Cation-anion Interaction Strength Research Articles

Aggregation of ionic liquids with organic anions, driven by cation–anion interaction strength

The properties of ionic liquids (ILs) are related to the structure of their ions, which determines the nature of the cation–anion interaction. However, the direct effect that the structure has on properties such as aggregation and surface activity is not precise, especially when involving dicationic ILs with organic anions. In this study, the aggregation behaviour of imidazolium-based dicationic (1,8-bis(3-methylimidazolium-1-yl)octane - [C8(MIM)2]) ILs containing ibuprofenate [Ibu] and ascorbate [Asc] were evaluated (using conductivity and surface tension) and this was related to the cation–anion interaction strength determined using hydrogen nuclear magnetic resonance and surface plasmon resonance. The results showed that both ILs had a critical aggregation concentration value that was quite low compared to the analogue with a bromide anion. The [C8(MIM)2][2Asc] IL, which has the less hydrophobic anion and has more polar sites, had a greater cation–anion interaction strength and a lower critical aggregation concentration than the [C8(MIM)2][2Ibu] analogue. The cation–anion strength reflected the aggregation properties; for example, [C8(MIM)2][2Asc] had thermodynamically favoured aggregation and better packing of IL monomers at the air–water interface. These results are considered to be relevant for future studies related to the transport and drug loading associated with the use of these ILs.

1-Alkyl-3-methylimidazolium cation binding preferences in hexafluorophosphate ionic liquid clusters determined using competitive TCID measurements and theoretical calculations.

Ionic liquids (ILs) exhibit unique properties that have led to their development and widespread use for a variety of applications. Development efforts have generally focused on achieving desired macroscopic properties via tuning of the IL through variation of the cations and anions. Both the macroscopic and microscopic properties of an IL influence its tunability and thus feasibility of use for selected applications. Works geared toward a microscopic understanding of the nature and strength of the intrinsic cation-anion interactions of ILs have been limited to date. Specifically, the intrinsic strength of the cation-anion interactions in ILs is largely unknown. In previous work, we employed threshold collision-induced dissociation (TCID) approaches supported and enhanced by electronic structure calculations to determine the bond dissociation energies (BDEs) and characterize the nature of the cation-anion interactions in a series of four 2 : 1 clusters of 1-alkyl-3-methylimidazolium cations with the hexafluorophosphate anion, [2Cnmim:PF6]+. To examine the effects of the 1-alkyl chain on the structure and energetics of binding, the cation was varied over the series: 1-ethyl-3-methylimidazolium, [C2mim]+, 1-butyl-3-methylimidazolium, [C4mim]+, 1-hexyl-3-methylimidazolium, [C6mim]+, and 1-octyl-3-methylimidazolium, [C8mim]+. The variation in the strength of binding among these [2Cnmim:PF6]+ clusters was found to be similar in magnitude to the average experimental uncertainty in the measurements. To definitively establish an absolute order of binding among these [2Cnmim:PF6]+ clusters, we extend this work again using TCID and electronic structure theory approaches to include competitive binding studies of three mixed 2 : 1 clusters of 1-alkyl-3-methylimidazolium cations and the hexafluorophosphate anion, [Cn-2mim:PF6:Cnmim]+ for n = 4, 6, and 8. The absolute BDEs of these mixed [Cn-2mim:PF6:Cnmim]+ clusters as well as the absolute difference in the strength of the intrinsic binding interactions as a function of the cation are determined with significantly improved precision. By combining the thermochemical results of the previous independent and present competitive measurements, the BDEs of the [2Cnmim:PF6]+ clusters are both more accurately and more precisely determined. Comparisons are made to results for the analogous [2Cnmim:BF4]+ and [Cn-2mim:BF4:Cnmim]+ clusters previously examined to elucidate the effects of the [PF6]- and [BF4]- anions on the binding.

Absolute Trends and Accurate and Precise Gas-Phase Binding Energies of 1-Alkyl-3-Methylimidazolium Tetrafluoroborate Ionic Liquid Clusters from Combined Independent and Competitive TCID Measurements.

