Interfacial excess

An adsorbed layer of water molecules at the interface separates hydrated ions from the solid surface. The interfacial electric double layer can be represented by a condenser model comprising three distinct layers a diffuse charge layer in the ionic solution, a compact layer of adsorbed water molecules, and a diffuse charge layer in the solid as shown in Fig. 5-8. The interfacial excess charge on the... 

Fig. 6-12. Interfacial excess charge in an compact double layer com-prising adsorbed water molecules and hydrated ions = potential of the compact layer (HL).

The interfacial excess charge of semiconductor electrodes consists of the space charge osc on the semiconductor side the charge of surface states o the charge Oh of interfacial hydroxyl groups the charge Oms of adsorbed ions in the compact... 

Figure D3.5.4 Interfacial excess concentration of a two-phase system containing surface-active material. The concentration of surfactant at the interface is larger than the concentration within either of the two bulk phases.

With Eq. (3.6) it is possible to define something like a surface concentration, the so called interfacial excess ... 

A is the interfacial area. The interfacial excess is given as a number of molecules per unit area (m 2) or in mol/m2. 

Until now we have considered the total energy quantities of the system. Now we turn to the interfacial excess quantities. We start with the internal interfacial or internal surface energy... 

There are two common and widely used definitions of the interfacial excess enthalpy. We can argue that enthalpy is equal to the internal energy minus the total mechanical work 7A-PVa. Since in the Gibbs convention PVa = 0 we define... 

What is the interfacial excess Gibbs energy The difference between IJ[Pg.34]

The choice of the ideal interface in the Gibbs adsorption isotherm (3.52) for a two-component system is, in a certain view, arbitrary. It is, however, convenient. There are two reasons First, on the right side there are physically measurable quantities (a, 7, T), which are related in a simple way to the interfacial excess. Any other choice of the interface would lead to a more complicated expression. Second, the choice of the interface is intuitively evident, at least for ci > C2. One should, however, keep in mind that different spatial distributions of the solute can lead to the same T. Figure 3.6 shows two examples of the same interfacial excess concentration In the first case the distribution of molecules 2 stretches out beyond the interface, but the concentration is nowhere increased. In the second case, the concentration of the molecules 2 is actually increased. 

Figure 3.6 Examples of two different concentration profiles leading to the same interfacial excess concentration I )1 ...

To apply the thermodynamic formalism to surfaces, Gibbs defined the ideal dividing plane which is infinitely thin. Excess quantities are defined with respect to a particular position of the dividing plane. The most important quantity is the interfacial excess which describes the amount of substance enriched or depleted at an interface. 

The first term refers to the electrolyte. Accordingly, the sum runs over all ion types present in the electrolyte. The second term contains the contribution of the electrons in the metal. T and Te are the interfacial excess concentrations of the ions in solution and of the electrons in the metal, respectively, /x is the chemical potential of the particle type i, Fa is Faradays constant, and /x is the electrochemical potential of the electrons. Substitution leads to... 

The interfacial tension decreases with increasing amount of surface potential. The reason is the increased interfacial excess of counterions in the electric double layer. In accordance with the Gibbs adsorption isotherms, the interfacial tension must decrease with increasing interfacial excess. At charged interfaces ions have an effect similarly to surfactants at liquid surfaces. 

An adsorption isotherm is a graph of the amount adsorbed versus the pressure of the vapor phase (or concentration in the case of adsorption from solution). The amounts adsorbed can be described by different variables. The first one is the surface excess I in mol/m2. We use the Gibbs convention (interfacial excess volume Va = 0). For a solid surface the Gibbs dividing plane is localized directly at the solid surface. Then we can convert the number of moles adsorbed Na to the surface excess by... 

For a compound to be qualified as a surfactant, it should also exhibit surface activity. It means that when the compound is added to a liquid at low concentration, it should be able to adsorb on the surface or interface of the system and reduce the surface or interfacial excess free energy. The surface is a boundary between air and liquid and the interface is a boundary between two immiscible phases (liquid-liquid, liquid-solid and solid-solid). Surface activity is achieved when the number of carbon atoms in the hydrophobic tail is higher than 8 [3]. Surfactant activities are at a maximum if the carbon atoms are between 10 and 18 at which level a surfactant has good but limited solubility in water. If the carbon number is less than 8 or more than 18, surfactant properties become minimal. Below 8, a surfactant is very soluble and above 18, it is insoluble. Thus, the solubility and practical surfactant properties are somewhat related [1]. 

Here we denote the solution or ambient phase as phase 1, the electrode as phase 2, and the new phase as phase 3. The three interfacial excess free energy densities between them are y12, yu, and y23, and a is the steady state rate of nucleation. In addition, o is a constant, R is the - gas constant, T is the absolute temperature, n is the number of electrons in the total reaction, F is the - Faraday constant, pm is the molar density of the depositing phase, and rj is the overpotential. 

Analogously we have to proceed if we are measuring in the orthogonal direction the results will be completely different (space charge profile, core effects). Even though the overall effect can be obtained more precisely by a superposition of bulk values and interfacial excess contributions, it is more convenient for large space... 

Is friction electrochemical also At least on moist surfaces, the distance between surface promontories—the protrusions of the metal-metal contacts—is controlled by the repulsion of like charges from ions adsorbed from electrolyte-containing moisture films onto surfaces. Indeed, if a pendulum swings on a fulcrum containing a metal-metal contact, its rate of decay (which is increased by the friction of the contact) maximizes when the interfacial excess electrical charge is a minimum the friction therefore is a maximum (because the metal contacts, unrepelled by charges, are in closer contact). 

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