High Dielectric Medium Research Articles

Toward a Lithium-‘air’ Battery: The Effect of CO2 on the Chemistry of a Lithium-Oxygen Cell

Lithium-oxygen chemistry offers the highest energy density for a rechargeable system; such a system is known as a “lithium-air battery”. Most studies of lithium-air batteries have focused on demonstrating battery operations in an environment of pure oxygen; such a battery should technically be described as a “lithium-dioxygen (O2) battery”. Consequently, the next step for the lithium-“air” battery is to understand how the reaction chemistry is affected by the constituents of ambient air. Among the components of air, CO2 is of particular interest because its solubility in organic solvents is about fifty times higher than that of O2, and it can react actively with O2 - ·, which is the key intermediate species in Li-O2 battery reactions. In this work, we investigated the reaction mechanisms in the Li-O2/CO2 cell under various electrolyte conditions using quantum mechanical simulations combined with experimental verification. Our most important finding is that the subtle balance among various reaction pathways influencing the potential energy surfaces can be modified by the electrolyte solvation effect. Thus, a low dielectric electrolyte tends to primarily form Li2O2, while a high dielectric electrolyte is effective in electrochemically activating CO2, yielding only Li2CO3. Most surprisingly, we further discovered that a high dielectric medium such as DMSO can result in the reversible reaction of Li2CO3 within a Li-air cell (contrary to conventional belief) over multiple cycles. We believe that the current mechanistic understanding of the chemistry of CO2 in a Li-air cell and the interplay of CO2 with electrolyte solvation will provide an important guideline for developing Li-air batteries. Furthermore, the newly discovered possibility for a rechargeable Li-O2/CO2 battery based on Li2CO3 formation chemistry may have merits in enhancing cyclability by minimizing side reactions.

On the Modeling of Polar Component of Solvation Energy using Smooth Gaussian-Based Dielectric Function.

Traditional implicit methods for modeling electrostatics in biomolecules use a two-dielectric approach: a biomolecule is assigned low dielectric constant while the water phase is considered as a high dielectric constant medium. However, such an approach treats the biomolecule-water interface as a sharp dielectric border between two homogeneous dielectric media and does not account for inhomogeneous dielectric properties of the macromolecule as well. Recently we reported a new development, a smooth Gaussian-based dielectric function which treats the entire system, the solute and the water phase, as inhomogeneous dielectric medium (J Chem Theory Comput. 2013 Apr 9; 9(4): 2126-2136.). Here we examine various aspects of the modeling of polar solvation energy in such inhomogeneous systems in terms of the solute-water boundary and the inhomogeneity of the solute in the absence of water surrounding. The smooth Gaussian-based dielectric function is implemented in the DelPhi finite-difference program, and therefore the sensitivity of the results with respect to the grid parameters is investigated, and it is shown that the calculated polar solvation energy is almost grid independent. Furthermore, the results are compared with the standard two-media model and it is demonstrated that on average, the standard method overestimates the magnitude of the polar solvation energy by a factor 2.5. Lastly, the possibility of the solute to have local dielectric constant larger than of a bulk water is investigated in a benchmarking test against experimentally determined set of pKa's and it is speculated that side chain rearrangements could result in local dielectric constant larger than 80.

Rationalizing Catalyst Inks for PEMFC Electrodes Based on Colloidal Interactions

A preliminary kinetic model was developed for polymer electrolyte membrane fuel cell (PEMFC) catalyst inks in order to understand their particle stability. The Derjaguin Landau Verwey Overbeek (DLVO) model containing van der Waals attractive and electrostatic repulsive interaction energy was applied to the aqueous ink dispersions, while a modified DLVO type interaction containing a Coulombic term instead of the electrostatic term was applied to the non-aqueous dispersions. Solvents were compared based on their particle size distribution and stability ratios. Results show that the carbon black particles are stable in a higher dielectric medium whereas they tend to aggregate in a lower dielectric medium. A low ionic concentration for the aqueous medium also helped to improve the ink stability by providing a better electrostatic particle repulsion. Experiments conducted with ethyl acetate, iso-propanol and deionized water agree with the model predictions.

