Asymmetric Ionic Liquids Research Articles

Mechanism of overscreening breakdown by molecular-scale electrode surface morphology in asymmetric ionic liquids

The interfacial nature of the electric double layer (EDL) assumes that electrode surface morphology significantly impacts the EDL properties. Since molecular-scale roughness modifies the structure of EDL, it is expected to disturb the overscreening effect and alter differential capacitance (DC). In this paper, we present a model that describes EDL near atomically rough electrodes with account for short-range electrostatic correlations. We provide numerical and analytical solutions for the analysis of conditions for the overscreening breakdown and DC shift estimation. Our findings reveal that electrode surface structure leads to DC decrease and can both break or enhance overscreening depending on the relation of surface roughness to electrostatic correlation length and ion size asymmetry.

Nanopore efficiently identifies hepatitis D virus antigens in vitro assay

Chronic hepatis B with HDV (Hepatitis D Virus) coinfection is considered as one of the most serious forms of human viral hepatitis that can lead to cirrhosis and hepatocellular carcinoma. However, the high GC ratios in HDV genome and the same surface antigen protein HBsAg (Hepatitis B surface Antigen) in virus particles assembly make it hard (or ignored) to diagnose HDV co-infection with HBV (Hepatitis B Virus) using traditional methods (i.e., antibody-surface antigen reaction), further limit to build epidemiologic information on HDV. Thus, developing efficient detection for key pathogenetic molecules of HDV becomes key experimental questions for HDV diagnosis. In these key molecules, specific HDAgs (Hepatitis D Antigens, including large and small HDAgs) are the solitary ribonucleoproteins encoded by HDV genome, which function in viral life cycle in significant ways. Herein, we propose a direct and precise detection for large and small HDAgs using nanopore sensor in asymmetric ionic liquids electrolyte condition. What's as obvious from nanopore measurement for either antigen is that the results allow discriminate large and small HDAgs as well as antigen-ribozyme complex in serological assay. More importantly, it is ascertained by nanopore test that HDAgs can be efficiently distinguished with HBsAg in HBV-infected cells, enabling simultaneous identification of HBV- and HDV-infected cells. In conclusion, the proposed nanopore can provide specific and efficient detection of physiological indicators for different hepatitis virus infections at molecular and cellular levels, and should benefit severer HBV/HDV co-infection diagnosis and therapy.

Revealing the causes of degradation of an asymmetric ionic liquid supercapacitor with a composite polyaniline/carbon electrode

The extended window of electrochemical stability of ionic liquids and the high specific capacitance values of activated carbon (AC)/conductive polymer (polyaniline, Pani) composite materials are promising approaches to improve the performance of supercapacitors (SC).In this work, we studied the stability of an asymmetric AC//Pani-C cell with 1 M solution of 1-butyl-3-methylimidazolium tetrafluoroborate (BMIMBF4) in acetonitrile as an electrolyte. Pani-C (56 wt% of Pani) composite was prepared by oxidative polymerization of aniline in the presence of high surface area AC (SBET = 2300 m2‧g−1). Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy, transmission and scanning electron microscopies data confirmed the formation of Pani-C composite. Combination of cyclic voltammetry, galvanostatic charge/discharge method, and electrochemical impedance spectroscopy in 2- and 3-electrode cell configurations gave information about anodic (1 V vs. Ti) and cathodic (−1.8 V vs. Ti) potential limits of electrochemical stability, and potential of zero charge of Pani-C and AC.From these data, optimal mass ratio of Pani-C and AC electrodes was obtained for asymmetric SC assembly. It was found that after 500 charge/discharge cycles, the capacitance of the asymmetric AC//Pani-C SC decreased by 15% from 46.8 to 39.9 F·g−1 as a result of Pani-C degradation. The specific capacitance of AC remained virtually unchanged (decreased from 144 to 141 F·g−1), while that of Pani-C decreased by 8.7% from 230 to 210 F·g−1. Electrochemical measurements showed that degradation is accompanied by an increase in the working potential window of Pani-C (from 0.85 V to 0.97 V) and a shift in Emin…Emax working potentials from 0.15 … 1 to 0.52 … 1.49 V vs. Ti in the beginning and in the end of stability test, respectively. The last region is related to the potential for non-conductive pernigraniline formation (1.3 V vs. Ti). It was shown that the main reason of AC//Pani-C SC capacitance decay is a gradual increase in the operating potential window with a constantly shifts towards anodic values of positive Pani-C electrode, which resulted in accumulation of nonconductive polymer form that does not participate in charge accumulation.

