Self-diffusion Coefficients Of Cations Research Articles

Transport properties of imidazolium-based room temperature ionic liquids in confinement of slit charged carbon nanopores: New insights from molecular simulations

The paper presents a study of the behavior of room-temperature ionic liquids with different alkyl chain lengths and anion types in slit negatively charged carbon nanopores of various widths (1–15 nm) using all-atom molecular dynamics simulations. The structure of the confined ionic liquids is investigated using number, mass, and charge density profiles and radial distribution functions. The paper also discusses such characteristics as mean squared displacement of ions in all directions, self-diffusion coefficients of cations and anions in the bulk, their temperature dependences, electrical conductivity. Some energy characteristics are obtained by quantum chemical calculations. It is established that there is a bulk region in the pore center for all the confined ionic liquids, starting from the pore width of 7 nm. The sums of the cationic and anionic self-diffusion coefficients of different ionic liquids can be arranged as follows: D[OMIM][BF4] < D[EMIM][BF4] < D[OMIM][NTf2] < D[EMIM][NTf2]. Due to layering structure inside the pore, the dynamics of the ions is heterogeneous. It is shown that the RTILs with longer alkyl chains have lower electrical conductivities regardless of temperature. The opposite effect associated with the species size is observed for RTILs with different anions.

Water Molecules' and Lithium Cations' Mobility in Sulfonated Polystyrene Studied by Nuclear Magnetic Resonance.

The hydration of ions and charge groups controls electro mass transfer through ion exchange systems. The self-diffusion and local mobility of water molecules as well as lithium cations in poly (4-styrenesulfonic acid) and its lithium, sodium and cesium salts were investigated for the first time using pulsed-field gradient NMR (PFG NMR) and NMR relaxation techniques. The temperature dependences of the water molecule and Li+ cation self-diffusion coefficients exhibited increasing self-diffusion activation energy in temperature regions below 0 °C, which is not due to the freezing of parts of the water. The self-diffusion coefficients of water molecules and lithium cations, as measured using PFG NMR, are in good agreement with the self-diffusion coefficients calculated based on Einstein's equation using correlation times obtained from spin-lattice relaxation data. It was shown that macroscopic water molecules' and lithium cations' transfer is controlled by local particles jumping between neighboring sulfonated groups. These results are similar to the behavior of water and cations in sulfonic cation exchanger membranes and resins. It was concluded that polystyrenesulfonic acid is appropriate model of the ionogenic part of membranes based on this polymer.

Hydration, self-diffusion and ionic conductivity of Li+, Na+ and Cs+ cations in Nafion membrane studied by NMR

Hydration of Nafion 117 perfluorinated sulfonic cation-exchange membrane in alkaline ion forms was investigated by high resolution 1H NMR. Hydration numbers of Li+, Na+ and Cs+ cations were 5 ± 1, 6 ± 1 and 1 ± 0.2, correspondingly for membrane equilibrated with water vapor at 98% RH. As opposed to Li+ and Na+, which form separate ion pair, Cs+ cation directly contacts with membrane sulfonate group. Cation self-diffusion coefficients were measured by pulsed field gradient NMR technique on 7Li, 23Na and 133Cs nuclei for the first time. Self-diffusion coefficients are changed in the next rows Li+ ≤ Na+ > Cs+. Self-diffusion activation energies of Li+ and Na+ cations are about 20 kJ/mol which is close to water self-diffusion activation energy in these membranes, but Cs+ self-diffusion activation energy is distinctly more (25 kJ/mol). Ionic conductivities calculated on the basis of Nernst–Einstein equation from cation self-diffusion coefficients 1.6∙10−2, 2∙10−2, 6∙10−3 S/cm for Li+, Na+, Cs+ cations, correspondingly, are closely approximating to conductivities measured by impedance spectroscopy: 1.3∙10−2, 1.1∙10−2, 2.3∙10−3 S/cm for Li+, Na+, Cs+ cations, correspondingly, but calculated values are appreciably more compared with experimental meanings.

