Thermoresponsive Ionic Liquid/Water Mixtures: From Nanostructuring to Phase Separation

The thermodynamics, structures, and applications of thermoresponsive systems, consisting primarily of water solutions of organic salts, are reviewed. The focus is on organic salts of low melting temperatures, belonging to the ionic liquid (IL) family. The thermo-responsiveness is represented by a temperature driven transition between a homogeneous liquid state and a biphasic state, comprising an IL-rich phase and a solvent-rich phase, divided by a relatively sharp interface. Demixing occurs either with decreasing temperatures, developing from an upper critical solution temperature (UCST), or, less often, with increasing temperatures, arising from a lower critical solution temperature (LCST). In the former case, the enthalpy and entropy of mixing are both positive, and enthalpy prevails at low T. In the latter case, the enthalpy and entropy of mixing are both negative, and entropy drives the demixing with increasing T. Experiments and computer simulations highlight the contiguity of these phase separations with the nanoscale inhomogeneity (nanostructuring), displayed by several ILs and IL solutions. Current applications in extraction, separation, and catalysis are briefly reviewed. Moreover, future applications in forward osmosis desalination, low-enthalpy thermal storage, and water harvesting from the atmosphere are discussed in more detail.

Search for an exothermic halogen bond between anions

A halogen-bonded complex containing a pair of anions can be made more stable than the isolated anions if the Lewis acid is a long carbon chain, fully substituted by CN groups, with an I atom on one end and a COO− group on the other, with Cl− as base.

Inter-anion chalcogen bonds: Are they anti-electrostatic in nature?

Inter-anion hydrogen and halogen bonds have emerged as counterintuitive linkers and inspired us to expand the range of this unconventional bonding pattern. Here, the inter-anion chalcogen bond (IAChB) was proposed and theoretically analyzed in a series of complexes formed by negatively charged bidentate chalcogen bond donors with chloride anions. The kinetic stability of IAChB was evidenced by the minima on binding energy profiles and further supported by ab initio molecular dynamic simulations. The block-localized wave function (BLW) method and its subsequent energy decomposition (BLW-ED) approach were employed to elucidate the physical origin of IAChB. While all other energy components vary monotonically as anions get together, the electrostatic interaction behaves exceptionally as it experiences a Coulombic repulsion barrier. Before reaching the barrier, the electrostatic repulsion increases with the shortening Ch⋯Cl^- distance as expected from classical electrostatics. However, after passing the barrier, the electrostatic repulsion decreases with the Ch⋯Cl^- distance shortening and subsequently turns into the most favorable trend among all energy terms at short ranges, representing a dominating force for the kinetic stability of inter-anions. For comparison, all energy components exhibit the same trends and vary monotonically in the conventional counterparts where donors are neutral. By comparing inter-anions and their conventional counterparts, we found that only the electrostatic energy term is affected by the extra negative charge. Remarkably, the distinctive (nonmonotonic) electrostatic energy profiles were reproduced using quantum mechanical-based atomic multipoles, suggesting that the crucial electrostatic interaction in IAChB can be rationalized within the classical electrostatic theory just like conventional non-covalent interactions.

Classical Electrostatics Remains the Driving Force for Interanion Hydrogen and Halogen Bonding

Interanion hydrogen bonding (IAHB) and halogen bonding (IAXB) have emerged as a counterintuitive linker in a range of fascinating applications. Despite the overall repulsive (positive) binding energy, anions are trapped in a local minimum with its corresponding transition state (TS) preventing dissociation. In other words, the adduct of anions is metastable. Seemingly, the electrostatic paradigm and force field description of hydrogen/halogen bonding (HB/XB) are challenged, because of the preconceived Coulombic repulsion. Aiming at an insightful understanding of these interanion phenomena, we employed the energy decomposition approach based on the block-localized wavefunction method (BLW-ED) to investigate a series of exemplary interanion complexes. As expected, the key distinction from the conventional HB/XB lies in the electrostatic interaction, which is not increasingly repulsive as anions gradually approach to each other. Rather, there is a Coulombic barrier at a certain point. After this point, the electrostatic repulsion diminishes with the decreasing distance between anions. Differently, other energy components vary monotonically just like in conventional cases. The nonmonotonic characteristic of the electrostatic interaction in interanion complexes was reproduced using the multipole expansion in AMOEBA polarizable force field in which the state-specified atomic multipoles were adopted. This suggests that the nonmonotonicity can be well interpreted by classical electrostatic theory and there is no conceptual difference between conventional HB/XB and IAHB/IAXB. The stability of IAHB/IAXB depends on the competition between the local attractive HB/XB and the global Coulombic repulsion of net charges, though there is cooperativity between these two contrasting forces. This concise model was supported by the attractive IAHB/IAXB in modified molecular capsules, which exhibit strong quadruple HB/XBs and a considerable distance between charged substituents.

