkyl-3-methylimidazolium alkylsulfonates, [C(n)H(2n+1)mim][C(m)H(2m+1)SO(3)]: mesomorphism and aggregation.

Anionic and cationic alkyl-chain effects on the self-aggregation of both neat and aqueous solutions of 1-alkyl-3-methylimidazolium alkylsulfonate salts ([C(n)H(2n+1)mim][C(m)H(2m+1)SO(3)]; n = 8, 10 or 12; m = 1 and n = 4 or 8; m = 4 or 8) have been investigated. Some of these salts constitute a novel family of pure catanionic surfactants in aqueous solution. Examples of this class of materials are rare; they are distinct from both mixed cationic-anionic surfactants (obtained by mixing two salts) and gemini surfactants (with two or more amphiphilic groups bound by a covalent linker). Fluorescence spectroscopy and interfacial tension measurements have been used to determine critical micelle concentrations (CMCs), surface activity, and to compare the effects of the alkyl-substitution patterns in both the cation and anion on the surfactant properties of these salts. With relatively small methylsulfonate anions (n = 8, 10 and 12, m = 1), the salts behave as conventional single chain cationic surfactants, showing a decrease of the CMC upon increase of the alkyl chain length (n) in the cation. When the amphiphilic character is present in both the cation and anion (n = 4 and 8, m = 4 and 8), novel catanionic surfactants with CMC values lower than those of the corresponding cationic analogues, and which exhibited an unanticipated enhanced reduction of surface tension, were obtained. In addition, the thermotropic phase behaviour of [C(8)H(18)mim][C(8)H(18)SO(3)] (n = m = 8) was investigated using variable temperature X-ray scattering, polarising optical microscopy and differential scanning calorimetry; formation of a smectic liquid crystalline phase with a broad temperature range was observed.

Phase equilibria in ionic liquid-aromatic compound mixtures, including benzene fluorination effects.

This work extends the scope of previous studies on the phase behavior of mixtures of ionic liquids with benzenes or its derivatives by determining the solid-liquid and liquid-liquid phase diagrams of mixtures containing an ionic liquid and a fluorinated benzene. The systems studied include 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide plus hexafluorobenzene or 1,3,5-trifluorobenzene and 1-ethyl-3-methylimidazolium triflate or N-ethyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide plus benzene. The phase diagrams exhibit different kinds of solid-liquid behavior: the (usual) occurrence of eutectic points; the (not-so-usual) presence of congruent melting points and the corresponding formation of inclusion crystals; or the observation of different ionic liquid crystalline phases (polymorphism). These different types of behavior can be controlled by temperature annealing during crystallization or by the nature of the aromatic compound and can be interpreted, at a molecular level, taking into account the structure of the crystals or liquid mixtures, together with the unique characteristics of ionic liquids, namely the dual nature of their interactions with aromatic compounds.

1-Alkyl-3-methylimidazolium alkanesulfonate ionic liquids, [C(n)H(2)(n)(+1)mim][C(k)H(2)(k)(+1)SO(3)]: synthesis and physicochemical properties.

A set of 1-alkyl-3-methylimidazolium alkanesulfonate ionic liquids, [C(n)mim][C(k)SO(3)], formed by the variation of the alkyl chain lengths both in the cation and the anion (n = 1-6, 8, or 10; k = 1-4, or 6), was synthesised, with sixteen of them being novel. The ionic liquids were characterised by (1)H and (13)C NMR spectroscopy, and mass spectrometry. Their viscosities and densities as a function of temperature, as well as melting points and decomposition temperatures, were determined. The molecular volumes, both experimental and calculated, were found to depend linearly on the sum (n + k).

On the self-aggregation and fluorescence quenching aptitude of surfactant ionic liquids.

The aggregation behavior in aqueous solution of a number of ionic liquids was investigated at ambient conditions by using three techniques: fluorescence, interfacial tension, and (1)H NMR spectroscopy. For the first time, the fluorescence quenching effect has been used for the determination of critical micelle concentrations. This study focuses on the following ionic liquids: [Cnmpy]Cl (1-alkyl-3-methylpyridinium chlorides) with different linear alkyl chain lengths (n=4, 10, 12, 14, 16, or 18), [C12mpip]Br (1-dodecyl-1-methylpiperidinium bromide), [C12mpy]Br (1-dodecyl-3-methylpyridinium bromide), and [C12mpyrr]Br (1-dodecyl-1-methylpyrrolidinium bromide). Both the influence of the alkyl side-chain length and the type of ring in the cation (head) on the CMC were investigated. A comparison of the self-aggregation behavior of ionic liquids based on 1-alkyl-3-methylpyridinium and 1-alkyl-3-methylpyridinium cations is provided. It was observed that 1-alkyl-3-methylpyridinium ionic liquids could be used as quenchers for some fluorescence probes (fluorophores). As a consequence, a simple and convenient method to probe early evidence of aggregate formation was established.

Salting-out effects in aqueous ionic liquid solutions: cloud-point temperature shifts.

The effects of the addition of three inorganic salts, namely, NaCl, Na(2)SO(4), and Na(3)PO(4), on the liquid-liquid (L-L) phase diagram of aqueous solutions containing the model ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate, [bmim][BF(4)], were investigated. All three inorganic salts trigger salting-out effects, leading to significant upward shifts of the L-L demixing temperatures of the systems. The magnitude of the shifts depends on both the water-structuring nature of the salt and its concentration; that is, the effects are correlated with the ionic strength of the solution and the Gibbs free energy of hydration of the inorganic salt. The pH effect and the occurrence of salt precipitation in concentrated solutions are also discussed.