Ionic liquid (IL) development efforts have focused on achieving desired properties via tuning of the IL through variation of the cations and anions. However, works geared toward a microscopic understanding of the nature and strength of the intrinsic cation-anion interactions of ILs have been rather limited such that the intrinsic strength of the cation-anion interactions in ILs is largely unknown. In previous work, we employed threshold collision-induced dissociation approaches supported and enhanced by electronic structure calculations to characterize the nature of the cation-anion interactions in and determine the bond dissociation energies (BDEs) of a series of four 2:1 clusters of 1-alkyl-3-methylimidazolium cations and tetrafluoroborate anions, [2Cnmim:BF4]+. The cation was varied over the series: 1-ethyl-3-methylimidazolium, [C2mim]+, 1-butyl-3-methylimidazolium, [C4mim]+, 1-hexyl-3-methylimidazolium, [C6mim]+, and 1-octyl-3-methylimidazolium, [C8mim]+, to determine the structural and energetic effects of the size of the 1-alkyl substituent on the binding. The variation in the strength of binding determined for these [2Cnmim:BF4]+ clusters was found to be similar in magnitude to the average experimental uncertainty in these determinations. To definitively establish an absolute order of binding among these [2Cnmim:BF4]+ clusters, we extend this work here to include competitive binding studies of three mixed 2:1 clusters of 1-alkyl-3-methylimidazolium cations and tetrafluoroborate anions, [Cn-2mim:BF4:Cnmim]+ for n = 4, 6, and 8. Importantly, the results of the present work simultaneously provide the absolute BDEs of these mixed [Cn-2mim:BF4:Cnmim]+ clusters and the absolute relative order of the intrinsic binding interactions as a function of the cation with significantly improved precision. Further, by combining the thermochemical results of the previous and present studies, the BDEs of the [2Cnmim:BF4]+ clusters are more accurately and precisely determined.

Effect of large anions in thermal properties and cation-anion interaction strength of dicationic ionic liquids

The properties of an ionic liquid (IL) are directly related to the structure of its ions, which determine the nature of intermolecular cation-anion interactions. In this paper, we determined the thermal properties and cation-anion interaction strength of a series of dicationic imidazolium-based ILs containing alkyl spacer with eight methylene cation [BisOct(MIM)2] and organic larger anions (ibuprofenate, hippurate, ascorbate, N-triflate, and docusate). Results showed that thermal properties do not depend on the volume or molecular weight of the anion, but on its organic nature, which is, the more organic, the less stable. The cation-anion interaction strength was determined by electrospray ionization mass spectrometry using collision-induced experiments. Results showed that the larger the anion, the weaker the cation-anion interaction. This tendency can be understood considering that the higher the anion volume, the smaller the charge density and, therefore, the cation-anion interaction weakens. On the other hand, our results showed that cation-anion interaction strength is related to more negative sites of the anion surface electrostatic potential. The more negative the sites, the stronger the cation- anion interaction.

Absolute energy level positions in tin- and lead-based halide perovskites

Metal halide perovskites are promising materials for future optoelectronic applications. One intriguing property, important for many applications, is the tunability of the band gap via compositional engineering. While experimental reports on changes in absorption or photoluminescence show rather good agreement for different compounds, the physical origins of these changes, namely the variations in valence and conduction band positions, are not well characterized. Here, we determine ionization energy and electron affinity values of all primary tin- and lead-based perovskites using photoelectron spectroscopy data, supported by first-principles calculations and a tight-binding analysis. We demonstrate energy level variations are primarily determined by the relative positions of the atomic energy levels of metal cations and halide anions and secondarily influenced by the cation-anion interaction strength. These results mark a significant step towards understanding the electronic structure of this material class and provides the basis for rational design rules regarding the energetics in perovskite optoelectronics.