Controlled Diffusion of Photoswitchable Receptors by Binding Anti-electrostatic Hydrogen-Bonded Phosphate Oligomers

Dihydrogen phosphate anions are found to spontaneously associate into anti-electrostatic oligomers via hydrogen bonding interactions at millimolar concentrations in DMSO. Diffusion NMR measurements supported formation of these oligomers, which can be bound by photoswitchable anion receptors to form large bridged assemblies of approximately three times the volume of the unbound receptor. Photoisomerization of the oligomer-bound receptor causes a decrease in diffusion coefficient of up to 16%, corresponding to a 70% increase in effective volume. This new approach to external control of diffusion opens prospects in controlling molecular transport using light.

Solute displacement in the aqueous phase of water-NaCl-organic ternary mixtures relevant to solvent-driven water treatment

Twelve water miscible organic solvents (MOS): acetone, tetrahydrofuran, isopropanol, acetonitrile, dimethyl sulfoxide, 1,4-dioxane, dimethylacetamide, N-methyl-2-pyrrolidone, trifluoroethanol, isopropylamine, dimethylformamide, and dimethyl ether (DME) were used to produce ternary mixtures of water-NaCl-MOS relevant to MOS-driven fractional precipitation. The aqueous-phase composition of the ternary mixture at liquid-liquid equilibrium and liquid-solid endpoint was established through quantitative nuclear magnetic resonance and mass balance. The results highlight the importance of considering the hydrated concentrations of salts and suggest that at high salt concentrations and low MOS concentration, the salt concentration is governed by competition between the salt ions and MOS molecules. Under these conditions a LS phase boundary is established, over which one mole of salt is replaced by one mole of MOS (solute displacement). At higher MOS concentrations, MOS with higher water affinity deviate from the one-to-one solute exchange but maintain a LS boundary with a homogenous liquid phase, while MOS with lower water affinity form a liquid-liquid phase boundary. DME is found to function effectively as an MOS for fractional precipitation, precipitating 97.7% of the CaSO₄ from a saturated solution, a challenging scalant. DME-driven water softening recycles the DME within the system improving the atom-efficiency over existing seawater desalination pretreatments by avoiding chemical consumption.

Recognition and applications of anion-anion dimers based on anti-electrostatic hydrogen bonds (AEHBs)

Based on Coulomb's Law alone, electrostatic repulsion between two anions is expected to prevent their dimerization. Contrary to that idea, this Tutorial Review will present evidence showing that anion-anion dimers of protic hydroxyanions can form readily, and describe conditions that facilitate their formation. From X-ray crystal structures, we learn that hydroxyanions dimerize and oligomerize by overcoming long-range electrostatic opposition. Common examples are hydroxyanions of phosphate, sulfate, and carbonate, often in partnership with charged and neutral receptors. Short-range hydrogen bonds between anionic donors and acceptors are defined as anti-electrostatic hydrogen bonds (AEHBs) with insight from theoretical studies. While anion dimers are difficult to identify unequivocally in solution, these solution dimers have recently been definitively identified. The development of the supramolecular chemistry of anion-anion dimers has led to applications in hierarchical assemblies, such as supramolecular polymers and hydrogen bonded organic frameworks.

Antielectrostatically hydrogen bonded anion dimers: counter-intuitive, common and consistent

A Cambridge Structural Database survey reveals that antielectrostatically hydrogen bonded dimers occur frequently between a wide range of anions.

