Depletion = negative adsorption

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. 

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... 

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. 

Adsorption — Adsorption is the enrichment (positive adsorption, or briefly, adsorption) or depletion (negative... 

Negative adsorption is in most cases very small compared to positive adsorption and therefore not easily detected directly. A sophisticated optical technique (evanescent-wave-induced fluorescence, EWIF) has been used to prove the reduction in segment concentration close to a non-adsorbing surface Depletion also has an effect on the flux of polymer solutions through pores since the viscosity of the liquid near the non-adsorbing surface is lower than that of the polymer solution, the flux is then higher than would be expected on the basis of the bulk viscosity. Negative adsorption at a liquid-air interface leads to a measurable increase In surface tenslon ... 

Adsorption The increase in quantity of a component at an interface or in an interfadal layer. In most usage, it is positive, but it can be negative (depletion) in this sense, negative adsorption is a different process from desorption. Adsorption may also denote the process of components accumulating at an interface. 

Figure 9.7 Origin of depletion flocculation. Polymer molecules are excluded from a volume V and an area A between the particles at a separation of H. In this region there is a negative adsorption given by —r,K. As H decreases, the adsorption becomes less negative (i.e. increases) and an attractive force results.

Negative adsorption, such as the depletion of the dissolved substance at the boundary of a solution, may occur as well as positive adsorption. The condition is that the surface layers of uncontaminated solvent should be more stable than layers into which solute penetrates. This might at first sight appear to imply also the condition of insolubility. But the boundary layers of a solvent differ quite a lot from the bulk phase in that they are often oriented and possess... 

In the case of like-charged polymers and surfaces, polymers are depleted from the surface (negative adsorption) unless non-electrostatic interactions aid adsorption (typically hydrophobic attraction in aqueous media). In such a case, adsorption increases with increasing salt concentration, due to the screening of repulsive electrostatic interactions between the polymer and the surface. 

The presence of nonadsorbing polymer molecules in the solution gives rise to a layer adjacent to the plates that is depleted of these polymer molecules in other words, the polymer is negatively adsorbed. When the plates approach down to a separation distance where the depletion layers overlap, the negative adsorption decreases dP/d/t < 0, which implies an attractive force leading to so-called depletion adhesion (cf Section 16.3.1). 

The nonadsorbing polymer molecules introduce a depletion zone adjacent to the surfaces, having a thickness of 25 nm (=/ g). Depletion implies negative adsorption, T(=c x 2R ) < 0, for the two surfaces. 

One can characterize the negative adsorption by the depletion layer thickness 6, which is defined as... 

Generally, the term adsorption is used to describe accumulation, i.e. a concentration rise close to a surface, or to an interface between neighbouring phases. In principle, the opposite of accumulation, i.e. the negative adsorption (depletion) of substances at a surface, also exists. However, this case is considered infrequently. 

The amount of anions adsorbed cannot be directly measured by the depletion in solution. Repulsion of the anions by the high density of negative surface charges (negative adsorption [57—59]) increases the anion concentration in the bulk and reduces the depletion by the (positive) adsorption. 

Metal coordination to the porphyrin negatively affects the interaction with the nanotubes, with adsorption depleting according to the metal used [77]. 

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