Electrical double layer electrostatic adsorption

In 2003, Tsionsky, Daikhin, Urbakh, and Gileadi [21] published a very thorough treatment of the metal/solution interface as examined by the electrochemical quartz crystal microbalance, with emphasis on the misinterpretations of data that can occur if the basic physics and chemistry at the interface are not understood. Topics covered include the electrical double-layer/electrostatic adsorption, the adsorption of organic and inorganic species, metal deposition, and the influence of roughness on the response of the QCM in liquids. 

The deviations from the Szyszkowski-Langmuir adsorption theory have led to the proposal of a munber of models for the equihbrium adsorption of surfactants at the gas-Uquid interface. The aim of this paper is to critically analyze the theories and assess their applicabihty to the adsorption of both ionic and nonionic surfactants at the gas-hquid interface. The thermodynamic approach of Butler [14] and the Lucassen-Reynders dividing surface [15] will be used to describe the adsorption layer state and adsorption isotherm as a function of partial molecular area for adsorbed nonionic surfactants. The traditional approach with the Gibbs dividing surface and Gibbs adsorption isotherm, and the Gouy-Chapman electrical double layer electrostatics will be used to describe the adsorption of ionic surfactants and ionic-nonionic surfactant mixtures. The fimdamental modeling of the adsorption processes and the molecular interactions in the adsorption layers will be developed to predict the parameters of the proposed models and improve the adsorption models for ionic surfactants. Finally, experimental data for surface tension will be used to validate the proposed adsorption models. 

In some cases, e.g., the Hg/NaF q interface, Q is charge dependent but concentration independent. Then it is said that there is no specific ionic adsorption. In order to interpret the charge dependence of Q a standard explanation consists in assuming that Q is related to the existence of a solvent monolayer in contact with the wall [16]. From a theoretical point of view this monolayer is postulated as a subsystem coupled with the metal and the solution via electrostatic and non-electrostatic interactions. The specific shape of Q versus a results from the competition between these interactions and the interactions between solvent molecules in the mono-layer. This description of the electrical double layer has been revisited by... 

Ionic compounds such as halides, carboxylates or polyoxoanions, dissolved in (generally aqueous) solution can generate electrostatic stabilization. The adsorption of these compounds and their related counter ions on the metallic surface will generate an electrical double-layer around the particles (Fig. 1). The result is a coulombic repulsion between the particles. If the electric potential associated with the double layer is high enough, then the electrostatic repulsion will prevent particle aggregation [27,30]. 

At present it is impossible to formulate an exact theory of the structure of the electrical double layer, even in the simple case where no specific adsorption occurs. This is partly because of the lack of experimental data (e.g. on the permittivity in electric fields of up to 109 V m"1) and partly because even the largest computers are incapable of carrying out such a task. The analysis of a system where an electrically charged metal in which the positions of the ions in the lattice are known (the situation is more complicated with liquid metals) is in contact with an electrolyte solution should include the effect of the electrical field on the permittivity of the solvent, its structure and electrolyte ion concentrations in the vicinity of the interface, and, at the same time, the effect of varying ion concentrations on the structure and the permittivity of the solvent. Because of the unsolved difficulties in the solution of this problem, simplifying models must be employed the electrical double layer is divided into three regions that interact only electrostatically, i.e. the electrode itself, the compact layer and the diffuse layer. 

It is assumed that the quantity Cc is not a function of the electrolyte concentration c, and changes only with the charge cr, while Cd depends both on o and on c, according to the diffuse layer theory (see below). The validity of this relationship is a necessary condition for the case where the adsorption of ions in the double layer is purely electrostatic in nature. Experiments have demonstrated that the concept of the electrical double layer without specific adsorption is applicable to a very limited number of systems. Specific adsorption apparently does not occur in LiF, NaF and KF solutions (except at high concentrations, where anomalous phenomena occur). At potentials that are appropriately more negative than Epzc, where adsorption of anions is absent, no specific adsorption occurs for the salts of... 

The inhibition of electrode processes as a result of the adsorption of electroinactive surfactants has been studied in detail at catalytically inactive mercury electrodes. In contrast to solid metal electrodes where knowledge of the structure of the electrical double layer is small, it is often possible to determine whether the effect of adsorption on the electrode process at mercury electrodes is solely due to electrostatics (a change in potential 02)... 

A full understanding of adsorption requires that the interaction of a solute with a surface be characterized in terms of the fundamental physical and chemical properties of the solute, the sorbent and the solvent (water) (Westall, 1987). The adsorption reactions of importance in waters, sediments and soils are listed in terms of intermolecular reactions in Table. 4.1. The fundamental chemical interactions of solutes with the surfaces by formation of coordinative bonds were already discussed in Chapter 2. The electrostatic interactions and the electric double layer were considered in Chapter 3. 

Fig. 6-53. Interfadal charges, electron levels and electrostatic potential profile across an electric double layer with contact adsorption of dehydrated ions on semiconductor electrodes ogc = space charge o = charge of surface states = ionic charge due to contact adsorption dsc = thickness of space charge layer da = thickness of compact la3rer.

