Non‐Ideal Mixing Behaviour of Hydrogen Bonding in Mixtures of Protic Ionic Liquids

Ionic liquids (ILs) attract interest in science and technology as a result of their unique properties. Binary and ternary mixtures of ILs significantly increase the number of possible cation/anion combinations, resulting in targeted physical and chemical properties. In this work, we study the mixing behaviour of two protic ILs: triethyl ammonium methylsulfonate [Et₃NH][CH₃SO₃] and triethylammonium triflate [Et₃NH][CF₃SO₃]. We find a characteristic deviation from ideal mixing by means of low‐frequency infrared spectroscopy. By using molecular dynamics simulations, we explain this behaviour as being the result of different strengths of anion/cation hydrogen bonding. This non‐ideality of non‐random H‐bond mixing is also reflected in macroscopic properties such as the viscosity. Mixing suitable ILs may, thus, result in new ILs with targeted physical properties.     

Hydrogen-Bonding-Driven Ion-Pair Formation in Protic Ionic Liquid Aqueous Solution

Protic ionic liquids (PILs) in solution especially in water have attracted more and more attention due to their unique properties. The solvation of PILs in water is important to their properties and applications. To explore the solvation of bio-based PILs in water, acidity of 49 [AA]X amino acid ionic liquids (AAILs) consisting of 7 different cations and 7 different anions was studied as a favorable probe. The pK_a values for [AA]X PILs containing same cations were obtained and discussed. The acidity strength of the [AA]X PILs varies with both cation and anion which does not follow the conventional assumption that the acidity for PILs is independent of anions. The acidic discrepancy of [AA]X PILs aqueous solution is probably mediated by the formation of ion pairs according to a revised solvation model of PILs. Quantum-chemistry calculation was employed to unpuzzle anion's different effects on the acid balance of cations where cation-anion hydrogen bonds play an important role. Such difference in acidity allows us to understand the formation of solvated ion pairs. This work provides an insight into the fundamental solvation of PILs from acid perspective and their influence on acidity properties for the first time.     

Potentiometric Determination of Non‐Ionic Surfactants by Liquid Membrane Electrodes

It is well known that non‐ionic surfactants (NIS) influence remarkably the potentiometric measurements with liquid membrane ion selective electrodes (ISEs), interfering particularly on performance of ISEs for earth‐alkali metals, for which the loss of selectivity with regard to alkali metals has been documented. These studies indicate that such interferences are due to the extraction of surfactants within the membrane, where a competition takes place between the originally present ionophore and the surfactant which also acts as a ligand for alkali metals. The interpretation of such phenomena enabled one to exploit this interference for analytical purposes by membrane/solution extraction experiment monitored by UV measurements and by impedance FRA analysis on coated wire electrodes. Using Ca/Mg ISEs based on the neutral ionophore ETH 4030, it has been established that the logarithm of the Ca/Mg over Na potentiometric selectivity constant is linearly correlated with the concentration of NIS like Tegopren 5863 and Triton X‐100. The proposed method has been applied for the development of a new potentiometric analytical procedure for the determination of Tegopren 5863 in synthetic seawater (SSW), ranging from 0.25 to 5 ppm. Our procedure consists in the exposure of the electrode to stirred SSW containing the surfactant; the progressive extraction of Tegopren 5863 causes a growth in electrode's sensitivity to Na⁺ and K⁺, losing selectivity for Ca²⁺ and Mg²⁺. In turn this induces an increase of EMF, as all these ions are present in the studied matrix. The potential drift was monitored for 15 hours, showing that the process reaches thermodynamic equilibrium after about 12 hours of exposure. This method presents a value of 210 ppb of Tegopren 5863 as detection limit.     

Effect of the Cationic Surfactant Moiety on the Structure of Water Entrapped in Two Catanionic Reverse Micelles Created from Ionic Liquid‐Like Surfactants