Models for understanding the structural effects on the cation-anion interaction strength of dicationic ionic liquids

Electrospray ionization mass spectrometry with variable collision-induced dissociation (CID) was used to conduct a comprehensive study of cation-anion interaction strength in dicationic ionic liquids (ILs). This investigation reports the influence of anion structure (Cl−, Br−, NO3−, BF4−, and SCN−) and spacer chain length (n=4, 6, 8, and 10) on the interaction strength of dicationic ILs derived from 1,n-bis(3-methylimidazolim-1-yl)alkane. The anion-cation interaction strength was determined from the binary mixture of ILs and from the center-of-mass energy (Ecm). Moreover, two novel models were proposed, based on the extended first order and zero order equations, which allow determining the order of stability from the constant dissociation (Kd) and decay rate (k0), respectively. Additionally, quantum chemical calculations of cation-anion energy were done in order to support experimental results. The order of energy of the anion-cation interaction determined by both Ecm,1/2 and Kd followed the same trend, while the order of the cation-anion interaction strength observed in the binary mixtures was in accordance with quantum chemical calculations. The model based on zero order process did not show good agreement with either the Ecm,1/2 or Kd, thus indicating that it is a very simplified model that does not result in an appropriate order for the cation-anion energy interaction. Orders were not comparable; however, it was the first time that binary mixture experiments and Ecm were used with the same set of ILs.

Local environment structure and dynamics of CO2 in the 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide and related ionic liquids.

Despite the innumerous papers regarding the study of the ionic liquids as a potential candidate for CO2 capture, many details concerning the structure and dynamics of CO2 in the system are still to be revealed, i.e., the correlation between the local environment structure and the dynamic properties of the substance. This present work relied on the performance of molecular dynamics both for the neat [C2mim][Tf2N] and [C2mim][Tf2N]/CO2 mixtures in an attempt to elucidate the local environment of CO2 and their effects on the dynamic properties of [C2mim][Tf2N]. A slight change in the orientation of the cation and anion could be observed, which was correlated to the cation and anion moving away from each other in order to receive the carbon dioxide. The gas molecules pushed both the cation and the anion away to create sufficient void to its accommodation. The diffusion coefficient of [C2mim]+ is higher than [Tf2N]- regardless the increase of the CO2 concentration. The addition of CO2 in the ionic liquid has shown an increase of 4-5 times for the diffusivity of ions, which was related to the decrease of cation-anion interaction strength. The transport properties' results showed that the addition of CO2 in the ionic liquid generates the fluidization of the system, decreasing the viscosity as a consequence of the local environment structure changing. Likewise, the effect of the type of anion and cation on the system properties was studied considering [Ac]- and [BMpyr]+ ions, showing large effects by the change of anion to [Ac]- which rise from the strong [C2mim]+-[Ac]- interaction, which conditions the solvation of ions by CO2 molecules.

Anion Effects on the Solid/Ionic Liquid Interface and the Electrodeposition of Zinc

The effect of anions on the solid/ionic liquid (IL) interface and the electrodeposition of zinc have been investigated. The employed ILs are composed of 1-ethyl-3-methylimidazolium ([EMIm]+), bis(trifluoromethylsulfonyl)imide (TFSI–), trifluoromethylsulfonate (TfO–), methylsulfonate (OMs–) and acetate (OAc–). These anions show an increasing cation–anion interaction strength in the order TFSI– < TfO– < OMs– < OAc–, as probed by far-infrared spectroscopy below 200 cm–1. It was shown by in situ AFM that the anion has a profound impact on the interfacial properties. Multilayered structures were observed at the electrode/IL interface for [EMIm]TFSI and [EMIm]TfO, respectively, while only a few layers with rather a low push-through force were found at the interface for [EMIm]OMs and [EMIm]OAc, respectively. The coordination of Zn(II) ions in these ILs by varying zinc salts was investigated by Raman spectroscopy. The differences in metal species and interfacial layers have a strong influence on the electrochemical process and on the quality of the deposits. Dense zinc deposits with nanowire-like and hexagonal plate-like structures were obtained from ILs with TFSI– and TfO– anions, respectively. Thin layers of zinc with porous and spongy structures were obtained in ILs with OMs– anion containing 0.2 mol/L zinc salts, while homogeneous and smooth deposits with a fine-grained structure were obtained with 1 mol/L zinc salts. However, no deposits were found in Zn(OAc)2/[EMIm]OAc under the same conditions. These results indicated that the anions of ILs strongly affected the solid/IL interface, the speciation of Zn(II) ions in ILs, and the morphology of zinc deposits.

