Surface electrical potential

Symbol for the bond angle beween the C2 (that is, a) carbon and the adjacent side-chain carbon of an amino acid in a peptide or protein. 2. Symbol for surface electric potential. 3. A parameter associated with a distribution in statistics, more commonly referred to as chi-squared distributions ( -distribution). Chi-square is a sum of terms in which each term is a quotient obtained by dividing the square of the difference between the observed and the theoretical value of a quantity by the theoretical value. 

The potential of each channel may be composed of two potentials. One is an oxidation-reduction potential generating at the boundary surface between the Ag electrode and the lipid membrane. The other is a Donnan potential at the boundary between the lipid membrane and the aqueous medium or more generally a Gouy-Chapman electrical double-layer potential formed in the aqueous medium [24]. Figure 7 shows a potential profile near the lipid membrane. The oxidation-reduction potential would not be affected by the outer solution in short time, because the lipid membrane had low permeability for water. Then the measured potential change by application of the taste solution is mainly due to the change in the surface electrical potential. 

In the second step the charge arrives at the internal phase passing through the interface. The associated potential is known as the surface potential jump (also called surface potential, surface electrical potential, etc.). It is determined by dipoles aligned at the interface and by surface charges. It is not identical with the Volta potential difference (also sometimes called the surface potential) that has so far been used for the description of the electrical double layer. For the treatment of the electrical double layer, dipoles did not play a role. In particular in water, however, the aligned water molecules contribute substantially to the surface potential jump x- The Galvani potential, Volta potential, and surface potential jump are related by... 

Surface electric potential control (or surface charge control) of the rate of flocculation is possible for any adsorptive that forms a surface complex with suspended particles, as discussed in Section 6.1 and in Chapter 4 (cf. Table 4.2). Among these adsorptives for soil colloids are oxyanions, such as phosphate or oxalate, and transition metal cations. An expression analogous to Eq. 6.78 can be developed to define points of zero charge for any such adsorptive, as illustrated in Fig. 6.9.42... 

The colloidal system is divided into a number of Wigner-Seitz cells. A large particle of radius R is located at the centa of each cell, and a liquid atmosphae is associated with the particle. The atmosphere contains an electrolyte solution and small particles of radius Rs (Fig. 1). Each of the small particles has a charge ze, and the large particles have a surface electrical potential Rq, which is assumed to be negative in the calculations. 

Fig. 7 shows that the electrical potential first inaeases with the distance from the surface of a large particle and then deaeases. The magnitude of the change of the electrical potential is, howeva, small, less than 10% of the surface electrical potential in the case examined. 

Figs. 4 and 5 present the chain surface electrical potentials surface charge density a at various electrolyte concentrations, and pll0 = 7, respectively. For the conditions employed, the charge of the polyelectrolyte... 

Fig. 4. The chain surface electrical potential at various electrolyte concentrations. The distance between the two plates is 100 A. The other parameters are as in Fig. 2.

The electrostatic free energy contribution is the sum of the electrostatic energy of the solution and the chemical contribution of the surface, if the potential determining ions are in thermodynamic equilibrium with those in solution. At constant surface electrical potential Pq, the chemical contribution is given by... 

Equation 3.7 points out that the variation in the electric field strength (-dy/dx) is related to the second power of the inverse of the thickness of the double layer times /, while Equation 3.8 shows that / decays exponentially with respect to distance (jc) from the surface (Fig. 3.23). A plot of ln( // (/0) versus x produces a straight line with slope k, which is the inverse of the double layer thickness. The assumption ij/0 < 25 mV is not applicable to all soil minerals or all soils. Commonly, clay minerals possess more than 25 mV in surface electrical potential, depending on ionic strength. The purpose of the assumption was to demonstrate the generally expected behavior of charged surfaces. 

Equation 3.11 shows that the surface charge is directly related to the surface electrical potential ( j/0) and number of ion pairs per cubic centimeter (n) or ionic strength (/) Note that the only terms in Equation 3.6 that are variable are the valence and concentration of the ions in the bulk solution. In further dissecting Equation 3.6, one finds that under constant temperature, 8Jtne2z2/ K is a constant and can be defined as A. Therefore,... 

Figure 3.26. Surface electrical potential as a function of distance from the surface of a constant charge or pH-dependent charge.

Equation 3.14 reveals that as surface electrical potential ( /0) increases, total surface charge (a) increases. The surface electrical potential /0 can be approximated by the Nernst equation ... 

Note that variably charged soil mineral surfaces do not exactly obey the Nernst equation. The assumption is only valid at approximately one pH unit above or belov the PZC or at approximately 25 mV of surface electrical potential. The assumption is only used for demonstration purposes. Since H+ and OH are considered to be potential determining ions (PDIs), Equation 3.15 can be rewritten as... 

Explain the relationship between variably charged soils and surface electric potential and the relationship between constant charge soils and surface electric potential. Define the role of potentially determining ions in variably and constant charged soils. Discuss the practical meaning of the above. 

Based on the classical double layer theory, name two assumptions that are now known not to be valid when one tries to predict ion adsorption on the basis of surface electrical potential. 

According to the Nernst equation, the surface electrical potential ( j/) can be approximated as a function of H+ activity or pH as follows ... 

Considering that surface coverage by a metal (-SM) is given in units of 0 (see box above), and 1-0 represents the uncovered surface (-S) (see Equation E), then based on Equation F and the pH-dependent surface electrical potential, j/ (Equation 4.27),... 

B. A surface that does not have the potential to form inner-sphere complexes prefers cations with higher valence. This preference depends on the magnitude of the surface-electrical potential. For example, a surface with high electrical potential shows lower preference for monovalent cations (e.g., Na+) in the presence of a divalent cation (e.g., Ca2+) than a surface with low electrical potential. 

C. Surfaces that exhibit pH-dependent electrical potential show various degrees of selectivity for the same cation. For example, kaolinite or kaolinitic soils at high pH (high negative surface electrical potential) shows increasing preference for divalent cations than monovalent cations, while at low pH (low negative electrical potential), kaolinite, or kaolinitic soils show increasing preference for monovalent cations than divalent cations. 

Colloid behavior in natural soil-water systems is controlled by dispersion-flocculation processes, which are multifaceted phenomena. They include surface electrical potential (El-Swaify, 1976 Stumm and Morgan, 1981), solution composition (Quirk and Schofield, 1955 Arora and Coleman, 1979 Oster et al., 1980), shape of particles, initial particle concentration in suspension (Oster et al., 1980), and type and relative proportion of clay minerals (Arora and Coleman, 1979). When suspended in water, soil colloids are classified according to their settling characteristics into settleable and nonsettleable solids. 

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