Molecular scale roughness effects on electric double layer structure in asymmetric ionic liquids

Molecular scale roughness effects on electric double layer structure in asymmetric ionic liquids

Exploring the electrosorption and surface charge amplification at the ionic liquid/cavity interface: Influence of imidazolium alkyl chain length and the size of the spherical cavities of the porous electrode

The surface charge amplification (SCA) phenomenon at the ionic liquid/cavity interface has been investigated as a result of co-ion electrosorption due to increasing the alkyl chain length of imidazolium. Three types of ionic liquids with different imidazolium alkyl chain lengths inside the different-sized spherical cavities of porous electrodes have been studied using the coarse-grained model within the framework of classical density functional theory (CDFT). Our results show that selectivity occurs at lower potentials for ionic liquids with longer cation chain lengths in smaller cavities. The significant point is that at low negative surface potentials for asymmetric ionic liquids, the electrosorption of anions as co-ions occurs in some spatial regions that are made possible by the accidental orientation of cations in the first layer in the vicinity of the cavity surface. This is evidence of the SCA phenomenon for asymmetric ionic liquids at the ionic liquid/cavity interface. Moreover, the comparison of results, especially the normalized screening strength in the first layer indicates that the smaller cavities containing the ionic liquid with cations that have longer alkyl chains amplify the electrode surface charge more.

General framework for the study of dynamical properties and arrested states of ionic liquids

In this work, we present a first-principles theoretical framework for the description of structural and dynamical behaviors of ionic liquids. This framework applies the self-consistent generalized Langevin equation (SCGLE) theory, which predicts the dynamically arrested states of several physical systems, including charge and size asymmetric ionic liquids. In its current form, the SCGLE is a particularly useful framework for predictions of arrested and partially arrested states in both classical and room-temperature molten salts, electrolytic solutions, and superionic conductors. Some illustrative examples of the predictive power of our framework are presented in this work, showing its potential application in design and development of novel conducting materials.

Synthesis of chiral iron-based ionic liquids: modelling stable hybrid materials

A simple method to prepare asymmetric ionic liquids combining the optical, magnetic and Lewis acidic properties of [FeX4]− anions with the chirality of imidazolium cations.

Differential capacitance of ionic liquid interface with graphene: The effects of correlation and finite size of ions

We use a mean field model for ionic liquids, which takes into account both the ion correlation and the finite ion size effects, in order to calculate the differential capacitance of the ionic liquid interface with single-layer graphene. Besides choosing ion packing fractions that give rise to the camel-shaped and bell-shaped capacitances of the diffuse layer in ionic liquids, we also explore small packing fractions in the regime of “dilute electrolytes”, as well as different packing fractions for different ion species in asymmetric ionic liquids. We find that the main effect of a graphene electrode arises due to a V-shaped minimum in its quantum capacitance at the point of zero charge (PZC), which is a manifestation of the Dirac cone structure of graphene's π electron bands. As a result, the total capacitance of a graphene–ionic liquid interface exhibits a camel-shaped dependence on the total applied potential, even for large ion packing fractions and finite ion correlation lengths. While the minimum at the PZC in the total capacitance is “inherited” from graphene's quantum capacitance, the two peaks that occur for applied potentials ∼±1 V are sensitive to the presence of the ion correlation and a Stern layer, which both tend to reduce the height and flatten the peaks in the camel-shaped total capacitance. Finally, we have found that the largest fraction of the applied potential goes to charging the graphene electrode.