Connecting molecular simulations and laboratory experiments for the study of time-resolved cation-exchange process in the interlayer of swelling clay minerals

In the context of element migration in clay-rich media, self-diffusion coefficients of interlayer cations in swelling clay minerals obtained from molecular simulations are rarely used by macroscopic models predicting cation-exchange processes. Based on experiments and simulations, this study aims at (i) making a connection between molecular and sample scale processes to predict the dynamics of cation-exchange reactions between the interlayer space of millimetre disks of vermiculite and aqueous reservoirs, and (ii) assessing the role played by both self-diffusion and selectivity coefficients on this process.Time-resolved cation exchange experiments were performed using Ca-saturated vermiculite disks immersed in aqueous reservoirs with different NaCl or SrCl2 salinities. The results were reproduced via a finite-volume model constrained by (i) cation self-diffusion coefficients calculated by molecular dynamics simulations and (ii) interlayer selectivity coefficients drawn from “batch” cation-exchange isotherms.Results showed that considering the averaged values for both the cation-exchange selectivity coefficients and self-diffusion coefficients of the slowest interlayer cation led to good agreement between the experiments and simulations, validating the modelling strategy for the connection between the molecular and laboratory time scales. A sensitivity test regarding the influence of the two input parameters on the overall results was then performed. This study underlined a constrained upscaling strategy to better assess the role played by different intrinsic parameters of the clay/water systems (molecular self-diffusion coefficients in the interlayer space vs. selectivity coefficient) on the diffusion of cations during cation-exchange reaction in clay-rich media.

Mobility of Cations and Water Molecules in Sulfocation-Exchange Membranes Based on Polyethylene and Sulfonated Grafted Polystyrene

The main patterns of the hydration of sulfo groups, the translational mobility of water molecules, alkali metal cations, and ionic conductivity in sulfocation-exchange membranes (MSC) based on polyethylene and sulfonated grafted polystyrene have been investigated using NMR and impedance spectroscopy techniques. It has been shown that at moisture contents λ < 4 (λ is the number of water molecules per sulfo group) the H+ counterions in the membranes form diaquahydrogen ions $${{{\text{H}}}_{{\text{5}}}}{\text{O}}_{{\text{2}}}^{ + }{\text{.}}$$ In the temperature range below 0°C at λ < 12, water molecules retain high mobility and do not form the ice phase. Water molecules diffusion coefficients (for the H+ form, the average diffusion coefficient of water molecules and acidic protons) and first for ion-exchange systems, and the diffusion coefficients of counterions Li+, Na+, and Cs+ have been measured by pulsed field gradient 1H, 7Li, 23Na, and 133Cs NMR spectroscopy. In MSC membranes in contact with water, the self-diffusion coefficients of cations increase in the Li+ < Na+ < Cs+ series. The cation conductivity values are in the same Li+ < Na+ < Cs+$$ \ll $$ H+ sequence. The cation conductivity values calculated from the self-diffusion coefficients based on the Nernst–Einstein equation are essentially higher than the experimental values.

Negative Pressure Regimes in Ionic Liquids: Structure and Interactions in Stretched Liquids as Probed by NMR

In this work, a broad pallete of macroscopically sized samples of ionic liquids was used, in which tensions of about -100 MPa were obtained in preliminary testing methodologies before cavitation would occur. Preliminary NMR studies have been performed on ionic liquids under homogeneous tension. Upon entering into these metastable regimes of negative pressure, it is possible to observe a sharp increase in the anion and cation self-diffusion coefficients as compared to the equilibrium saturated liquid conditions at identical temperatures.