The behavior of water entrapped in reverse micelles (RMs) formed by two catanionic ionic liquid‐like surfactants, benzyl‐n‐hexadecyldimethylammonium 1,4‐bis‐2‐ethylhexylsulfosuccinate (AOT‐BHD) and cetyltrimethylammonium 1,4‐bis‐2‐ethylhexylsulfosuccinate (AOT‐CTA), was investigated by using dynamic (DLS) and static (SLS) light scattering, FTIR, and ¹H NMR spectroscopy techniques. To the best of our knowledge, this is the first report in which AOT‐CTA has been used to create RMs and encapsulate water. DLS and SLS results revealed the formation of RMs in benzene and the interaction of water with the RM interface. From FTIR and ¹H NMR spectroscopy data, a difference in the magnitude of the water–catanionic surfactant interaction at the interface is observed. For the AOT‐BHD RMs, a strong water–surfactant interaction can be invoked whereas for AOT‐CTA this interaction seems to be weaker. Consequently, more water molecules interact with the interface in AOT‐BHD RMs with a completely disrupted hydrogen‐bond network, than in AOT‐CTA RMs in which the water structure is partially preserved. We suggest that the benzyl group present in the BHD⁺ moiety in AOT‐BHD is responsible for the behavior of the catanionic interface in comparison with the interface created in AOT‐CTA. These results show that a simple change in the cationic component in the catanionic surfactant promotes remarkable changes in the RMs interface with interesting consequences, in particular when using the systems as nanoreactors.     

Self‐Assembly of Imidazolium‐Based Surfactants in Magnetic Room‐Temperature Ionic Liquids: Binary Mixtures

The phase behaviour of binary mixtures of ionic surfactants (1‐alkyl‐3‐imidazolium chloride, C_nmimCl with n=14, 16 and 18) and imidazolium‐based ionic liquids (1‐alkyl‐3‐methylimidazolium tetrachloroferrate, C_nmimFeCl₄, with n=2 and 4) over a broad temperature range and the complete range of compositions is described. By using many complementary methods including differential scanning calorimetry (DSC), polarised microscopy, small‐angle neutron and X‐ray scattering (SANS/SAXS), and surface tension, the ability of this model system to support self‐assembly is described quantitatively and this behaviour is compared with common water systems. The existence of micelles swollen by the solvent can be deduced from SANS experiments and represent a possible model for aggregates, which has barely been considered for ionic‐liquid systems until now, and can be ascribed to the rather low solvophobicity of the surfactants. Our investigation shows that, in general, C_nmimCl is a rather weak amphiphile in these ionic liquids. The amphiphilic strength increases systematically with the length of the alkyl chain, as seen from the phase behaviour, the critical micelle concentration, and also the level of definition of the aggregates formed.     

Ion‐Responsive Behavior of Ionic‐Liquid Surfactant Aggregates with Applications in Controlled Release and Emulsification

We propose a simple but efficient, rapid, and quantitative ion‐responsive micelle system based on counter‐anion exchange of a surfactant with an imidazolium unit. The ion‐exchange reaction results in the amphiphilic‐to‐hydrophobic transition of the imidazolium salt, leading to the destruction of the micelles, which has been successfully applied to controlled release and emulsification. The proposed design offers a novel alternative stimulus to control these smart physical aggregates besides pH, temperature and light—with extra advantages. Our finding greatly benefits both fundamental research and industry.     

Hydrogen Bonding in Protic Ionic Liquids: Reminiscent of Water

Similarities and differences : Far‐infrared spectra of protic ionic liquids could be assigned to intermolecular bending and stretching modes of hydrogen bonds. The characteristics of the low‐frequency spectra resemble those of water. Both liquids form three‐dimensional network structures, but only water is capable of building tetrahedral configurations. EAN: ethylammonium nitrate, PAN: propylammonium nitrate, DMAN: dimethylammonium nitrate.

     

Role of the Surfactant Structure in the Behavior of Hydrophobic Ionic Liquids within Aqueous Micellar Solutions

The behavior of an ionic liquid (IL) within aqueous micellar solutions is governed by its unique property to act as both an electrolyte and a cosolvent. The influence of the surfactant structure on the properties of aqueous micellar solutions of zwitterionic SB‐12, nonionic Brij‐35 and TX‐100, and anionic sodium dodecyl sulfate (SDS) in the presence of the “hydrophobic” IL 1‐butyl‐3‐methylimidazolium hexafluorophosphate ([bmim][PF₆]) is assessed along with the possibility of forming oil‐in‐water microemulsions in which the IL acts as the “oil” phase. The solubility of [bmim][PF₆] within aqueous micellar solutions increases with increasing surfactant concentration. In contrast to anionic SDS, the zwitterionic and nonionic surfactant solutions solubilize more [bmim][PF₆] at higher concentrations and the average aggregate size remains almost unchanged. The formation of IL‐in‐water microemulsions when the concentration of [bmim][PF₆] is above its aqueous solubility is suggested for nonionic Brij‐35 and TX‐100 aqueous surfactant solutions.     

