Stern adsorption

It is apparent from the correlation between the rankings of monatomic ions and their placement in the periodic table that some systematic effect is responsible for this ordering. This was the focus of Section 11.8 on Stern adsorption. 

In summary, it is not difficult to formulate Stern adsorption isotherm equations. The main problem is to determine y, for which assumptions have to be made. 

Figure 21. A schematic diagram of the Stern adsorption layer (top) and the average potential profile of the Stern layer and Gouy-Chapman diffuse double layer.

The adsorption of ions is determined by the potential of the inner Helmholtz plane 0n while the shift of Epzc to more negative values with increasing concentration of adsorbed anions is identical with the shift in 0(m). Thus, the electrocapillary maximum is shifted to more negative values on an increase in the anion concentration more rapidly than would follow from earlier theories based on concepts of a continuously distributed charge of adsorbed anions over the electrode surface (Stern, 1925). Under Stern s assumption, it would hold that 0(m) = 0X (where, of course, 0X no longer has the significance of the potential at the inner Helmholtz plane). 

Gouy-Chapman, Stern, and triple layer). Methods which have been used for determining thermodynamic constants from experimental data for surface hydrolysis reactions are examined critically. One method of linear extrapolation of the logarithm of the activity quotient to zero surface charge is shown to bias the values which are obtained for the intrinsic acidity constants of the diprotic surface groups. The advantages of a simple model based on monoprotic surface groups and a Stern model of the electric double layer are discussed. The model is physically plausible, and mathematically consistent with adsorption and surface potential data. 

The representation of the data for TiC in terms of the monoprotic surface group model of the oxide surface and the basic Stern model of the electric double layer is shown in Figure 5. It is seen that there is good agreement between the model and the adsorption data furthermore, the computed potential Vq (not shown in the figure) is almost Nernstian, as is observed experimentally. 

Co/pH and V o/pH results are sensitive to different aspects of the surface chemistry of oxides. Surface charge data allow the determination of the parameters which describe counterion complexation. Surface potential data allow the determination of the ratio /3 —< slaDL- Given assumptions about the magnitude of the site density Ns and the Stern capacitance C t, this quantity can be combined with the pHp2C to yield values of Ka and Ka2. Surface charge/pH data contain direct information about the counterion adsorption capacitances in their slope. To find the equilibrium constants for adsorption, a plot such as those in Figures 7 and 8 can be used, provided that Ka and Kai are independently known from V o/pH curves. 

Newsome A, Stern R. Pilocarpine adsorption by serum and ocular tissues. Am J Ophthalmol 77 918-922 (1974). 

Adsorption and ElectroKlnetic Behavior of Rutile. Isotherms for the adsorption of lysine, prollne and glutamic acid on rutile (1102) are given in Figure 1. There is no simple relationship between the adsorption density and the equilibrium concentration. The adsorption does not obey the Langmiur, Freundllch or Stern-Grahame relationships. The leveling-off of the adsorption... 

Sterling Jr MC, Bonner JS, Page CA, Ernest ANS, Autenrieth RL (2003) Partitioning of crude oil polycyclic aromatic hydrocarbons in aquatic systems. Environ Sci Technol 37 4429-4434 Stern O (1924) Zur theorie der elecktrolytischen doppelschict. Z Electrochem 30 508-516 Stollenwerk KC, Grove DB (1985) Adsorption and desorption of hexavalent chromium in an alluvial aquifer near Telluride, Colorado. J Environ Qual 14 150-155 Stumm W, Morgan JJ (1996) Aquatic chemistry, 3rd edn. WUey, New York... 

The zeta potential is also modified by the ionic strength. When ionic strength increases, the absolute value of Zeta potential reduces. This observed phenomenon can be explained by both the presence of more counterions in the shear layer due to the decreasing double-layer thickness and to the increasing counterion adsorption into the stern layer. 