Toward a Lithium–“Air” Battery: The Effect of CO2 on the Chemistry of a Lithium–Oxygen Cell

Lithium-oxygen chemistry offers the highest energy density for a rechargeable system as a "lithium-air battery". Most studies of lithium-air batteries have focused on demonstrating battery operations in pure oxygen conditions; such a battery should technically be described as a "lithium-dioxygen battery". Consequently, the next step for the lithium-"air" battery is to understand how the reaction chemistry is affected by the constituents of ambient air. Among the components of air, CO2 is of particular interest because of its high solubility in organic solvents and it can react actively with O2(-•), which is the key intermediate species in Li-O2 battery reactions. In this work, we investigated the reaction mechanisms in the Li-O2/CO2 cell under various electrolyte conditions using quantum mechanical simulations combined with experimental verification. Our most important finding is that the subtle balance among various reaction pathways influencing the potential energy surfaces can be modified by the electrolyte solvation effect. Thus, a low dielectric electrolyte tends to primarily form Li2O2, while a high dielectric electrolyte is effective in electrochemically activating CO2, yielding only Li2CO3. Most surprisingly, we further discovered that a high dielectric medium such as DMSO can result in the reversible reaction of Li2CO3 over multiple cycles. We believe that the current mechanistic understanding of the chemistry of CO2 in a Li-air cell and the interplay of CO2 with electrolyte solvation will provide an important guideline for developing Li-air batteries. Furthermore, the possibility for a rechargeable Li-O2/CO2 battery based on Li2CO3 may have merits in enhancing cyclability by minimizing side reactions.

Enhanced Charge Carrier Mobility in Two‐Dimensional High Dielectric Molybdenum Oxide

We demonstrate that the energy bandgap of layered, high-dielectric α-MoO(3) can be reduced to values viable for the fabrication of 2D electronic devices. This is achieved through embedding Coulomb charges within the high dielectric media, advantageously limiting charge scattering. As a result, devices with α-MoO(3) of ∼11 nm thickness and carrier mobilities larger than 1100 cm(2) V(-1) s(-1) are obtained.

Ionic Polymers Act as Polyelectrolytes in Nonpolar Media.

Polyelectrolytes are ubiquitous materials, and their unique properties originate from dissociation of ionic groups to the small number of macromolecular ions and the large number of small counterions. They have been exploited only in water or high-dielectric media and scarcely in nonpolar ones (ε < 10). Herein, we demonstrate that poly(octadecyl acrylate) bearing tetraalkylammonium tetraarylborate as ionic groups behaves as a polyelectrolyte in the common nonpolar organic solvents such as chloroform, THF, and 1,2-dichloroethane. Conductivity measurement, DOSY NMR spectroscopy, and viscosity measurements clearly indicate that they form the extended conformation in them. This result emphasizes that the ionic polymers bearing suitable ion pairs ionizable in the given media act as polyelectrolytes. Various characteristic properties and processes of polyelectrolytes should be realized in nonpolar media by designing ion pairs and polymer chains in the ionic polymers. Moreover, our results imply that electrostatic interaction is readily available as a long-range repulsive force even in the nonpolar media.

Comparison of three-dimensional Poisson solution methods for particle-based simulation and inhomogeneous dielectrics

Particle-based simulation represents a powerful approach to modeling physical systems in electronics, molecular biology, and chemical physics. Accounting for the interactions occurring among charged particles requires an accurate and efficient solution of Poisson's equation. For a system of discrete charges with inhomogeneous dielectrics, i.e., a system with discontinuities in the permittivity, the boundary element method (BEM) is frequently adopted. It provides the solution of Poisson's equation, accounting for polarization effects due to the discontinuity in the permittivity by computing the induced charges at the dielectric boundaries. In this framework, the total electrostatic potential is then found by superimposing the elemental contributions from both source and induced charges. In this paper, we present a comparison between two BEMs to solve a boundary-integral formulation of Poisson's equation, with emphasis on the BEMs' suitability for particle-based simulations in terms of solution accuracy and computation speed. The two approaches are the collocation and qualocation methods. Collocation is implemented following the induced-charge computation method of D. Boda et al. [J. Chem. Phys. 125, 034901 (2006)]. The qualocation method is described by J. Tausch et al. [IEEE Transactions on Computer-Aided Design of Integrated Circuits and Systems 20, 1398 (2001)]. These approaches are studied using both flat and curved surface elements to discretize the dielectric boundary, using two challenging test cases: a dielectric sphere embedded in a different dielectric medium and a toy model of an ion channel. Earlier comparisons of the two BEM approaches did not address curved surface elements or semiatomistic models of ion channels. Our results support the earlier findings that for flat-element calculations, qualocation is always significantly more accurate than collocation. On the other hand, when the dielectric boundary is discretized with curved surface elements, the two methods are essentially equivalent; i.e., they have comparable accuracies for the same number of elements. We find that ions in water--charges embedded in a high-dielectric medium--are harder to compute accurately than charges in a low-dielectric medium.