Advanced Nuclear Magnetic Resonance Techniques for Characterizing Ionic Liquids for Lithium Ion Battery Applications; High Presssure NMR and Fast Field Cycling Relaxometry

Ionic liquids (ILs) have received widespread acclaim for their potential use as a safe and effective alternative to unstable and flammable organic solvents used in electrolytes for lithium-ion secondary batteries. The thermal and chemical stability of ILs, as well as the tunable properties of ILs by altering cation and anion species, have made them promising materials for use in multiple applications: such as textile dyeing, nuclear waste recycling, and solar cells. A limiting characteristic for IL use in secondary lithium-ion batteries is a notable lower ionic conductivity in most known species. Characterizing the effect of cation and anion structure on ionic diffusion in ILs, as well as studying molecular dynamics in IL species is of great importance for increasing their potential utility in battery chemistries. To this end, the techniques of Fast Field Cycling Nuclear Magnetic Resonance (FFCNMR) and pressure dependent NMR diffusometry are used to study ionic liquid species of interest. Fast field cycling measurements of spin-lattice relaxation times (T1) allow for access to time dependent molecular dynamics in ionic systems, as functions of both magnetic field strength and temperature. FFC measurements are complementary to high-pressure NMR experiments, which reveal pressure dependence of reorientation and tumbling processes, as well as the effects of alkyl chain length on ionic mobility in specific species. Presented here is the application of these two techniques to 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide (CnMIM TFSA) ILs. Specifically, a selectively deuterated species of 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide (BMIM TFSA) is observed and characterized. FFC results were used to extract cation self-diffusion coefficients, showing good agreement with Pulse Field Gradient (PFG) NMR measurements. High pressure T1 data on selectively deuterated cation isotopologues revealed site dependent interactions on the cation alkyl chain. The work at BNL (JFW and JH) was supported by the US-DOE Office of Science, Division of Chemical Sciences, Geosciences and Biosciences under contract DE-SC0012704.

CO2 Absorption in the Ionic Liquids Immobilized on Solid Surface by Molecular Dynamics Simulation.

Based on our previous experimental research, we studied the absorption of CO2 in the ionic liquid, 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([hmim][Tf2N]), immobilized on TiO2 [rutile (110) ] with different thickness by molecular dynamics simulation. The effects of the properties (hydrophobicity and hydrophilicity) of solid interfaces were also studied with IL immobilized on graphite and TiO2, respectively. We studied the influence of the thickness of IL immobilized on TiO2 on the absorption of CO2 via structural and dynamical properties. The results show that the self-diffusion coefficients of IL and CO2 increase as the thickness of immobilized IL decreases. And the CO2 absorption capacity increases as the thickness of immobilized IL decreases as well. Additionally, more CO2 molecules are absorbed in the region near the solid interface as the thickness of IL decreases. For IL immobilized on graphite, the self-diffusion coefficients of cations and anions are larger than that of IL immobilized on TiO2 with the same thickness. They are also larger than nonimmobilized cations and anions.Besides, the CO2 absorption capacity of IL immobilized on TiO2 is the largest compared with IL immobilized on graphite and nonimmobilized IL with the same thickness. From our simulation work, we try to explore the microscopic mechanism that is unexplored by experimental work, and we found the important role of IL/solid interface for CO2 absorption in immobilized ILs.

Density Functional Theory Modeling of A-site Cation Diffusion in Bulk LaMnO3 ±δ for Solid Oxide Fuel Cell Cathodes

Solid oxide fuel cells (SOFC)s performances and long term degradation are directly and indirectly influenced by cation diffusion in electrodes, electrolytes, and across their interfaces. For SOFC cathodes, due to the sluggish rates of cation diffusion and the complex coupling between the defect chemistry and cation diffusion pathways, the dependences of the cation diffusivities on temperature and O2 partial pressure in the state-of-the-art La1-xSrxMnO3±δ (LSM) remain insufficiently understood based on the available theoretical models. In this work, the interactions of the defect complexes for cation transport and the associated cation diffusion mechanisms in bulk LSM are investigated based on density functional theory simulations. While the active cation diffusion pathways in bulk LSM have been suggested to involve the transport of the cation defect clusters[3,4], this work assessed stability of a number of possible cation vacancy clusters as a function of temperature and P(O2) as well as the associated cation migration barriers, and unveiled new cation diffusion mechanisms with lower migration barriers than those proposed in the previous studies[1-2]. The temperature and P(O2) dependences of the cation self-diffusion coefficients in bulk LSM are further assessed, by incorporating the defect energetics and the transport carrier concentration obtained from the ab initio bulk defect model[5], and the migration barriers of the investigated cation diffusion pathways[6]. The predicted temperature and P(O2) dependences of the cation diffusivities obtained using this model were compared with the experimental tracer diffusion coefficients reported in the literature [2-3], and implications of the new cation diffusion mechanisms will be discussed. The trends in the migration barriers vs. various types of metal cations relevant to SOFC applications, including La3+, Sr2+, Zr4+, Y3+, etc,are also investigated, and the results obtained suggest that both the ionic charge and the ionic radius can correlate with the calculated cation migration barriers [6].

