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Debye Length Dependence of the Anomalous Dynamics of Ionic Double Layers in a Parallel Plate Capacitor R Article pubs.acs.org/JPCC Debye Length Dependence of the Anomalous Dynamics of Ionic Double Layers in a Parallel Plate Capacitor R. J. Kortschot, A. P. Philipse, and B. H. Erne*́ Van ’tHoff Laboratory for Physical and Colloid Chemistry, Debye Institute for Nanomaterials Science, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands ABSTRACT: The electrical impedance spectrum of simple ionic solutions is measured in a parallel plate capacitor at small applied ac voltage. The influence of the ionic strength is investigated using several electrolytes at different concentrations in solvents of different dielectric constants. The electric double layers that appear at the electrodes at low frequencies are not perfectly capacitive. At moderate ionic strength, ion transport agrees with a model based on the Poisson−Nernst−Planck (PNP) equations. At low ionic strength, double layer dynamics deviate from the PNP model, and the deviation is well described by an empirical function with only one fit parameter. This deviation from the PNP equations increases systematically with increasing Debye length, possibly caused by the long-range Coulomb interaction. 1. INTRODUCTION This paper starts with a theoretical discussion of electrode polarization in section 2 and a description of our experimental The accumulation of charged species near an electrode, so-called fi electrode polarization, occurs in and complicates dielectric methods in section 3. Then data are presented and tted in spectroscopy measurements of, for instance, biological materials,1 section 4. Finally, a general discussion is made in section 5 and solids,2 and colloidal dispersions.3 Additionally, electrode polar- conclusions are drawn in section 6. ization is also a fundamental process for applications such as fuel cells,4 supercapacitors,4 and electrophoretic displays,5 and it can be 2. THEORY 6 used for the manipulation and characterization of red blood cells. As a simple model for electrode polarization, we consider the Measurements of electrode polarization often exhibit response of a symmetric electrolyte in an isotropic solvent, “capacitance dispersion”; that is, the double layer capacitance − confined between two blocking electrodes of a parallel plate appears to be frequency dependent.7 10 A satisfying physical model capacitor, to which a small ac voltage is applied. Ionic transport in to describe this effect, however, is still lacking. Experimental data of the direction perpendicular to the electrodes (z-axis) as a relaxation behavior are often described by (semi)empirical function of time t is assumed to occur according to the Poisson− functions,11,12 and capacitance dispersion is usually modeled by a Nernst−Planck (PNP) model, which consists of the follow- constant phase element (CPE). This equivalent element in an −1 α ing three equations. First, the fluxes J± of positive or negative electric circuit has an impedance Z = Aα(iω) ,with0<α ≤ 1and −1 α ions with concentrations n±(z, t) are due to diffusion in a Aα aconstantwithunitsΩ s . The presence of fractal units when α 13 concentration gradient ∂n±(z,t)/∂z and drift in a gradient of the < 1 complicates the physical interpretation of a CPE. Whether ∂ ∂ the term “double layer capacitance” then still applies to a CPE electrostatic potential V(z, t)/ z: − response has been debated.14 19 ⎛ ∂nzt(,) ⎞ Our objective here is to enhance the physical and quantitative ⎜ ± q ∂Vzt(,)⎟ Jzt±(,)=− D ± nzt±(,) understanding of the frequency dependence of the double layer ⎝ ∂z kT ∂z ⎠ (1) capacitance. Our approach is to perform a systematic study of electrode polarization of simple ionic solutions with several Second, the gradient in the electric field is determined by the electrolytes of known diffusion coefficients in solvents with Poisson equation: different dielectric constants. The electrical impedance is measured in the limit of a small applied voltage. In particular, ∂2Vzt(,) q =− − we examine how capacitance dispersion is affected by the ionic 2 ((,)nzt+− n (,)) zt ∂z ϵϵ (2) strength, and we focus on electrode polarization in the frequency 0s range above the characteristic frequency of double layer relaxation. Our data are compared with a simple model that Received: March 13, 2014 takes into account diffusion and drift20 and an empirical Revised: April 22, 2014 extension of this model with a specific form of a CPE.21 Published: May 9, 2014 © 2014 American Chemical Society 11584 dx.doi.org/10.1021/jp5025476 | J. Phys. Chem. C 2014, 118, 11584−11592 The Journal of Physical Chemistry C Article Third, conservation of particles is formulated in the equation of continuity: ∂nzt(,) ∂Jzt(,) ±±=− ∂t ∂z (3) In these equations, k