Adsorption negative

It must be emphasized that the definition of negative adsorption epitomized by Eqs. 3.50 and 3.51 is strictly macroscopic and does not depend in any way on the concepts of DDL theory applied in Sec. 1.4. If a DDL theory interpretation of Eq. 3.50 is desired, it can be developed through the definitions  

Equation 3.53 provides a molecular-level interpretation of Eq. 3.50. The exclusion volume, introduced in Eq. 1.9, is related to Eq. 3.53 through the macroscopic definition 

Equation 3,54 can be interpreted on the molecular level with the help of Eq. 3.53  

In conventional DDL theory applied to an electrolyte solution bathing solid particles with planar surfaces, Eq. 3.55 simplifies to 

As with electrokinetic phenomena, the existence of negative adsorption implies the existence of an electrified interface. The behavior of this interface toward charged particles can always be investigated with the help of a particular molecular model, such as DDL theory, but it is useful to see how much information can be obtained without a detailed model, in keeping wih the spirit of the previous sections in this chapter. Consider, for example, the application of thermodynamics to the prototypical two-chamber (experiment on negative adsorption. If the very small osmotic pressure created by the suspended soil clay is neglected, the activity of any electrolyte in the two chambers is the same in both the suspension and the supernatant solution  

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Most adsorption processes are exothermic (AH is negative). Adsorption processes involving nonspecific interactions are referred to as physical adsorption, a relatively weak, reversible interaction. Processes with stronger interactions (electron transfer) are termed chemisorption. Chemisorption is often irreversible and has higher heat of adsorption than physical adsorption. Most dispersants function by chemisorption, in contrast to surfactants, which... 

According as is = 0 there will be positive, zero, or negative adsorption, respectively. 

The adsorption equation shows that a solute may very strongly lower the surface tension of a solvent, but cannot strongly raise it, since although T may reach high values by positive adsorption (in some cases, as with solutions of some aniline dyes, the pure solute appears as a thin skin on the surface), it can never sink below that of the pure solvent by negative adsorption. 

Permeation When a fluid contacts one side of an elastomer membrane, it can permeate right through the membrane, escaping on the far side. The process again combines adsorption and diffusion as above, but with the additional process eventually of evaporation—treated mathematically as negative adsorption. (Permeation could also be viewed as combining one-way absorption and evaporation.) Wherever these conditions for permeation exist the phenomenon occurs, whatever the shape of the elastomer barrier— but the associated mathematics becomes complex for irregular barrier shapes. 

When the two phases in contact are condensed phases and the entire volume is taken up by incompressible substances, positive adsorption of one component must be attended by negative adsorption (desorption) of other components. This phenomenon is called adsorptive displacement. 

When the Gibbs equation is used for an electrode-electrolyte interface, the charged species (electrons, ions) are characterized by their electrochemical potentials, while the interface is regarded as electroneutral that is, the surface density, 2, of excess charges in the metal caused by positive or negative adsorption of electrons ... 

The second terms on the right-hand side of Eqs. (4)-(6), however, are not always negligible. The negative adsorption, i.e., the depletion, of nonsurface active ions is associated with the contribution of the second term, making the value on the left-hand side negative [31]. In this case, the interfacial tension at the potential of zero charge, pzc. 

Furthermore, one often finds that best fits of data may give rise to negative adsorption equilibrium constants. This result is clearly impossible on the basis of physical arguments. Nonetheless, reaction rate models of this type may be entirely suitable for design purposes if they are not extrapolated out of the range of the experimental data on which they are based. 

The impedance factor is strictly empirical, accounting primarily for the geometry of the soil pore network bnt also for ion exclusion by negative adsorption from narrow pores, and for the increased viscosity of water near charged surfaces. It is similar for all simple ions and molecules. It can be measured by following the self diffusion of a nonadsorbed ion, such as Cl , for which C = 0lCl and hence D =... 

Negative adsorption occurs when a charged solid surface faces an ion in an aqueous suspension and the ion is repelled from the surface by Coulomb forces. The Coulomb repulsion produces a region in the aqueous solution that is depleted of the anion and an equivalent region far from the surface that is relatively enriched. Sposito (1984) characterized this macroscopic phenomenon through the definition of the relative surface excess of an anion in a suspension, by... 

The film thickness calculated from F would appear to vary with the concentration of the salt as indicated by the following data in which the negative adsorption in gm. mols per sq. cm. and the thickness of the layer r in A. are given. 

The study of the interfacial liquid-liquid phase however is complicated by several factors, of which the chief is the mutual solubility of the liquids. No two liquids are completely immiscible even in such extreme cases as water and mercury or water and petroleum the interfacial energy between two pure liquids will thus be affected by such inter-solution of the two homogeneous phases. In cases of complete intersolubility there is evidently no boundary interface and consequently no interfacial energy. On addition of a solute to one of the liquids a partition of the solute between all three phases, the two liquids and the interfacial phase, takes place. Thus we obtain an apparent interfacial concentration of the added solute. The most varied possibilities, such as positive or negative adsorption from both liquids or positive adsorption from one and negative adsorption from the other, are evidently open to us. In spite of the complexity of such systems it is necessary that information on such points should be available, since one of the most important colloidal systems, the emulsions, consisting of liquids dispersed in liquids, owe their properties and peculiarities to an extended interfacial phase of this character. 

Williams Trans. Farad. Soo. I. 1914) bas shown that on the assumption that both solvent and solute are adsorbed by the adsorbing agent we may obtain positive, zero or negative adsorption as the solute is adsorbed more strongly, equally or less strongly than the solvent and that as an alteration in concentration of the solution takes place the adsorption may pass through all these separate phases. 

Or, in general, positive adsorption rising to a maximum will be followed by zero and eventually negative adsorption, due to the fact that solvent and solute are both adsorbed. 

Similar results were obtained with magnesium sulphate whilst with ammonium chloride a maximum was found, this was not followed by negative adsorption over the concentration range examined. 

Osaka Mem. Coll. Sci. Kyoto Univ. vi. 257, 1915) has obtained positive adsorption in the case of sodium and potassium nitrates and for potassium bromide and iodide, and negative adsorption in the case of sodium and potassium sulphate as well as potassium iodide. 

The Motomura analysis in Figure 2 shows the effects of monolayer compression. As expected, compression causes some ejection of surfactant, particularly at low surfactant concentrations. There even appears to be negative adsorption at 0.6 mmol kg" but while this result is plausible in a qualitative sense, the depth from which surfactant would need to be excluded is unacceptably great. 

Figure 2.16. Calculated dissociative nitrogen ( ), carbon monoxide ( ), and oxygen ( ) chemisorption energies over different 3d transition metals plotted as a function of the center of the transition metal rf-bands. A more negative adsorption energy indicates a stronger adsorbate-metal bond. Reproduced from [32].

Surface tensions for the interface between air and aqueous solutions generally display one of the three forms indicated schematically in Figure 7.14. The type of behavior indicated by curves 1 and 3 indicates positive adsorption of the solute. Since dy/dc and therefore dy/d In c are negative, E must be positive. On the other hand, the positive slope for curve 2 indicates a negative surface excess, or a surface depletion of the solute. Note that the magnitude of negative adsorption is also less than that of positive adsorption. 

One of the most significant results is that negative adsorption of similions (their partial expulsion from the surface region), which has been recognized but not emphasized, even in the earliest days of Guoy-... 

Figure 1. Schematic Diffuse double layer formed as a result of anion adsorption, showing the effect of negative adsorption of similions on the bulk concentration. A. Initial electrolyte concentration. B. Final electrolyte concentration. C. Final concentration if there were no negative adsorption of similions...

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