1,721,814 research outputs found
The impact of nonelectrostatic physisorption of ions on free energies and forces between redox electrodes: ion-specific repulsive peaks
No full textThe DLVO-style theory of surface energies and surface forces is extended to describe the charge transfer energy due to redox processes at an electrode, including nonelectrostatic ion interactions. Formulae for the corresponding disjoining pressure are given. Illustrative calculations are made for a coupled 0.01M Fe||Cu concentration cell. Nonelectrostatic interactions are found to reduce electrode potentials but increase electrode charges. Forces between graphite electrodes are calculated in two geometries: between two cathodes in the Fe cell and between two anodes in the Cu cell. Nonelectrostatic redox ion interactions, in particular strong nonelectrostatic physisorption of the Cu+ ion combined with a peak in the electrode charge, introduce a repulsive peak at a Debye-length separation (2 nm), that transforms the cathode-cathode interaction from attractive to repulsive and magnifies an existing repulsive peak in the anode-anode interaction. The strength of the peak varies with a Hofmeister series in the "inert" counterion
Effect of Nonelectrostatic Ion Interactions on Surface Forces Involving Ion Adsorption Equilibria
No full textThe chemical, or chemisorption, part of colloidal interaction free energy is revisited. Consistent incorporation of nonelectrostatic interactions in the chemical potential for the constant potential and charge regulation boundary conditions is developed. This gives rise to shifted adsorption equilibria, and thereby a shift in the predicted surface electrostatic potential. It also results in an additional component previously unaccounted for in the total double layer interaction force. The altered force leads to the need of recalibrating electrostatic surface potentials and equilibrium constants when fitting to experimental force data. A numerical illustration is presented using ionic dispersion potentials for mica surfaces interacting across NaCl at various concentrations. The new force component due to ionic dispersion is typically repulsive and exceeds entropic repulsion in magnitude. These results suggest that the effect of ionic dispersion is more profound than previously believed, even at low electrolyte concentrations
An advanced continuum medium model for treating solvation effects: Nonlocal electrostatics with a cavity
No full textThe Born-Kirkwood-Onsager (BKO) model of solvation, where a solute molecule is positioned inside a cavity cut into a solvent, which is considered as a dielectric continuum, is studied within the bounds of nonlocal electrostatics. The nonlocal cavity model is explicitly formulated and the corresponding nonlocal Poisson equation is reduced to an integral equation describing the behavior of the charge density induced in the medium. It is found that the presence of a cavity does not create singularities in the total electrostatic potential and its normal derivatives. Such singularities appear only in the local limit and are completely dissipated by nonlocal effects. The Born case of a spherical cavity with a point charge at its centre is investigated in detail. The corresponding one-dimensional integral Poisson equation is solved numerically and values for the solvation energy are determined. Several tests of this approach are presented: (a) We show that our integral equation reduces in the local limit to the chief equation of the local BKO theory. (b) We provide certain approximations which enable us to obtain the solution corresponding to the preceding nonlocal treatment of Dogonadze and Kornyshev (DK). (c) We make a comparison with the results of molecular solvation theory (mean spherical approximation), as applied to the calculation of solvation energies of spherical ions
Nonlocal continuum solvation model with oscillating susceptibility kernels: A nonrigid cavity model
No full textA nonlocal continuum theory of solvation is applied using an oscillating dielectric function with spatial dispersion. It is found that a convergent solution cannot be calculated using a model of a fixed solute cavity inside the solvent continuum. This is attributed to the fact that the dielectric oscillations appear as a result of coupling between polarization and density fluctuations, contradicting the concept of a fixed cavity, The theory is corrected by allowing the cavity size to vary. A cavitation energy and an interaction between the medium reaction field and the cavity size are added to the solvation free energy, and a new theory obtained by a variational treatment. The interaction term enables convergent solutions to become attainable, resulting in an oscillating electrostatic solvation energy as a function of cavity radius, the cavitation term enables these oscillations to be smoothed out, resulting in a regular, monotonic solvation free energy
Implications of rotation-inversion-permutation invariance for analytic molecular-potential energy surfaces
No full textA molecular potential energy surface has the symmetry properties of invariance to rotation of the whole molecule, inversion of all atomic coordinates, and permutation of indistinguishable nuclei. While some of this invariance character can be easily incorporated in a local description of the surface, a formal application of these symmetry restrictions is useful in considering the form of the global surface which must account for large amplitude changes of the atomic coordinates. The form of a global molecular potential energy surface as a properly symmetrized analytic function of Cartesian coordinates is derived by extending Molien's theorem of invariants for finite groups to cover the continuous rotation-inversion group. O(3), and the product of O(3) with the complete nuclear permutation group. The role of so-called redundant internal coordinates in molecular potential energy surfaces is clarified
The impact of ionic solvation energy and water structure on forces
No full textThe structure of water in an interfacial region differs from bulk due to
surface-induced water ordering. The difference is seen both in the local
water density and in the water polarisability, particularly orientational
polarisability as water dipoles are oriented towards the interface. The
impact of this phenomenon in continuum models is to replace the
dielectric constant of the medium with a position-dependent spatial
dielectric function. The relationship that this kind of dielectric function
bears with the electrostatic potential is known in the Poisson equation.