Calculation of self-diffusion coefficients of the [BMIM][TFSA]/water system by molecular dynamics simulation

We performed a molecular dynamics simulation to calculate the self-diffusion coefficients of 1-Butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide and water in a water–ionic liquid mixture. We then compared the simulated self-diffusion coefficients of cation, anion and water molecules with experimental data and with simulated data from the literature. Although the simulation overestimated the self-diffusion coefficients of ions, the simulated results qualitatively reproduced the enhancement of the self-diffusion coefficients of water as the water molar fraction increased. We also calculated the radial distribution functions to investigate the solution structure, i.e. the clustering of water molecules. The clustering of water in ionic liquid was found to play an important role in the enhancement of the diffusion of water molecules in the ionic liquid.

Investigation of solvation of iron nanoclusters in ionic liquid 1-butyl-1,1,1-trimethylammonium methane sulfonate using molecular dynamics simulations: Effect of cluster size at different temperatures

The systems composed of metal nanoclusters in ionic liquids are relevant for applications in lubrication, electrochemical devices, catalysis, and chemical processes. The mechanism of solvation and interactions of these systems are not understood at present. In this work, we have simulated iron nanoclusters with different sizes in ionic liquid 1-butyl-1,1,1-trimethylammonium methane sulfonate [N1114][C1SO3] at two temperatures (300 and 500K) and at atmospheric pressure. We have investigated the effects of cluster size and the temperature on some of the thermodynamics, structural and dynamical properties of the systems. Our results also show that the absolute value of solvation energy increases as the nanocluster size increases. Also the absolute solvation energy increases as the temperature increases. It is also shown that the effect of the cluster size is much more than the effect of the temperature. Our structural investigations indicate at least two shells (a double layer) around the nanocluster and the anions are closer to the cluster surface than the cations. The self-diffusion coefficients of cations, anions, and iron clusters have been also presented and discussed in this work.

Transport Properties of the 1-Hexyl-3-methylimidazolium Bis(trifluoromethylsulfonyl)amide-Trichloromethane Binary System: Indication of Trichloromethane Segregation.

Self-diffusion coefficients and electrical conductivity were studied for the binary system 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide-trichloromethane ([C6mim][NTf2]-CHCl3) as a function of composition and temperature. Self-diffusion coefficients of cation and anion are identical for ionic liquid mole fractions xIL < 0.95. The self-diffusion coefficient of CHCl3 is consistently larger than that of the ions by a factor of 4. A double logarithmic plot for the ratio of self-diffusion coefficient and temperature versus viscosity is linear for ionic liquid mole fractions 0.1 < xIL < 0.9 indicating (a) a fractional Stokes-Einstein applies where self-diffusion is inverse proportional to some power b of viscosity (D ∼ η(-b)) and (b) single average length scales are associated with the mass transport of [C6mim][NTf2] and CHCl3. However, the obtained length scale for CHCl3 is unreasonably small, which is indicative of CHCl3 segregation. The molar conductivity displays a maximum near xIL = 0.2. Evaluation of the ionicity from molar conductivity and self-diffusion coefficients indicates a gradual speciation change from charged species to neutral species for xIL < 0.5. The temperature dependencies of self-diffusion and molar conductivity follow Arrhenius behavior. The obtained xIL-dependent activation energies are found to be linear for molar conductivity and largest for the cation and anion self-diffusion coefficients. The activation energies for the self-diffusion of CHCl3 appear to be identical with those obtained from fluidity data.