is the Boltzmann constant, T is the ff Figure 2. Schematic overview of equivalent circuits of (a) the complex temperature, q is the absolute ion charge, D is the di usion ϵ′ ω coefficient, ϵ is the vacuum permittivity, and ϵ is the dielectric permittivity in terms of frequency-dependent parameters ( ) and 0 s σ(ω), (b) the PNP model (eq 7), and (c) the empirical model (eq 10). constant of the solvent. The response of the electric cell, as a result of the PNP ϵ′ ϵ equations (eqs 1−3), can be expressed as the impedance Z(ω), hence the permittivity is equal to that of the solvent, ∞ = s, and or alternatively as a complex capacity C(ω) or a complex relative the conductivity is σ∞. In the limit of low frequency, ions of one permittivity ϵ(ω): type accumulate at one electrode. The flux of ions due to diffusion and the flux due to drift cancel, and an equilibrium ionic 1 d Z()ω == double layer is present at each electrode, which strongly i()iωωC ωAϵϵ () ω ϵ′ ϵ κ 0 (4) enhances the permittivity: ω→0/ s = d/2. with i =(−1)1/2, ω =2πf the angular frequency, A the surface area Above the double layer relaxation frequency, electrode polarization of order 1 or more (ϵ′/ϵ∞ − 1 ≥ 1) occurs with a of the electrodes, and d the distance between the electrodes. The − 23 fi ω 2 frequency dependence, as indicated in Figure 1 by the frequency response of the PNP model was rst derived, in a more − general form, by Macdonald,20 through linearization of the ion slope of 2. In the limit of high frequency, a very small contribution to the permittivity (ϵ′/ϵ∞ − 1 ≪ 1) is present with concentrations and assuming blocking electrodes and con- ω−3/2 23 servation of the number of ions (derivation in refs 20 and 22): a frequency dependence, though it is not visible in Figure 1. Experimental observations of strong electrode polarization with a − ϵ≡ϵ′−ϵ′′()ωω () i () ω ω 3/2 frequency dependence have on occasion been ascribed to the 1 2 PNP model, but this is incorrect, as was recently commented on by dϵsλ 24 = 2 Hollingsworth. 2 κ−1 ≪ (/)tanh(/2)i(/2)κλ λddD+ ω (5) The exact solution (eq 5) is, for d (which is valid for our data), to a very good approximation equal to the Debye ϵ′ ω ϵ″ ω − with ( ) and ( ), respectively, the real and imaginary parts relaxation function:25 28 of ϵ(ω) and κ−1 the Debye length, for a symmetric electrolyte: Δϵ −1 2 ϵ=ϵ+()ω s κ =ϵϵ0skT/(2 nq ) (6) 1i+ ωτ (7) Δϵ ϵ κ 25,26 τ and with 2n the average free ion number density and λ = with = s d/2 and the double layer relaxation time = (κ2 +iω/D)1/2. The frequency dependent conductivity is defined d/(2Dκ). The response of this Debye function can also be as σ(ω)=ωϵ ϵ″(ω). In the limit of high frequency, the produced by the equivalent electrical circuit shown in Figure 2b. 0 ϵ ϵ κ conductivity converges to a constant value σ∞. For a symmetric In this circuit, Cdl = A 0 s /2 accounts for the double layers at σ μ μ σ electrolyte, the conductivity ∞ =2nq i with i the ionic mobility both electrodes, R∞ = d/(A ∞) accounts for the electrolyte σ ϵ ϵ κ2 ϵ ϵ can conveniently be written as ∞ = 0 sD , where the Einstein conductivity, and C∞ = A 0 s/d accounts for the dielectric relation D = μ kT/q was used. Equation 5 is plotted in terms of response of the solvent. i 21 ϵ′(ω)/ϵ∞ and σ(ω)/σ∞ as a function of frequency in Figure 1 One empirical model in particular, an extension of the PNP model just presented, appears successful in describing our experimental data. It has been found useful before to fit electrode polarization data,29 and here, it is tested for a wider set of dielectric constants and ionic strengths. This model is, to a very good approximation, equal to an equivalent circuit (Figure 2c) of electrode polarization in which the double layer capacitance is replaced by a CPE with an impedance of the specific form −1 Cdl α ZCPE ()ω = (iωτ∞ ) τ∞ (8) with 0 < α ≤ 1 and the Maxwell−Wagner relaxation time τ∞ = ϵ ϵ σ R∞C∞ = 0 s/ ∞. The total impedance of the equivalent circuit (Figure 2c) is then given by ZR+ ω = CPE ∞ Figure 1. Theoretical response of a monovalent electrolyte with Z() − − − 1i++ωCZ ( R ) (9) diffusion coefficient D =1× 10 10 m2 s 1, Debye length κ 1 = 5 nm, and ∞∞CPE electrode spacing d = 100 μm according to the PNP model (eq 7) (solid and the corresponding complex permittivity, using eq 4, by line), or that of the empirical model (eq 10) with α = 0.75 (dashed line), or α = 0.5 (dotted line). Δϵ ϵ=ϵ+()ω s 1−α (iωτ )+ i ωτ (10) (solid lines), corresponding to the equivalent circuit in Figure 2a. ∞ In this representation, the absence of electrode polarization at In the case of α = 1, the CPE reduces to a double layer high frequencies is characterized by ϵ′(ω)/ϵ∞ = σ(ω)/σ∞ =1; capacitance, and the equivalent circuit reduces to that of the 11585 dx.doi.org/10.1021/jp5025476 | J.
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