The main, currently used, surface complexation models (SCMs) are the constant capacitance, the diffuse double layer (DDL) or two layer, the triple layer, the four layer and the CD-MUSIC models. These models differ mainly in their descriptions of the electrical double layer at the oxide/solution interface and, in particular, in the locations of the various adsorbing species. As a result, the electrostatic equations which are used to relate surface potential to surface charge, i. e. the way the free energy of adsorption is divided into its chemical and electrostatic components, are different for each model. A further difference is the method by which the weakly bound (non specifically adsorbing see below) ions are treated. The CD-MUSIC model differs from all the others in that it attempts to take into account the nature and arrangement of the surface functional groups of the adsorbent. These models, which are fully described in a number of reviews (Westall and Hohl, 1980 Westall, 1986, 1987 James and Parks, 1982 Sparks, 1986 Schindler and Stumm, 1987 Davis and Kent, 1990 Hiemstra and Van Riemsdijk, 1996 Venema et al., 1996) are summarised here. 

Polymeric polyamines are also strongly adsorbed in the compact region of the electric double layer as a combination of multisite electrostatic and hydrophobic interactions. The adsorption results in masking the silanol groups and the other adsorption active sites on the capillary wall and in altering the EOF, which is lowered and in most cases reversed from cathodic to anodic. One of the most widely employed polyamine coating agents is polybrene (or hexadimetrine bromide), a linear hydrophobic polyquaternary amine polymer of the ionene type [129]. 

As the electrode surface will, in general, be electrically charged, there will be a surplus of ionic charge with opposite sign in the electrolyte phase in a layer of a certain thickness. The distribution of jons in the electrical double layer so formed is usually described by the Gouy— Chapman—Stern theory [20], which essentially considers the electrostatic interaction between the smeared-out charge on the surface and the positive and negative ions (non-specific adsorption). An extension to this theory is necessary when ions have a more specific interaction with the electrode, i.e. when there is specific adsorption of ions. 

Reaction 5.1 is meant to represent a nonspecific electrostatic interaction (presumably responsible for double-layer charge accumulation) Reaction 5.2 symbolizes specific adsorption (e.g., ion/dipole interaction) Reaction 5.3 represents electron transfer across the double layer. Together, these three reactions in fact symbolize the entire field of carbon electrochemistry electric double layer (EDL) formation (see Section 5.3.3), electrosorption (see Section 5.3.4), and oxidation/reduction processes (see Section 5.3.5). The authors did not discuss what exactly >C, represents, and they did not attempt to clarify how and why, for example, the quinone surface groups (represented by >CxO) sometimes engage in proton transfer only and other times in electron transfer as well. In this chapter, the available literature is scrutinized and the current state of knowledge on carbon surface (electrochemistry is assessed in search of answers to such questions. 

The potential in the diffuse layer decreases exponentially with the distance to zero (from the Stem plane). The potential changes are affected by the characteristics of the diffuse layer and particularly by the type and number of ions in the bulk solution. In many systems, the electrical double layer originates from the adsorption of potential-determining ions such as surface-active ions. The addition of an inert electrolyte decreases the thickness of the electrical double layer (i.e., compressing the double layer) and thus the potential decays to zero in a short distance. As the surface potential remains constant upon addition of an inert electrolyte, the zeta potential decreases. When two similarly charged particles approach each other, the two particles are repelled due to their electrostatic interactions. The increase in the electrolyte concentration in a bulk solution helps to lower this repulsive interaction. This principle is widely used to destabilize many colloidal systems. 

Selective adsorption of ions on an electrode surface contributes to the formation of an electrical double layer. This layer is due to the electrostatic forces of adsorption between the charge of the electrode and the ions of opposite charge present in the solution. The electrical double layer generates a difference in potential and forms an electric condenser as a first approximation. 

Cantwell and co-workers submitted the second genuine electrostatic model the theory is reviewed in Reference 29 and described as a surface adsorption, diffuse layer ion exchange double layer model. The description of the electrical double layer adopted the Stem-Gouy-Chapman (SGC) version of the theory [30]. The role of the diffuse part of the double layer in enhancing retention was emphasized by assigning a stoichiometric constant for the exchange of the solute ion between the bulk of the mobile phase and the diffuse layer. However, the impact of the diffuse layer on organic ion retention was danonstrated to be residual [19],... 

When the electrostatic stabilization of the emulsion is considered, the electrolytes (monovalent and divalent) added to the mixture are the major destabilizing species. The zeta potential of the emulsion particles is a function of the concentration and type of electrolytes present. Two types of emulsion particle-electrolyte (ions) interaction are proposed non-specific and specific adsorption.f H non-specific adsorption the ions are bound to the emulsion particle only by electrical double-layer interactions with the charged surface. As the electrolyte concentration is increased, the zeta potential asymptotes to zero. As the electrostatic repulsion decreases, a point can be found where the attractive van der Waals force is equal to the repulsive electrostatic force and flocculation of the emulsion occurs (Fig. 9A). This point is called the critical flocculation concentration (CFC). 

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