Use of Electron Paramagnetic Resonance Spectroscopy to Study Dielectric Properties of Liquids

The signal response of an EPR active species is attenuated by the medium it is in. Keeping all other parameters the same, the higher the dielectric constant of the medium, the lower the EPR signal response. This behavior is problematic in studying EPR active species in high dielectric media but can be capitalized upon to monitor changes in the dielectric constant or estimate the dielectric constant of the medium. Using a coaxial EPR cell design, the EPR signal of a stable nitroxyl radical compound (2,2,6,6-tetramethyl-piperidin-1-oxyl radical) in a low dielectric constant solvent in an inner tube is attenuated by the solvent present between the inner and outer tubes (jacket medium). The attenuation increases monotonically with an increase in the dielectric constant of the jacket medium. Calibration curves can be constructed using jacket media of known dielectric constants ranging from 2 to 80 and the dielectric constant of a sample used as the jacket medium can be estimated by interpolation. This technique is applied to estimate the dielectric constants and/or composition of mixed solvents and to monitor the rate of a reaction.

Coulombic Interactions and Crystal Packing Effects in the Folding of Donor−Acceptor Oligorotaxanes

The folding of a series of donor-acceptor oligorotaxanes is studied by means of molecular dynamics simulations. The oligorotaxanes consist of tetracationic cyclobis(paraquat-p-phenylene) rings threaded onto oligomers of 1,5-dioxynaphthalene linked by polyethers. Minimum energy structures are isolated using simulated annealing from which insights into the interactions responsible for the folding are extracted. The folding in vacuum is primarily driven by Coulombic interactions between the ionic species. In turn, in a high dielectric medium, the folding is the result of a series of conflicting interactions primarily orchestrated by the rings. In both cases, folded globular structures result. The observed secondary structures extend a previously proposed pleated π-stacked model for the folding that has been, in fact, observed in crystal samples of a [3]Rotaxane, the smallest of the oligorotaxanes considered. Simulations using two different force fields in the crystal environment, in vacuum, and in explicit solvent indicate that the π-stacked conformation adopted by the [3]Rotaxane is stable due to crystal packing effects rather than intracomplex interactions. The π-stacked conformation is also shown to be thermodynamically improbable in longer oligomers.

Dual-beam-reflection phenomenon due to leaky modes in a photonic band gap

We show a dual-beam-reflection phenomenon for a Gaussian beam illuminating a hybrid structure of a dielectric waveguide and photonic crystal (WG-PC) inside the photonic band gap by numerical simulations. One reflection beam has a giant negative lateral shift, but the other has a positive lateral shift. The finite-difference time-domain (FDTD) simulations show that this phenomenon has a time delay effect and comes from the leaky surface mode of the hybrid structure. Field profile of the leaky mode demonstrates a strong localized stationary field in the higher dielectric medium. Furthermore, the maximum lateral shift is almost two times of the waist of the incident beam.

Single-molecule pulling and the folding of donor-acceptor oligorotaxanes: Phenomenology and interpretation

The thermodynamic driving force in the folding of a class of oligorotaxanes is elucidated by means of molecular dynamics simulations of equilibrium isometric single-molecule force spectroscopy by atomic force microscopy experiments. The oligorotaxanes consist of cyclobis(paraquat-p-phenylene) rings threaded onto an oligomer of 1,5-dioxynaphthalenes linked by polyethers. The simulations are performed in a high dielectric medium using MM3 as the force field. The resulting force versus extension isotherms show a mechanically unstable region in which the molecule unfolds and, for selected extensions, blinks in the force measurements between a high-force and a low-force regime. From the force versus extension data the molecular potential of mean force is reconstructed using the weighted histogram analysis method and decomposed into energetic and entropic contributions. The simulations indicate that the folding of the oligorotaxanes is energetically favored but entropically penalized, with the energetic contributions overcoming the entropy penalty and effectively driving the folding. In addition, an analogy between the single-molecule folding/unfolding events driven by the atomic force microscope (AFM) tip and the thermodynamic theory of first-order phase transitions is discussed. General conditions (on the molecule and the AFM cantilever) for the emergence of mechanical instabilities and blinks in the force measurements in equilibrium isometric pulling experiments are also presented. In particular, it is shown that the mechanical stability properties observed during the extension are intimately related to the fluctuations in the force measurements.