Enhancing the Basicity of Ionic Liquids by Tuning the Cation–Anion Interaction Strength and via the Anion-Tethered Strategy

Ionic liquids (ILs) with relatively strong basicity often show impressive performance in chemical processes, so it is important to enhance the basicity of ILs by molecular design. Here, we proposed two effective ways to enhance the basicity of ILs: by weakening the cation-anion interaction strength and by employing the anion-tethered strategy. Notably, two quantum-chemical parameters, the most negative surface electrostatic potential and the lowest surface average local ionization energy, were adopted as powerful tools to demonstrate the electrostatic and covalent aspects of basicity, respectively, at the microscopic level. It was shown that, for the ILs with the same anion (acetate or trifluoroacetate), the basicity of the ILs could be enhanced when the cation-anion interaction strength was weakened. For the acetate anion-based ILs, the hydrogen-bonding basicity scale (β) increased by 29% when the cation changed from 1-butyl-3-methylimidazolium ([Bmim]) to tetrabutylphosphonium ([P4444]), achieving one of the highest reported β values for ILs. Moreover, it was also demonstrated that, when an amine group was tethered to the anion of the IL, its basicity was stronger than when it was tethered to the cation. These results are highly instructive for designing ILs with strong basicity and for improving the efficiency of IL-based processes, such as CO2 capture, SO2 and acetylene absorption, dissolution of cellulose, extraction of bioactive compounds, and so on.

Phase Boundaries, Structural Characteristics, and NMR Spectra of Ionic Liquid-in-Oil Microemulsions Containing Double Chain Surface Active Ionic Liquid: A Comparative Study

A method developed for the first time, to create a huge number of ionic liquid (IL)-in-oil microemulsions has been discussed in our earlier publication (Rao, V. G.; Ghosh, S.; Ghatak, C.; Mandal, S.; Brahmachari, U.; Sarkar, N. J. Phys. Chem. B 2012, 116, 2850-2855). Here, we present facile methods to adjust the structural parameters of microemulsions using different ionic liquids (ILs) as additives (polar phase). We have characterized ILs/[C(4)mim][AOT]/benzene ternary system by performing a phase behavior study, dynamic light scattering (DLS) measurements, and (1)H NMR measurements. The IL loading capacity of microemulsions (area of single phase region) (i) increases with increase in alkyl chain length of cation of ILs and follows the trend [C(6)mim][TF(2)N] > [C(4)mim][TF(2)N] > [C(2)mim][TF(2)N], (ii) increases with decrease in cation anion interaction strength of added ILs and follows the trend [C(4)mim][TF(2)N] > [C(4)mim][PF(6)] > [C(4)mim][BF(4)]. So depending on the IL used, the amount of IL within the core of microemulsions can be easily manipulated to directly affect the size of aggregates in microemulsions. The size increase with increasing R value (R value is defined as the molar ratio of RTILs to [C(4)mim][AOT]) was found to be maximum in the case of [C(2)mim][TF(2)N]/[C(4)mim][AOT]/benzene microemulsions and follows the trend [C(2)mim][TF(2)N] > [C(4)mim][TF(2)N] > [C(6)mim][TF(2)N]. However, the size increase was almost the same with increase in R value in the case of ILs with different anions. The most promising fact about IL-in-oil microemulsions is their high thermal stability compared to that of aqueous microemulsions, so we investigated the effect of temperature on size of aggregates in microemulsions at R = 1.0. It is evident from dynamic light scattering measurements that the aggregates in microemulsions remain monodisperse in nature with increasing temperature, and in all the cases, the size of aggregates in microemulsions decreases with increasing temperature. The effect of water addition on IL-in-oil (IL/O) microemulsions was also studied in detail. By far, this is the first report where the effect of water addition on microemulsions containing hydrophobic ILs is being reported and compared with microemulsions containing hydrophilic ILs. We observed that the added water has a prominent effect on the microstructure of the microemulsions. In all the cases, (1)H NMR spectra provide more detailed information about intra/intermolecular interactions thus affording a clear picture of locations of (i) the RTILs in RTILs/[C(4)mim][AOT]/benzene microemulsions and (ii) the added water molecules in microemulsions.