Ionic Conduction Mechanism Analysis for Neodymium Complex in Phosphonium Ionic Liquids and Potassium Bis(trifluoromethylsulfonyl)amide Melts

It is important to investigate the ionic conduction mechanism for neodymium complex; Nd(III) in ionic liquids and low-temperature molten salts. The temperature dependence of ionic conductivities in [P2225][TFSA] dissolving 0.05-0.50 M Nd(III) was evaluated from Vogel-Fulcher-Tammann (VFT) equation in this study. The concentration dependence of ionic conductivities for Nd(III) in [P2225][TFSA] was evaluated from Kohlrausch law. Considering from Walden plot, this result indicated that the ionic conduction was attributed to the transfer of [P2225]+ and Nd3+ cations, and [TFSA]- anions even in the presence of Nd(III). Moreover, The VFT parameters of the ionic conductivities in [Nd1/3,K][TFSA] melts with molar fraction of Nd; x Nd=0.010-0.075 were also estimated in this study. Furthermore, the self-diffusion coefficients of cation and anion in [P2225][TFSA] were calculated from molecular dynamics simulation. This tendency of diffusivity for each ion was consistent with the ionic conductivity.

Self-Diffusion Coefficients and Related Transport Properties for a Number of Fragile Ionic Liquids

Ion self-diffusion coefficients (DSi) for [EMIM][CF3SO3] and [chol][Tf2N] and cation self-diffusion coefficients for [CH3SO3]−, [TCB]−, and [FAP]− salts of [EMIM]+ are reported together with ancillary data allowing calculation of velocity cross-correlation (VCC or fij), distinct diffusion (Ddij) and Laity resistance coefficients (rij). Comparison of ion Stokes–Einstein–Sutherland (SES) exponents is a useful test of experimental consistency. New analysis shows the viscosity (η) and molar conductivity (Λ) of [EMIM][CH3SO3] and [EMIM][TCB] do not show the dynamic crossovers previously reported. Nor do DS and η for the comparison substance propylene carbonate. For all the ionic liquids, DS+ > DS-. Data show fractional SES (DSi-η, Ddij-η) and Walden (Λ–η) plots. The Nernst–Einstein parameter, Δ, for [EMIM][FAP] is approximately zero, implying the relative velocity of any ion pair selected randomly is uncorrelated with those of other distinct pairs. For the other ionic liquids, the relative pair velocities are ...

Effect of additives on self-diffusion and creep of UO2

The creep of UO2 doped with Nb2O5 and Cr2O3 has been assessed using a point defect model based on the law of mass action, and the diffusional creep according to the Nabarro-Herring mechanism, which relates the creep rate to the lattice self-diffusivity, the inverse of grain area and the applied stress. The self-diffusion coefficients of cation (U) and anion (O) are directly proportional to the concentrations of ions, which in turn are functions of dopant concentrations. The model has been used to evaluate past creep experiments on UO2 doped with Nb2O5 and Cr2O3 in concentrations up to about 1mol%, with a varying grain size at different temperatures and applied stresses. The creep rate increases significantly with the dopant concentration and the putative model, after a modification of the creep rate coefficient, retrodict the measured data satisfactorily. A number of factors affecting creep rate and thereby our model computations are discussed.

Analysis of the Ionic Conduction Behavior in a Few of Room Temperature Molten Fluorides

The average self-diffusion coefficients of cations and anions increased with increasing the HF-concentration and temperature in (CH3)3N·mHF, (CH3)4NF·mHF, (C2H5)3N·mHF, and (C2H5)4NF·mHF melts. The behavior of the self-diffusion coefficients of cations and anions in every melt is in good agreement with an increase in ionic conductivity and a decrease in viscosity. The transference number of (FH)nF− anions estimated from the values of self-diffusion coefficients for each ion in every melt was almost constant and their values were around 0.6 in the (CH3)3N·mHF melt and around 0.7 in the (CH3)4NF·mHF, the (C2H5)3N·mHF, and the (C2H5)4NF·mHF melts. The value of theoretical conductivity (σNMR) obtained from the average self-diffusion coefficients of cations and anions by using Nernst-Einstein equation was higher than the value of experimental ionic conductivity (σ). Therefore, it is concluded that the diffusion of (FH)nF− anions is faster than that of each cation in every melt and that a part of both cation and anion in each melt may associate by interaction between both ions.