Effect of Stacking Interactions on the Spectra of the Monomer of PFBT: A Theoretical Study

Conjugated polymers (CPs) contain one pi-conjugated backbone and functional groups that could be ionized in high dielectric media. These materials combine the semiconducting and photon harvesting properties of electronically delocalized polymers with the charge-mediated behavior of polyelectrolytes. CPs can be used as highly responsive optical sensors for chemical and biological targets. The density functional theory (DFT) and the time-dependent density functional theory (TDDFT) approach were employed to simulate the absorption and emission spectra of poly[9,9'-bis(6''-N,N,N-trimethylammonium)hexyl]fluorene-co-alt-4,7-(2,1,3-benzothiadiazole) dibromide] (PFBT) in the present study. The influences on the spectra of the monomer unit F(BT)F due to stacking with the fluorene (F) and 2,1,3-benzothiadiazole (BT) units have been explored. The results suggest that stacking lowers the excitation and emission energy, facilitating detections of the polymers.

Transmission and directionality control of light emission from a nanoslit in metallic film flanked by periodic gratings

We investigate the transmission and directional properties of light emission from a nanoslit film flanked by periodic gratings, formed on a silver film, embedded in a high index dielectric medium. Using FDTD calculations it is demonstrated that the transmission has strong dependence on dimension of the dielectric film. The directional property of the emitted beam is controlled by tapering height of output grating elements and this effect is explained by standard theory of antenna array. We propose and examine the potential of such slit grating structure as a Plasmonic antenna in axial injection of light from single mode fiber to photonic crystal waveguide.

Theoretical Studies on the Addition Reactions of Ketene with NH3in the Gas Phase and in Non-Aqueous Solutions

Theoretical studies on the un-catalyzed and catalyzed aminations of ketene with <TEX>$NH_3$</TEX> and <TEX>$(NH_3)_2$</TEX>, respectively, were studied using MP2 and hybrid density functional theory of B3LYP at the 6-31+G(d,p) and 6- 311+G(3df,2p) basis sets in the gas phase and in benzene and acetonitrile solvents. In the gas phase reaction, the un-catalyzed mechanism was the same as those previously reported by others. The catalyzed mechanism, however, was more complicated than expected requiring three transition states for the complete description of the C=O addition pathways. In the un-catalyzed amination, rate determining step was the breakdown of enol amide but in the catalyzed reaction, it was changed to the formation of enol amide, which was contradictory to the previous findings. Starting from the gas-phase structures, all structures were re-optimized using the CPCM method in solvent medium. In a high dielectric medium, acetonitrile, a zwitterions formed from the reaction of <TEX>$CH_2$</TEX>=C=O with <TEX>$(NH_3)_2$</TEX>, I(d), exists as a genuine minimum but other zwitterions, I(m) in acetonitrile and I(d) in benzene become unstable when ZPE corrected energies are used. Structural and energetic changes induced by solvation were considered in detail. Lowering of the activation energy by introducing additional <TEX>$NH_3$</TEX> molecule amounted to ca. −20 <TEX>$\sim$</TEX> −25 kcal/mol, which made catalyzed reaction more facile than un-catalyzed one.

Permanganate Oxidation of Alkenes. Substituent and Solvent Effects. Difficulties with MP2 Calculations

The permanganate oxidation of alkenes has been studied both experimentally and computationally. Transition state structures were located for the reaction of permanganate ion with a variety of monosubstituted alkenes at the B3LYP/6-311++G** level. Although the calculated activation energy for the reaction with ethene was reasonable, the calculated effect of substituents, based on the energies of the reactants, was much larger than that experimentally found. This was shown to be due to the formation of an intermediate charge-dipole complex which led to the transition state. Reaction field calculations found the complex to disappear in a high dielectric constant medium, and the range of activation energies for the reaction in solution became quite small. MP2 calculations were carried out in order to have a comparison with the DFT results. MP2-MP4 gave unusual results for calculations on permanganate ion as well as chromate ion and iron tetraoxide. They also gave markedly unreasonable results for the activation energy of the reaction of permanganate with ethane. CCSD/6-311++G** calculations gave satisfactory results for permanganate ion and chromate ion. At this level of theory, the reaction of permanganate with ethene was found to have a very early transition state, when the bond lengths of the reactants just began to change. The reaction was calculated to be very exothermic (-69 kcal/mol), and this was confirmed via calorimetry. The rates of permanganate oxidation of allyl alcohol and acrylonitrile were determined, and they had similar reactivities. The kinetics and the products of the reaction of permanganate with crotonate ion were examined in some detail.

An Analytical Electrostatic Model for Salt Screened Interactions between Multiple Proteins.