Structure Effects on the Ionicity of Protic Ionic Liquids

We report on the characterisation of 16 protic ionic liquids (PILs) prepared by neutralisation of primary or tertiary amines with a range of simple carboxylic acids, or salicylic acid. The extent of proton transfer was greater for simple primary amine ILs compared to tertiary amines. For the latter case, proton transfer was increased by providing a better solvation environment for the ions through the addition of a hydroxyl group, either on the tertiary amine, or by formation of PIL/molecular solvent mixtures. The library of PILs was characterised by differential scanning calorimetry and a range of transport properties (i. e. viscosity, conductivity and diffusivity) were measured. Using the (fractional) Walden rule, the conductivity and viscosity results were analysed with respect to their deviation from ideal behaviour. The validity of the Walden plot for PILs containing ions of varying sizes was also verified for a number of samples by directly measuring self-diffusion coefficients using pulsed-field gradient spin-echo (PGSE) NMR. Ionicity was found to decrease as the alkyl chain length and degree of branching of both the cations and anions was increased. These results aim to develop a better understanding of the relationship between PIL properties and structure, to help design ILs with optimal properties for applications.     

Liquid Crystalline Period Variations in Self‐Assembled Block Copolypeptides–Surfactant Ionic Complexes

We investigate the complexation of ampholytic poly(N‐isopropylacylamide)‐block‐poly‐ (L ‐glutamic acid)‐block‐poly(L ‐lysine) (PNiPAM‐b‐PLG‐b‐PLLys) triblock copolymers and PNiPAM‐block‐(PLG‐co‐PLLys) diblock copolymers with counter charged anionic and cationic surfactants. Both triblock and diblock copolymers are able to selectively form complexes through either L ‐glutamic acid–cationic surfactant or L ‐lysine–anionic surfactant ionic pairs, depending on the protonated or deprotonated states of the ampholytic peptide units. The complexes show ordering at multiple length scales: i) the block copolymer length scale (10¹ nm), ii) the liquid crystalline length scale (10⁰ nm), and, iii) the peptidic secondary structures length scale (10⁰ nm). We show that the liquid crystalline period can be tuned by varying the random/block copolypeptide architectures and the composition of the ampholytic amino acid species.

     

Aqueous Networks and Toroids of Amphiphilic Block Copolymer with Non‐ionic Surfactants

Network formation: Comicellization of polystyrene‐block‐polyethylene oxide (PS–PEO) with non‐ionic surfactants leads to the formation of networks and chains of spheres. The aqueous network solutions are responsive to dilution, undergoing morphological transformations to cylinders with Y junctions and both two‐ and three‐dimensional toroids (see figure).

     

Supramolecular Self‐Assembly of Cucurbit[6]uil and Ionic Liquid in Non‐aqueous System

A novel [2]pseudorotaxane of cucurbit[6]uril(CB[6]) and 1‐butyl‐3‐methyl‐imidazolium bromide ([C₄mim]Br) was synthesized by directly mixing the host and the guest molecules in non‐aqueous system. Structural characterizations of the [2]pseudorotaxane were carried out by 1D, 2D NMR and X‐ray crystallography techniques both in solution and in crystal structure. The crystal structure demonstrated that CB[6] and [C₄mim]Br formed a complex with the ratio 1:1, in which one guest [C₄mim]Br was included inside the CB[6], while two other [C₄mim]Br molecules were free and surrounded the [2]pseudorotaxane as solvent molecules, which could stabilize the crystal structure through hydrogen bonds. Moreover, parallel solvent channels consisting by free [C₄mim]Br molecules occupied the pores among the frame of the pseudorotaxanes and formed zigzag lines in the crystal structure. [C₄mim]Br can serve as not only the guest reactant but also the solvent in the formation of [2]pseudorotaxane formation.     

Solvation Structure of Uracil in Ionic Liquids

The local solvation environment of uracil dissolved in the ionic liquid 1‐ethyl‐3‐methylimidazolium acetate has been studied using neutron diffraction techniques. At solvent:solute (ionic liquid:uracil) ratios of 3:1 and 2:1, little perturbation of the ion–ion correlations compared to those of the neat ionic liquid are observed. We find that solvation of the uracil is driven predominantly by the acetate anion of the solvent. While short distance correlations exist between uracil and the imidazolium cation, the geometry of these contacts suggest that they cannot be considered as hydrogen bonds, in contrast to other studies by Araújo et al. (J. M. Araújo, A. B. Pereiro, J. N. Canongia‐Lopes, L. P. Rebelo, I. M. Marrucho, J. Phys. Chem. B 2013, 117, 4109–4120 ). Nevertheless, this combination of interactions of the solute with both the cation and anion components of the solvents helps explain the high solubility of the nucleobase in this media. In addition, favourable uracil–uracil contacts are observed, of similar magnitude to those between cation and uracil, and are also likely to aid dissolution.