Influence of Electronic Polarization on the Structure of Ionic Liquids

The liquid structure and electrical screening ability of ionic liquids are fundamentally intertwined. The molecular nature of the charge carriers means that screening distances of external fields depend sensitively on the ion packing and structure of the ionic liquid. In this work, we quantitatively illustrate how the liquid structure itself is directly modulated by electrostatic screening conditions. In particular, electronic polarization fundamentally relaxes long-range ion structuring in asymmetric ionic liquids such as [BMIM+][BF4-], with the influence propagating to short-range ion-ion correlation. A consequence of the exact Stillinger-Lovett second moment condition is that, at fixed density, any pairwise-additive, nonpolarizable force field will necessarily predict artificially enhanced long-range ion structuring. This is because the screening condition is set by the infinite-frequency dielectric response. There is no ad-hoc fix: One has to use polarizable force fields to correctly reproduce the optical dielectric constant. Our illustration of this fundamental effect significantly clarifies interpretation of previous work comparing property prediction using polarizable and nonpolarizable force fields.

A mean-field theory on the differential capacitance of asymmetric ionic liquid electrolytes. II. Accounts of ionic interactions.

A size-asymmetric mean-field theory with ionic interactions is developed for the electric double layer of room temperature ionic liquid (IL) electrolytes, based on the previous work on non-interacting ions (Y. Han et al., J. Phys.: Condens. Matter, 2014, 26, 284103). By solving the modified Poisson-Boltzmann equation with some simplified assumptions following a recent work (Z. A. H. Goodwin et al., Electrochim. Acta, 2017, 225, 190-197), an analytical expression of differential capacitance can be derived, with a scaled electrode potential due to the ionic interactions. Compared with non-interacting ions, the main effect of ionic interactions is the insufficient screening of the electrode potential, and this depresses the differential capacitance compared with the non-interactive case.

Electric double layer capacitance for ionic liquids in nanoporous electrodes: Effects of pore size and ion composition

The energy density of an electric double layer (EDL) capacitor, a type of supercapacitor, depends on the ion distribution within the micropores of electrodes that are typically made of amorphous carbon. By using coarse-grained models and the classical density functional theory, we investigate the distributions of ionic species among different idealized nanopores in contact with an asymmetric ionic liquid mixture and the effects of the bulk electrolyte composition on capacitive energy storage. We find that a charged pore is always small-ion selective, provided all ions have the same valence and similar non-electrostatic interactions. While small ions enhance both the EDL capacitance and the accessibility of micropores, an ionic mixture containing ions of different sizes may yield a capacitance higher than those corresponding to pure ionic liquids. The increased capacitance may be attributed to more efficient ion packing near the charged surface. At certain conditions, the improvement is on a par with the anomalous capacitance rise for pure ionic liquids in electrodes with ultranarrow pores.

A mean-field theory on the differential capacitance of asymmetric ionic liquid electrolytes

The size of ions significantly influences the electric double layer structure of room temperature ionic liquid (IL) electrolytes and their differential capacitance (Cd). In this study, we extended the mean-field theory (MFT) developed independently by Kornyshev (2007J. Phys. Chem. B 111 5545–57) and Kilic, Bazant, and Ajdari (2007 Phys. Rev. E 75 021502) (the KKBA MFT) to take into account the asymmetric 1:1 IL electrolytes by introducing an additional parameter ξ for the anion/cation volume ratio, besides the ionic compressibility γ in the KKBA MFT. The MFT of asymmetric ions becomes KKBA MFT upon ξ = 1, and further reduces to Gouy–Chapman theory in the γ → 0 limit. The result of the extended MFT demonstrates that the asymmetric ILs give rise to an asymmetric Cd, with the higher peak in Cd occurring at positive polarization for the smaller anionic size. At high potential, Cd decays asymptotically toward KKBA MFT characterized by γ for the negative polarization, and characterized by ξγ for the positive polarization, with inverse-square-root behavior. At low potential, around the potential of zero charge, the asymmetric ions cause a higher Cd, which exceeds that of Gouy–Chapman theory.

