169 research outputs found
Analytical interfacial layer model for the capacitance and electrokinetics of charged aqueous interfaces
We construct an analytical model to account for the influence of the subnanometer-wide interfacial layer on the differential capacitance and the electro-osmotic mobility of solidâelectrolyte interfaces. The interfacial layer is incorporated into the PoissonâBoltzmann and Stokes equations using a box model for the dielectric properties, the viscosity, and the ionic potential of mean force. We calculate the differential capacitance and the electro-osmotic mobility as a function of the surface charge density and the salt concentration, both with and without steric interactions between the ions. We compare the results from our theoretical model with experimental data on a variety of systems (graphite and metallic silver for capacitance and titanium oxide and silver iodide for electro-osmotic data). The differential capacitance of silver as a function of salinity and surface charge density is well reproduced by our theory, using either the width of the interfacial layer or the ionic potential of mean force as the only fitting parameter. The differential capacitance of graphite, however, needs an additional carbon capacitance to explain the experimental data. Our theory yields a power-law dependence of the electro-osmotic mobility on the surface charge density for high surface charges, reproducing the experimental data using both the interfacial parameters extracted from molecular dynamics simulations and fitted interfacial parameters. Finally, we examine different types of hydrodynamic boundary conditions for the power-law behavior of the electro-osmotic mobility, showing that a finite-viscosity layer explains the experimental data better than the usual hydrodynamic slip boundary condition. Our analytical model thus allows us to extract the properties of the subnanometer-wide interfacial layer by fitting to macroscopic experimental data
Breakdown of linear dielectric theory for the interaction between hydrated ions and graphene
Many vital processes taking place in electrolytes, such as nanoparticle self-assembly, water purification, and the operation of aqueous supercapacitors, rely on the precise many-body interactions between surfaces and ions in water. Here we study the interaction between a hydrated ion and a charge-neutral graphene layer using atomistic molecular dynamics simulations. For small separations, the ionâgraphene repulsion is of nonelectrostatic nature, and for intermediate separations, van der Waals attraction becomes important. Contrary to prevailing theory, we show that nonlinear and tensorial dielectric effects become non-negligible close to surfaces, even for monovalent ions. This breakdown of standard isotropic linear dielectric theory has important consequences for the understanding and modeling of charged objects at surfaces
Power-law electrokinetic behavior as a direct probe of effective surface viscosity
An exact solution to the Poisson-Boltzmann and Stokes equations is derived to describe the electric double layer with inhomogeneous dielectric and viscosity profiles in a lateral electric field. In the limit of strongly charged surfaces and low salinity, the electrokinetic flow magnitude follows a power law as a function of the surface charge density. Remarkably, the power-law exponent is determined by the interfacial dielectric constant and viscosity, the latter of which has eluded experimental determination. Our approach provides a novel method to extract the effective interfacial viscosity from standard electrokinetic experiments. We find good agreement between our theory and experimental data
The effects of ion adsorption on the potential of zero charge and the differential capacitance of charged aqueous interfaces
Using a box profile approximation for the non-electrostatic surface adsorption potentials of anions and cations, we calculate the differential capacitance of aqueous electrolyte interfaces from a numerical solution of the PoissonâBoltzmann equation, including steric interactions between the ions and an inhomogeneous dielectric profile. Preferential adsorption of the positive (negative) ion shifts the minimum of the differential capacitance to positive (negative) surface potential values. The trends are similar for the potential of zero charge; however, the potential of zero charge does not correspond to the minimum of the differential capacitance in the case of asymmetric ion adsorption, contrary to the assumption commonly used to determine the potential of zero charge. Our model can be used to obtain more accurate estimates of ion adsorption properties from differential capacitance or electrocapillary measurements. Asymmetric ion adsorption also affects the relative heights of the characteristic maxima in the differential capacitance curves as a function of the surface potential, but even for strong adsorption potentials the effect is small, making it difficult to reliably determine the adsorption properties from the peak heights
Impurity effects at hydrophobic surfaces