A three dimensional hydrogen bonded organic framework assembled through antielectrostatic hydrogen bonds

A 3D hydrogen bonded organic framework was assembled in water from tetraamidinium cations and antielectrostatically hydrogen bonded bicarbonate dimers.

Cationic clustering influences the phase behaviour of ionic liquids

“Unlike charges attract, but like charges repel”. This conventional wisdom has been recently challenged for ionic liquids. It could be shown that like-charged ions attract each other despite the powerful opposing electrostatic forces. In principle, cooperative hydrogen bonding between ions of like-charge can overcome the repulsive Coulomb interaction while pushing the limits of chemical bonding. The key challenge of this solvation phenomenon is to establish design principles for the efficient formation of clusters of like-charged ions in ionic liquids. This is realised here for a set of well-suited ionic liquids including the same hydrophobic anion but different cations all equipped with hydroxyethyl groups for possible H-bonding. The formation of H-bonded cationic clusters can be controlled by the delocalization of the positive charge on the cations. Strongly localized charge results in cation-anion interaction, delocalized charge leads to the formation of cationic clusters. For the first time we can show, that the cationic clusters influence the properties of ILs. ILs comprising these clusters can be supercooled and form glasses. Crystalline structures are obtained only, if the ILs are dominantly characterized by the attraction between opposite-charged ions resulting in conventional ion pairs. That may open a new path for controlling glass formation and crystallization. The glass temperatures and the phase transitions of the ILs are observed by differential scanning calorimetry (DSC) and infrared (IR) spectroscopy.

Phosphate-phosphate oligomerization drives higher order co-assemblies with stacks of cyanostar macrocycles

The importance of phosphate in biology and chemistry has long motivated investigation of its recognition. Despite this interest, phosphate's facile oligomerization is only now being examined following the discovery of complexes of anion-anion dimers of hydroxyanions. Here we address how oligomerization dictates phosphate's recognition properties when engaged with planar cyanostar macrocycles that can also oligomerize by stacking. The crystal structure of cyanostar with phosphate shows an unprecedented tetrameric stack of cyanostar macrocycles threaded by a phosphate trimer, [H₂PO₄···H₂PO₄···H₂PO₄]^3-. The solution behaviour, studied as a function of solvent quality, highlights how dimers and trimers of phosphate drive formation of higher order stacks of cyanostar into dimer, trimer and tetramer co-assemblies. Solution behaviors differ significantly from simpler complexes of bisulfate hydroxyanion dimers. Phosphate oligomerization is: (1) preferred over ion pairing with tetrabutylammonium cations, (2) inhibits disassembly of the complexes upon dilution, and (3) resists interference from competitive anion solvation. The phosphate oligomers also appear critical for stability; complexation of just one phosphate with cyanostars is unfavored. The cyanostar's ability to self-assemble is found to create a tubular, highly electropositive cavity that complements the size and shape of the phosphate oligomers as well as their higher charge. When given the opportunity, phosphate will cooperate with the receptor to form co-assembled architectures.

Theoretical Prediction of Robust Second-Row Oxyanion Clusters in the Metastable Domain of Antielectrostatic Hydrogen Bonding

We provide ab initio and density functional theory evidence for a family of surprisingly robust like-charged clusters of common HSO₄^- and H₂PO₄^- oxyanions, ranging up to tetramers of net charge 4-. Our results support other recent theoretical and experimental evidence for "antielectrostatic" hydrogen-bonded (AEHB) species that challenge conventional electrostatic conceptions and force-field modeling of closed-shell ion interactions. We provide structural and energetic descriptors of the predicted kinetic well-depths (in the range 3-10 kcal/mol) and barrier widths (in the range 2-4 Å) for simple AEHB dimers, including evidence of extremely strong hydrogen bonding in the fluoride-bisulfate dianion. For more complex polyanionic species, we employ natural-bond-orbital-based descriptors to characterize the electronic features of the cooperative hydrogen-bonding network that are able to successfully defy Coulomb explosion. The computational results suggest a variety of kinetically stable AEHB species that may be suitable for experimental detection as long-lived gas-phase species or structural units of condensed phases, despite the imposing electrostatic barriers that oppose their formation under ambient conditions.