Does decreasing ion–ion association improve cation mobility in single ion conductors?

We report on poly(ethylene oxide) based single ion conductors for solid polymer electrolytes. The widely agreed upon vehicle for cation movement in PEO-based solid polymer electrolytes is the single cation, in which the cation is solvated by PEO ether oxygens. Here we report a different vehicle that becomes active with strong anion-cation interactions. In the common perspective, increasing ion-ion interactions would increase ion association, decrease cation solvation, and disable cation movement. Decreasing these interactions would have the opposite effect. We vary cation-anion interaction strength, using anion charge delocalization in molecular dynamics simulations. This creates a series of systems with levels of ion aggregation from single cations (weak interaction) to mostly ion aggregates (strong interaction). Although in the weak model single cations are faster than those in ion pairs and aggregates, with stronger interactions a different mechanism emerges. Paired cations move the fastest by visiting different anion partners in succession. The importance of this observation lies in the possibility of decoupling cation movement from polymer motion, which is required to prevent dendrite formation in both Li and Na ion batteries.

NMR Study of Cation Dynamics in Three Crystalline States of 1-Butyl-3-methylimidazolium Hexafluorophosphate Exhibiting Crystal Polymorphism

We investigate the cation rotational dynamics of a room temperature ionic liquid (RTIL) 1-butyl-3-methylimidazolium hexafluorophosphate ([C(4)mim]PF(6)) in its three crystalline states by (1)H NMR spectroscopy. Spin-lattice and spin-spin relaxation time (T(1) and T(2), respectively) measurements as a function of temperature confirm the presence of three polymorphic crystals of [C(4)mim]PF(6): crystals α, β, and γ, which we previously discovered using Raman spectroscopy and calorimetry. Second moment calculations of (1)H NMR spectra reveal that certain segmental motions of the butyl group in addition to the rapid rotation of the two methyl groups in the cation occur in all the crystals. The trend in the mobility of the segmental motions is γ < β ≤ α, which is consistent with the strength of cation-anion interactions (or crystal packing density) estimated from high-frequency Raman scattering experiments. T(1) measurements demonstrate two types of rotational motions on the nanosecond time scale in all three crystals: fast and slow motions. The three crystals have similar activation energies of 12.5-15.1 kJ mol(-1) for the fast motion, which is assigned to the rotation of the methyl group at the terminal of the butyl group. These observed activation energies were consistent with that estimated by quantum chemical calculations in the gas phase (11.9 kJ mol(-1)). In contrast, the slow motions of crystals α and γ are attributed to different segmental motions of the butyl group and that of crystal β to either a little segmental motion or a certain PF(6)(-) rotational motion. These nanosecond rotational motions obtained from the T(1) measurements do not appear to be affected by crystal packing density because local interactions in the crystalline state rather than packing density govern such nanosecond motions. With respect to the segmental motions, the mobility is likely to change significantly with the conformation of the butyl group. On the basis of these findings, crystal γ, which is the only crystalline phase previously determined using single-crystal X-ray diffraction, is considered to be the most stable phase because of the slowest segmental motions and the strongest cation-anion interactions.

Novel pseudo-delocalized anions for lithium battery electrolytes

A novel anion concept of pseudo-delocalized anions, anions with distinct positive and negative charge regions, has been studied by a computer aided synthesis using DFT calculations. With the aim to find safer and better performing lithium salts for lithium battery electrolytes two factors have been evaluated: the cation-anion interaction strength via the dissociation reaction LiAn ⇌ Li(+) + An(-) and the anion oxidative stability via a vertical ionisation from anion to radical. Based on our computational results some of these anions have shown promise to perform well as lithium salts for modern lithium batteries and should be interesting synthetic targets for future research.