The effective charge of hydrophobic surfaces and in particular of the airâwater interface is a crucial parameter for electrochemistry, colloidal chemistry and interfacial science, but different experiments give conflicting estimates. Zeta-potential and disjoining-pressure measurements point to a strongly negative surface charge, often interpreted as being due to adsorbing hydroxide ions. In contrast, surface tension measurements of acids and bases suggest the hydronium ion to be surface active, in agreement with some surface-specific non-linear spectroscopy results. The airâelectrolyte interfacial tension exhibits a characteristic minimum at millimolar electrolyte concentration for all salts, the so-called JonesâRay effect, which points to competitive adsorption mechanisms present in dilute electrolyte solutions. We show that all these puzzling experimental findings can be explained by the presence of trace amounts of surface-active charged impurities, most likely anionic surfactants
Molecular dynamics simulations of the evaporation of hydrated ions from aqueous solution
Although important for atmospheric processes and gas-phase catalysis, very little is known about the hydration state of ions in the vapor phase. Here we study the evaporation energetics and kinetics of a chloride ion from liquid water by molecular dynamics simulations. As chloride permeates the interface, a water finger forms and breaks at a chloride separation ofâââ2.8ânm from the Gibbs dividing surface. For larger separations from the interface, about 7 water molecules are estimated to stay bound to chloride in saturated water vapor, as corroborated by continuum dielectrics and statistical mechanics models. This ion hydration significantly reduces the free-energy barrier for evaporation. The effective chloride diffusivity in the transition state is found to be about 6 times higher than in bulk, which reflects the highly mobile hydration dynamics as the water finger breaks. Both effects significantly increase the chloride evaporation flux from the quiescent interface of an electrolyte solution, which is predicted from reaction kinetic theory
Nanomolar Surface-Active Charged Impurities Account for the Zeta Potential of Hydrophobic Surfaces
The electrification of hydrophobic surfaces is an intensely debated subject in physical chemistry. We theoretically study the ζ potential of hydrophobic surfaces for varying pH and salt concentration by solving the PoissonâBoltzmann and Stokes equations with individual ionic adsorption affinities. Using the ionic surface affinities extracted from the experimentally measured surface tension of the airâelectrolyte interface, we first show that the interfacial adsorption and repulsion of small inorganic ions such as H3O+, OHâ, HCO3â, and CO32â cannot account for the ζ potential observed in experiments because the surface affinities of these ions are too small. Even if we take hydrodynamic slip into account, the characteristic dependence of the ζ potential on pH and salt concentration cannot be reproduced. Instead, to explain the sizable experimentally measured ζ potential of hydrophobic surfaces, we assume minute amounts of impurities in the water and include the impuritiesâ acidic and basic reactions with water. We find good agreement between our predictions and the reported experimental ζ potential data of various hydrophobic surfaces if we account for impurities that consist of a mixture of weak acids (pKa = 5â7) and weak bases (pKb = 12) at a concentration of the order of 10â7 M
Optimization of classical nonpolarizable force fields for OHâ and H3O+
We optimize force fields for H3O+ and OHâ that reproduce the experimental
solvation free energies and the activities of H3O+ Clâ and Na+ OHâ solutions
up to concentrations of 1.5 mol/l. The force fields are optimized with respect
to the partial charge on the hydrogen atoms and the Lennard-Jones parameters
of the oxygen atoms. Remarkably, the partial charge on the hydrogen atom of
the optimized H3O+ force field is 0.8 ± 0.1|e|âsignificantly higher than the
value typically used for nonpolarizable water models and H3O+ force fields. In
contrast, the optimal partial charge on the hydrogen atom of OHâ turns out to
be zero. Standard combination rules can be used for H3O+ Clâ solutions, while
for Na+ OHâ solutions, we need to significantly increase the effective anion-
cation Lennard-Jones radius. While highlighting the importance of
intramolecular electrostatics, our results show that it is possible to
generate thermodynamically consistent force fields without using atomic
polarizability
Dielectric properties of aqueous electrolytes at the nanoscale
Despite the ubiquity of nanoconfined aqueous electrolytes, a theoretical
framework that accounts for the nonlinear coupling of water and ion
polarization is still missing. We introduce a nonlocal and nonlinear field
theory for the nanoscale polarization of ions and water and derive the
electrolyte dielectric properties as a function of salt concentration to first
order in a loop expansion. Classical molecular dynamics simulations are
favorably compared with the calculated dielectric response functions. The
theory correctly predicts the dielectric permittivity decrement with rising
salt concentration and furthermore shows that salt induces a Debye screening in
the longitudinal susceptibility but leaves the short-range water organization
remarkably unchanged.Comment: 6 pages, 3 figure
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