But an additional effect of the spatially dependent dielectric function
on ionic solvation energies (including a change in the Born energy of
the ion) has not been widely recognised. The dielectric constant in an
interfacial region has been reported to fall to values around 5, far from
the bulk value of 78. The corresponding spatially-dependent ionic
solvation energy therefore introduces a strongly repulsive ion-surface
interaction which must be included as an additional “nonelectrostatic
potential” in the Boltzmann factor, determining ion concentrations
in a Poisson-Boltzmann model. Consequently a strongly repulsive
surface force—a primary hydration force—is obtained with a range
corresponding to the range of the surface-induced water ordering,
usually the thickness of several water layers
Supercapacitors have an asymmetric electrode potential and charge due to nonelectrostatic electrolyte interactions
No full textIn recent years we have built up a theoretical framework which aims to explain ion specific effects observed in colloids at high salt concentrations. Our approach adds nonelectrostatic ion interactions (ion dispersion energies) alongside the usual electrostatic interactions of the ions.Using these techniques we have explored the impact that ion specificity may have on supercapacitors. Our model uses graphite electrodes at constant potential difference in 1.2M Li salt dissolved in propylene carbonate. For the counterion we used the common battery anions, PF6 -, BF4 - and ClO4 - along with BrO4 -, IO4 - and Cl-.When nonelectrostatic ionic interactions are included, a potential difference V will not be split symmetrically between the two electrodes. We address two alternative mechanisms for partitioning the potential difference. If the circuit is isolated, then the charge at each electrode must be equal. This defines a potential ψeq≠V/2 at the positive electrode and therefore ψeq-V at the negative. Alternatively, if the circuit is connected to an external environment (ground, defining zero potential), then the state of the system is determined by minimisation of the total free energy, resulting in asymmetric electrode charges as well as potentials. The potentials determined by the two mechanisms are different, ψmin≠ψeq, with both the difference (averaging about 10%) and the direction of the difference between them depending on the ions. The free energy of the system with equalised charge exceeds the minimised free energy by less than 10%. Total free energies follow the Hofmeister series Cl->PF6 ->BF4 ->ClO4 ->BrO4 ->IO4 -.Despite the differences in electrode potential and charge under the two potential partitioning mechanisms, the capacitances under both mechanisms are similar. Local electrode capacitances C1=dσ/dψ relative to the positive electrode potential follow the same Hofmeister series as the energy. The energy-capacitance slope is nonlinear, becoming smaller as C1 increases. The Hofmeister series in the differential capacitance C2=dσ/dV relative to the potential difference V swaps BF4 ->PF6 -.The asymmetry in capacitance between positive and negative electrodes indicates that the capacitance of a two-electrode supercapacitor or battery ought be treated as a two-value quantity rather than as a single value, similar to the matrix of mutual capacitances used in multielectrode devices
Predicting ion specific capacitances of supercapacitors due to quantum ionic interactions
No full textA new theoretical framework is now available to help explain ion specific (Hofmeister) effects. All measurements in physical chemistry show ion specificity, inexplicable by classical electrostatic theories. These ignore ionic dispersion forces that change ionic adsorption.We explored ion specificity in supercapacitors using a modified Poisson-Boltzmann approach that includes ionic dispersion energies. We have applied ab initio quantum chemical methods to determine required ion sizes and ion polarisabilities. Our model represents graphite electrodes through their optical dielectric spectra. The electrolyte was 1.2. M Li salt in propylene carbonate, using the common battery anions, PF6-,BF4- and ClO4- We also investigated the perhalate series with BrO4- and IO4. The capacitance C = dσ / dψ was calculated from the predicted electrode surface charge σ of each electrode with potential ψ between electrodes. Compared to the purely electrostatic calculation, the capacitance of a positively charged graphite electrode was enhanced by more than 15%, with PF6- showing > 50 % increase in capacitance. IO4- provided minimal enhancement. The enhancement is due to adsorption of both anions and cations, driven by ionic dispersion forces. The Hofmeister series in the single-electrode capacitance was PF6->BF4->ClO4->BrO4->IO4- When the graphite electrode was negatively charged, the perhalates provided almost no enhancement of capacitance, while PF6- and BF4- decreased capacitance by about 15%.Due to the asymmetric impact of nonelectrostatic ion interactions, the capacitances of positive and negative electrodes are not equal. The capacitance of a supercapacitor should therefore be reported as two values rather than one, similar to the matrix of mutual capacitances used in multielectrode devices
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