We present a new general analytical solution for computing the screened electrostatic interaction between multiple macromolecules of arbitrarily complex charge distributions, assuming they are well described by spherical low dielectric cavities in a higher dielectric medium in the presence of a Debye-Hückel treatment of salt. The benefits to this approach are 3-fold. First, by exploiting multipole expansion theory for the screened Coulomb potential, we can describe direct charge-charge interactions and all significant higher-order cavity polarization effects between low dielectric spherical cavities containing their charges, while treating these higher order terms correctly at all separation distances. Second, our analytical solution is general to arbitrary numbers of macromolecules, is efficient to compute, and can therefore simultaneously provide on-the-fly updates to changes in charge distributions due to protein conformational changes. Third, we can change spatial resolutions of charge description as a function of separation distance without compromising the desired accuracy. While the current formulation describes solutions based on simple spherical geometries, it appears possible to reformulate these electrostatic expressions to smoothly increase spatial resolution back to greater molecular detail of the dielectric boundaries.

Light transmission through a high index dielectric-filled sub-wavelength hole in a metal film

We investigate transmission of a normally incident, linearly polarized plane wave through a circular sub-wavelength hole in a metal film filled by a high index dielectric medium. We demonstrate for the first time that the trans-mission efficiency of such holes exhibits a Fabry-Pérot like behaviour versus thickness of the metal film, similar to that exhibited by sub-wavelength slits in metal films illuminated by TM-polarized plane waves. We show that by reducing the imaginary part of the propagation constant of the hybrid HE11 mode and by fortifying the Fabry-Pérot resonance, the high index dielectric filling can greatly enhance light transmission through a circular sub-wavelength hole.

Wollaston prism-like devices based on blazed dielectric subwavelength gratings

A Wollaston prism-like binary dielectric grating is presented and analyzed. It behaves like a transmission grating, differentially and symmetrically blazed for the two crossed polarization states, TE and TM. The phase profile is obtained by means of subwavelength structures etched in a high optical index isotropic dielectric medium (gallium arsenide, for instance). The performance of the device is illustrated by numerical examples and sketched in terms of spectral bandwidth and of extinction ratio. Some practical issues related to the fabrication are discussed.

Macroscopic electrostatic models for protonation states in proteins.

The use of macroscopic electrostatic models to calculate the relative energetics of protonation states and the pH-titration properties of ionizable groups in proteins is described. These methods treat the protein as an irregularly-shaped low-dielectric object containing embedded atomic charges immersed in a high-dielectric (solvent) medium. The energetics of altering protonation states then involves the electrostatic work of altering the embedded atomic charges. The governing electrostatic equation is either the Poisson or linearized Poisson-Boltzmann equation, which generally requires numerical solution. A tutorial approach is taken, the main aim of which is a thorough understanding of the method.

Vibrational Spectra of Lumazine in Water at pH 2−13: Ab Initio Calculation and FTIR/Raman Spectra

Harmonic vibrational frequencies and transition intensities of lumazine (2,4-(1H,3H)pteridinedione in the neutral state) have been calculated in a self-consistent reaction field of high dielectric medium (ε = 78.54) using ab initio Hartree−Fock (HF) and hybrid HF/density functional theory (DFT) methods. For the DFT method, the 4-31G basis set and the three parameter exchange functional of Becke combined with the Lee, Yang, and Parr correlational functional are used. The 6-31+G* basis set is used in the HF calculations. Both the spherical cavity model (SCM) and the self-consistent isodensity polarizable continuum model (SCIPCM) are used to simulate an aqueous environment for the N-protonated lumazines. For the N-deuterated lumazines, only the SCM is used. Simple scaling of the vibrational frequencies resulting from the DFT calculations incorporating the reaction field models of neutral lumazine, lumazine A1/N3-H monoanion, and lumazine A1,A3-dianion compare closely with Raman and Fourier transform infrared spectra of lumazine taken in aqueous media (both H2O and D2O) over a wide pH/pD range. The mean deviation between calculated and experimental vibrational frequencies is 9.10 cm-1 for neutral lumazine (9.37 cm-1 in D2O), 9.18 cm-1 for the monoanion (10.77 cm-1 in D2O), and 16.06 cm-1 for the dianion (16.00 cm-1 when prepared in D2O). Calculated vibrational energy shifts with changes in ionization exhibit many trends that are evident in the experimental data although the absolute magnitudes of many of the individual shifts do not agree exactly. The calculated H/D isotopic shifts for the neutral and monoanionic species of lumazine agree well with those seen experimentally. Correlating shifts with ionization and the H/D isotopic shifts of the vibrational modes of each species of lumazine investigated have resulted in normal mode assignments of all in-plane vibrational modes observed in the 300−1750 cm-1 range in aqueous solution.