On the temperature dependence of several physicochemical properties for aqueous solutions of the ionic liquid 1-butyl-3-methylimidazolium methanesulfonate ([C4mim][MeSO3

The viscosity, conductivity, proton T1 relaxation, and the self-diffusion coefficients of cation, anion and water are presented for the binary system water–1-butyl-3-methylimidazolium methanesulfonate ([C4mim][MeSO3]) for the temperature and composition ranges of (298.15–343.14) K and ionic liquid mole fractions 0<xIL<0.5. The temperature dependence of these properties at fixed compositions was analyzed to obtain Arrhenius activation energies, which were found to be linearly dependent to xIL for xIL greater than about 0.15 for the self-diffusion coefficients, viscosity and proton T1 relaxation times (except for the water proton T1 relaxation times). The specific xIL dependencies of the various Arrhenius activation energies are qualitatively different from what was observed in a prior study of the water–1-ethyl-3-methylimidazolium methanesulfonate ([C2mim][MeSO3]) binary system [J. Mol. Liq. 160 (2011) 166–179]. In particular, the T1 relaxation behavior is markedly different und is discussed in detail along with other comparisons between these two water–IL binary systems.

Ionization dynamics in ionic liquids probed via self-diffusion coefficient measurements

Ionic liquids contain ions and ion pairs with fast exchange between them. We propose a novel process to deduce the fraction of ions present in an ionic liquid and the equilibrium constants of ionization processes from measured ion self-diffusion coefficients using Pulsed Field Gradient (PFG) NMR. The enthalpy and entropy changes of ionization and ion self-diffusion processes are obtained for a series of ionic liquids using this method. These data were used to explain the interactions between cations and anions of ionic liquids. The interactions are also interpreted by the delocalization of the ion’s charge densities. The self-diffusion coefficients of cations and anions for measured ionic liquids are discussed.

Translational diffusion in mixtures of imidazolium ILs with polar aprotic molecular solvents.

Self-diffusion coefficients of cations and solvent molecules were determined with (1)H NMR in mixtures of 1-n-butyl-3-methylimidazolium (Bmim(+)) tetrafluoroborate (BF4(-)), hexafluorophosphate (PF6(-)), trifluoromethanesulfonate (TfO(-)), and bis(trifluoromethylsulfonyl)imide (TFSI(-)) with acetonitrile (AN), γ-butyrolactone (γ-BL), and propylene carbonate (PC) over the entire composition range at 300 K. The relative diffusivities of solvent molecules to cations as a function of concentration were found to depend on the solvent but not on the anion (i.e., IL). In all cases the values exhibit a plateau at low IL content (x(IL) < 0.2) and then increase steeply (AN), moderately (γ-BL), or negligibly (PC) at higher IL concentrations. This behavior was related to the different solvation patterns in the employed solvents. In BmimPF6-based systems, anionic diffusivities were followed via (31)P nuclei and found to be higher than the corresponding cation values in IL-poor systems and lower in the IL-rich region. The inversion point of relative ionic diffusivities was found around equimolar composition and does not depend on the solvent. At this point, a distinct change in the ion-diffusion mechanism appears to take place.

Defect Structure of the NiS2 Disulphide

AbstractDefect structure of NiS2 disulphide has been studied using marker technique and sulphidation rate measurements of NiS to NiS2, as a function of temperature and sulphur vapor pressure. It has been found that cation sublattice of the discussed compound is predominantly disordered and the prevailing defects are doubly ionized interstitial cations and quasi-free electrons. Using kinetic results, the self-diffusion coefficient of cations in NiS2 has been calculated basing on Wagner's theory of metal oxidation.