Adsorbed depletion

Fig. 2. STM image (78 A x 76 A) of nitrogen atom adsorbates on an Fe(l 00) surface. Because the nitrogen adsorbates deplete the LDOS at the Fermi level, the nitrogen atoms are imaged as depressions in accord with the Tersoff-Hamann model. From the STM image it is concluded that nitrogen atoms adsorb in fourfold hollow sites on Fe(l 0 0). This is just one of many examples illustrating how the STM contrast may depend on the details of the LDOS around an adsorbate and produce a somewhat counterintuitive picture. Adapted from Reference (dd).

The experimental e values of binary solvents in thin-layer systems often appear low because of solvent demixing. Preferential adsorption of B by the adsorbent depletes the advancing solvent front of B, leaving a solvent which is weaker than the original mixture. The seriousness of this effect can be estimated from the likelihood of extensive solvent demixing (see below). 

One of the manifestations of depletion effects in a colloidal dispersion is that its fluid structure is affected by the presence of non-adsorbing depletants (for instance polymer chains). This is reflected in the radial distribution function g r) the local concentration of particle centers from a distance r to a fixed particle center. Statistical mechanics links g(r) to the potential of mean force W f [90],... 

Fig. 4.1 Types of instability that occur after mixing a colloidal dispersion with a polymer solution. When the polymer chains do not adsorb depletion leads to partitioning of colloids and polymers over different phases (2). In case of adsorption (and low polymer concentrations) bridging between different particles can induce flocculation (1)...

Figure 4.10 shows that the ratio of flocculated to unflocculated droplets varies exponentially with the concentration of HEC. Data for similar emulsions of 20% alkane with the same droplet size distribution but containing xanthan are also shown. The same dependence on polymer concentration is observed, although the larger xanthan molecules act as an effective flocculant at lower concentration than HEC. Both are behaving as non-adsorbing (depletion) floccu-lants. 

A quantitative treatment for the depletive adsorption of iogenic species on semiconductors is that known as the boundary layer theory [84,184], in which it is assumed that, as a result of adsorption, a charged layer is formed. Doublelayer theory is applied, and it turns out that the change in surface potential due to adsorption of such a species is proportional to the square of the amount adsorbed. The important point is that very little adsorption, e.g., a 0 of about 0.003, can produce a volt or more potential change. See Ref. 185 for a review. 

The second case involves non-adsorbing polymer chains in solution. It was realized by Asakura aird Oosawa (AO) [50] aird separately by Vrij [51] tlrat tlrese chains will give rise to air effective attraction between colloidal particles. This is kirowir as depletion attraction (see figure C2.6.4. We will summarize tire AO tlreory to explain tlris. 

In section C2.6.4.3 it was shown how tlie addition of non-adsorbing polymer chains induces a depletion attraction between colloidal particles. If sufficient polymer is added, tliese attractions can be strong enough to induce a phase separation of tire colloidal particles. An early application of tliis was tire creaming of mbber latex [93]. 

Rust inhibitors usually are corrosion inhibitors that have a high polar attraction toward metal surfaces and that form a tenacious, continuous film which prevents water from reaching the metal surface. Typical mst inhibitors are amine succinates and alkaline-earth sulfonates. Rust inhibitors can be used in most types of lubricating oils, but factors of selection include possible corrosion of nonferrous metals or formation of emulsions with water. Because mst inhibitors are adsorbed on metal surfaces, an oil can be depleted of its mst inhibitor. In certain cases, it is possible to correct the depletion by adding more inhibitor. 

Adsorption generally gives one highly pure product (adsorbate) and one product depleted in adsorbate. 

The escaping bubbles from the top of a bubble-fractionation column can carry off an appreciable quantity of adsorbed material in an aerosol of very fine film drops [various papers, J. Geophys. Res., Oceans Atmos., 77(27), (1972)]. If the residu solute is thus appre-ciablv depleted, Cj in Eq. (22-57) should be replaced with the average residual concentration. 

Exhaustion The state in which the adsorbent is no longer capable of useful ion exchange the depletion of the exchanger s supply of available ions. The exhaustion point is determined arbitrarily in terms of (1) a value in parts per million of ions in the effluent solution and (2) the reduction in quality of the effluent water determined by a conductivity bridge which measures the resistance of the water to the flow of an electric current. 

Adsorptive Properties. Substances such as silica gel and activated charcoal can be used to collect (adsorb) certain solids from solution. The adsorber bed may be discarded when depleted or recycled by washing and heating. 

A non-adsorbing polymer in solution can also destabilise a dispersion through a mechanism called depletion flocculation. When polymer molecules do not interact favourably with the particle surfaces from an enthal-pic perspective, they are repelled from the surface regions due to entropic reasons. A depletion zone around the particles is created which has a lower average polymer concentration than the bulk solution. The osmotic... 

In case of small density of adsorbed particles if contrasted to the density of charged BSS the adsorption of donors can be accompanied by non-monotonous kinetics change in 4s t) which is caused by fast ASS depletion with subsequent slow BSS recharging (see Fig. 1.10, curve J). The use of typical values of parameters in absorbate-adsorbent systems shows that depletion of donor levels is characterized by the times of the order of seconds whereas the relaxation of charge in BSS takes hours. 

At temperatures of the order 700 - 900 K the surface point defects play the dominant role in controlling the various eledrophysical parameters of adsorbent on the content of ambient medium [32]. As it has been mentioned in section 1.6, these defects are being formed in the temperature domain in which the respective concentration of volume defects is very small. In fact, cooling an adsorbent down to room temperature results violation of uniform distribution due to redistribution of defects. The availability of non-homogeneous defect distribution led to creation of a new model of depleted surface layer based on the phenomenon of oxidation of surface defects [182] which is an alternative to existing model of the Shottky barrier [183]. 

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