Impact of the cation symmetry on the mutual solubilities between water and imidazolium-based ionic liquids

Aiming at the evaluation of the impact of the ionic liquids (ILs) cation symmetry on their phase behaviour, in this work, novel mutual solubilities with water of the symmetric series of [CnCnim][NTf2] (with n=1–5) were determined and compared with their isomeric forms of the asymmetric [CnC1im][NTf2] group. While the solubility of isomeric ILs in water was found to be similar, the solubility of water in ILs follows the same trend up to a maximum cation alkyl side chain length. For n≥4 in [CnCnim][NTf2] the solubility of water in the asymmetric ILs is slightly higher than that observed in the symmetric counterparts. The thermodynamic properties of solution and solvation derived from the experimental solubility data of ILs in water at infinite dilution, namely the Gibbs energy, enthalpy and entropy were used to evaluate the cation symmetry effect on the ILs solvation. It is shown that the solubility of ILs in water is entropically driven and highly influenced by the cation size. Accordingly, it was found that the ILs solubility in water of both symmetric and asymmetric series depends on their molecular volume. Based on these findings, a linear correlation between the logarithm of the solubility of ILs in water and their molar volume is here proposed for the [NTf2]-based ILs at a fixed temperature.

Interfacial properties of charge asymmetric ionic liquids

We report molecular dynamics simulations of the coexistence and interfacial properties of ionic liquids as a function of cation/anion, (z +:z −) = (2:−1), (4:−1), charge asymmetry. Our results correct previous computations of the coexistence curve of (2:−1) charge asymmetric systems, obtained via the fine-lattice discretisation method. In agreement with previous computations we report a reduction in the critical temperature and an increase in the critical density with charge asymmetry. We have quantified the interphase potential resulting from the charge asymmetry, by analysing the charge density across the liquid–vapour interface. We show that the Debye–Hückel theory predicts reasonably well the magnitude and temperature dependence of the interphase potential for moderate charge asymmetries (2:−1) but it underestimates, by about 50, the electrostatic potential of ionic liquids with high charge asymmetries (4:−1).

Synthesis of dicationic symmetrical and asymmetrical ionic liquids and their tribological properties as ultrathin films

A new series of novel dicationic symmetrical and asymmetrical ionic liquids (ILs) consisting of tributylalkyl phosphonium and alkylimidazolium were synthesized. Their tribological properties in the form of spin-coated ultra-thin films sliding against AISI-52100 steel ball were studied in a ball-on-plate configuration on a Universal Micro-tribometer-2MT tester, using perfluoropolyether (PFPE) and normal ILs as comparisons. The best result is obtained for asymmetrical 1-(1-tributylphosphine-yl-hexyl)-3methylimidazolium dihexafluorophosphate, which has very high decomposition temperature-about 450 degrees C and good tribological properties.

Structure and thermodynamic properties of molten strontium chloride

Self-consistent calculations of pair distribution functions and thermodynamic properties are presented for a pair potentials model of molten strontium chloride. The calculations extend to a strongly asymmetric ionic liquid, an earlier assessment of bridge diagrams in a modified hypernetted chain approach to the liquid structure of alkali halides. Good agreement is found with computer simulation data obtained by de Leeuw (1978) with the same set of pair potentials, showing that the present approach incorporates genuine general features of liquid structure theory for multicomponent liquids with strong relative ordering of the component species. It is further shown that the strong correlations between the divalent cations, both in the model and in real molten strontium chloride, can be approximately reproduced on the basis of a simple one-component-plasma model, provided that dielectric screening is allowed for in the real liquid. This allows the authors tentatively to attribute the significant level of disagreement between a pair potentials model of this liquid and the neutron diffraction data of McGreevy and Mitchell (1982) to many-body distortions of the electronic shells of the ions.