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 solid–electrolyte interfaces. The interfacial layer is incorporated into the Poisson–Boltzmann 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

Viscous interfacial layer formation causes electroosmotic mobility reversal in monovalent electrolytes

We study the ion density, shear viscosity and electroosmotic mobility of an aqueous monovalent electrolyte at a charged solid surface using molecular dynamics simulations. Upon increasing the surface charge density, ions are displaced first from the diffuse layer to the outer Helmholtz layer, increasing its viscosity, and subsequently to the hydrodynamically stagnant inner Helmholtz layer. The ion redistribution causes both charge inversion and reversal of the electroosmotic mobility. Because of the surface-charge dependent interfacial hydrodynamic properties, however, the charge density of mobility reversal differs from the charge density of charge inversion, depending on the salt concentration and the chemical details of the ions and the surface. Mobility reversal cannot be described by an effective slip boundary condition alone – the spatial dependence of the viscosity is essential

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 ion–graphene 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 Poisson–Boltzmann 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 air–water 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 air–electrolyte interfacial tension exhibits a characteristic minimum at millimolar electrolyte concentration for all salts, the so-called Jones–Ray 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

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