The specihc adsorption of counterions at the interface between the surface and the electrolyte solution results in a drastic variation of the charge density in the Stern layer, which reduces the zeta potential and hence the EOF. If the charge density of the adsorbed counterions exceeds the charge density on the surface, the zeta potential changes sign and the direction of the EOF is reversed. 

In Eq. 30, Uioo and Fi are the activity in solution and the surface excess of the zth component, respectively. The activity is related to the concentration in solution Cioo and the activity coefficient / by Uioo =fCioo. The activity coefficient is a function of the solution ionic strength I [39]. The surface excess Fi includes the adsorption Fi in the Stern layer and the contribution, f lCiix) - Cioo] dx, from the diffuse part of the electrical double layer. The Boltzmann distribution gives Ci(x) = Cioo exp - Zj0(x), where z, is the ion valence and 0(x) is the dimensionless potential (measured from the Stern layer) obtained by dividing the actual potential, fix), by the thermal potential, k Tje = 25.7 mV at 25 °C). Similarly, the ionic activity in solution and at the Stern layer is inter-related as Uioo = af exp(z0s)> where tps is the scaled surface potential. Given that the sum of /jz, is equal to zero due to the electrical... 

Regarding the adsorption isotherm, the Eriunkin isotherm is usually used for surfactant ions and the Stern isotherm for the coimterion adsorption in the Stern layer. For these isotherms, the following equations can be derived. 

In this combined approach, water does not have any contribution to the entropy of mixing. In addition, this model considers only one interaction coefficient p, which presents an average value for all the interactions in the adsorption and Stern layers, p is determined by the molecular interactions... 

Table 3 presents the results for the analysis of the homologue series of the alkyl sulfate surfactants. The maximum adsorption, Poo, increases, together with the increasing munber of carbon atoms in the hydrophobic tail. Consequently, there is an increase in the attraction forces the stronger attractions lead to smaller areas occupied by the surfactant ions. This increases the number of the counterion bindings (except the last homologue-tetradecyl sulfate). The model has not been able to best fit the data for tetradecyl sulfate in the presence as well as in the absence (A/r = 0) of a Stern layer. 

The adsorption of ionic surfactants creates an adsorption layer of surfactant ions, a Stern layer of counterions and a diffusive layer distributed by the electric field of the charged surface. Every layer has its own contribution to surface tension. For example, the adsorption of dodecyl sulfate (DS") ions from the sodium dodecyl sulfate solution is described by the modified Frumkin isotherm as... 

Stern (1924) introduced the concept of specific ion adsorption at surfaces. 

Finally, a brief overview of the structure of the inner edge of the double layer (the so-called Stern layer), which accounts for the preferential adsorption of ions and the finite size of the ions, is presented in Section 11.8, along with a discussion of how the developments in previous sections can be modified to accommodate the variation in the surface potential because of the Stern layer. 

The constant in Equation (94) is easily shown to be proportional to a Boltzmann factor in which the exponential energy consists of two contributions zei/ s, the electrical energy associated with the ion in the Stern layer, and , the specific chemical energy associated with the adsorption ... 

It is the outer portion of the double layer that interests us most as far as colloidal stability is concerned. The existence of a Stern layer does not invalidate the expressions for the diffuse part of the double layer. As a matter of fact, by lowering the potential at the inner boundary of the diffuse double layer, we enhance the validity of low-potential approximations. The only problem is that specific adsorption effects make it difficult to decide what value to use for J/6. 

In subsequent chapters it will be the potential in the diffuse double layer that concerns us. It can be described relative to its value at the inner limit of the diffuse double layer, which may be either the actual surface or the Stern surface. We continue to use the symbol p0 for the potential at this inner limit. It should be remembered, however, that specific adsorption may make this quantity lower than the concentration of potential-determining ions in the solution would indicate. We see in Chapter 12 how the potential at some (unknown) location close to this inner limit can be measured. It is called the zeta potential. 

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