Electrolytes, double-layer properties

Non-situ and ex situ studies can provide important information for understanding the properties of metal/electrolyte interfaces. The applicability of these methods for fundamental studies of electrochemistry seems to be firmly established. The main differences between common electrochemical and UHV experiments are the temperature gap (ca. 300 vs. 150 K) and the difference in electrolyte concentration (very high concentrations in UHV experiments). In this respect, experimental research on double-layer properties in frozen electrolytes can be treated as a link between in situ experiments. The measurements of the work functions... 

When we discussed the double-layer properties of metal electrodes in contact with an electrolyte solution, we introduced the notion of an ideally polarizable interface, which is marked by the absence of charge-... 

Davis, J. A., James, R. O. and Leckie, J. O. (1978). Surface ionization and complexation at the oxide/water interface. I. Computation of electrical double layer properties in simple electrolytes, J. Coll. Inter/. Sci., 63, 480-499. 

From the preceding contributions in this volume it is evident that the techniques of modelling the electrical double layer properties at the oxide/electrolyte interface have been well developed (2, 11). However, the problem still contains a certain amount of " art form in the sense that there is more than one school of thought as to how the various modelling techniques should be applied. 

Since the metal can be treated as a nearly perfect conductor, C is high compared with C, and cannot influence the value of the measured doublelayer capacitance. The role of the metal in the double layer structure was discussed by Rice, who suggested that the distribution of electrons inside the metal decides the properties of the double-layer. This concept was later used to describe double-layer properties at the semiconductor/electrolyte interface. As shown later, the electron density on the metal side of the interface can be changed under the influence of charged solution species (dipoles, ions). ... 

The charge density, Volta potential, etc., are calculated for the diffuse double layer formed by adsorption of a strong 1 1 electrolyte from aqueous solution onto solid particles. The experimental isotherm can be resolved into individual isotherms without the common monolayer assumption. That for the electrolyte permits relating Guggenheim-Adam surface excess, double layer properties, and equilibrium concentrations. The ratio u0/T2N declines from two at zero potential toward unity with rising potential. Unity is closely reached near kT/e = 10 for spheres of 1000 A. radius but is still about 1.3 for plates. In dispersions of Sterling FTG in aqueous sodium ff-naphthalene sulfonate a maximum potential of kT/e = 7 (170 mv.) is reached at 4 X 10 3M electrolyte. The results are useful in interpretation of the stability of the dispersions. 

Electrical double layer properties at the solid/electrolyte solution interface were analyzed by potentiometric titration and electrophoresis measurements. Potentiometric titration and electrokinetic measurements were performed for three different concentrations 1 x 10 3, 1 x 10 2, and 1 x 10 1 M of NaClCXt solutions. The initial concentrations of Cd(II) and oxalate or citrate ions were 1 x 10 6, 1 x 105, 1 x 10 4, and 1 x 10 3 M, respectively. Double distilled water was used to prepare all solutions. All reagents used for experiments were analytical grade. 

J. A. Davies, R. O. James, and J. O. Leckie, Surface Ionization and Complexation at the Oxide/Water Interface. I. Computation of Electrical Double Layer Properties in Simple Electrolytes, J. Colloid Interface Sci. 63, 480-499 (1978). 

In this chapter the structure of the electrolyte double layer, and the consequences of adsorption on the electrode surface, are described. The effect of differences in structure and electronic distribution of different metals are indicated. The space-charge region in semiconductors is then discussed. Finally some properties of colloids are mentioned, given that they possess an interfacial region very similar to an electrode. 

In the study of impedance plots, we may observe the depression of semicircles. This is the so-called semicircle rotation of the impedance. This phenomenon is associated with electrode/electrolyte interface double-layer properties. For example, the rough surface of the electrodes or porous electrodes can result in an uneven distribution of the double-layer electric field. This semicircle rotation can be explained using the equivalent circuit presented in Figure 3.10, where R is inversely proportional to the frequency CO (and b is a constant). 

Even though this equation is based on an overly simple model, it illustrates well the double-layer properties that govern the electrosorption valency in the absence of pet. In particular, it shows that a fractional value of / need not necessarily indicate pet. We shall return to the hard-sphere electrolyte model when we discuss dipole moments of adsorbates. 

Computation of electrical double layer properties in simple electrolytes. J. Coll. Interface Sci. (3), 480-499 (1978). 

The present section concerns double layers In liquids with such low dielectric permittivities that dissociated electrolytes are all but absent. Under such conditions, the charge formation and double layer properties obey rules that differ quantitatively. If not qualitatively from their aqueous counterparts. Apart from these academic Issues such "apolar" double layers are relevant for a number of applications Including several types of paints, emulsions, lacquers and cosmetics. 

Lyons, J.S., Furlong, D.N., and Healy, T.W.. The electrical double-layer properties of the mica (muscovite)-aqueous electrolyte interface, Aust. J. Chem., 34, 1177, 1981. 

Double-layer properties of porous carbon materials have been widely investigated in relation to the development of the electrochemical capacitors. For detailed information the reader should consult specialized literature. For porous carbons materials, the double-layer capacitance depends on their specific snrface area [82,83], pore stmcture (notably, the pore size distribntion) [84-87], and their crystalline stmctnre and snrface chemistry [83,88,89], Shi [84] measnred the dc capacitance of varions carbons in a KOH electrolyte and noticed that the overall capacitance may reasonably be described as a sum of the capacitance of micro- and mesopores. Assuming that the electrical double layer propagates into micropores accessible for N2 adsorption, the author estimated the differential donble-layer capacitance per unit of micropore surface area as 15 to 20 p,F/cm. Lower values were reported by Vilinskaya... 

The method developed here for the description of chemical equilibria including adsorption on charged surfaces was applied to interpret phosphate adsorption on iron oxide (9), and to study electrical double-layer properties in simple electrolytes (6), and adsorption of metal ions on iron oxide (10). The mathematical formulation was combined with a procedure for determining constants from experimental data in a comparison of four different models for the surface/solution interface a constant-capacitance double-layer model, a diffuse double-layer model, the triplelayer model described here, and the Stem model (11). The reader is referred to the Literature Cited for an elaboration on the applications. 

D. E. Yates, S. Levine, and T. W. Healy, Site-binding model of the electrical double layer at the oxide/water interface, J.C.S. Faraday I 70 1807 (1974). J. A. Davis, R. O. James, and J. O. Leckie, Surface ionization and complexation at the oxide/water interface. I Computation of electrical double layer properties in simple electrolytes, J. Colloid Interface Sci. 63 480 (1978). J. A. Davis and J. O. Leckie, Surface ionization and complexation at the oxide/water interface. II Surface properties of amorphous iron oxyhydroxide and adsorption of metal ions, J, Colloid Interface Sci, 67 90 (1978). 3 Adsorption of anions, J. Colloid Interface Sci. 74 32 (1980). 

---