Adsorption, preferential

The electrostatic repulsion between the colloids can also be strengthened by adsorption of polyelectrolytes with the same net charge as the colloids. Such adsorption has been observed experimentally by several groups [55,56]. Another example is adsorption of polyelectrolytes on clay particles and in Fig. 13 it is shown that more salt must be added to coagulate the clay particles when the polyelectrolyte concentration has been increased (except for very low concentrations of polyelectrolytes, which has been described above). The polyelectrolytes only adsorb on equally charged clay particles in the presence of salt [51]. There are many explanations to this phenomenon and one theory is that the adsorption preferentially takes place at edges of the clay particles and it has been found that the probability for adsorption is higher for short polymers [56]. 

Chemical probing of powder Probe fluid qO Parti ate phase Inverse gas chromatography Preferential adsorption with probe gases Electrokinetics Zeta potential and charge Surfactant adsorption Preferential adsorption with probe surfactants References Lloyd et al. (eds.), ACS Symposium Series 391, ACS, Washington, 1989. Aveyard and Haydon, An Introduction to the Principles of Surface Chemistry, Cambridge University Press, 1973. Shaw, Introduction to Colloid and Surface Chemistry, Butterworths Co. Ltd., 1983. 

Materials suitable for the production of lubricating oils are comprised principally of hydrocarbons containing from 25 to 40 carbon atoms per molecule, whereas residual stocks may contain hydrocarbons with 50-60 or more (up to 80) carbon atoms per molecule. The composition of a lubricating oil may be substantially different from that of the lubricant fraction from which it was derived, because wax (normal paraffins) is removed by distillation or refining by solvent extraction and adsorption preferentially... 

Adsorption Preferential adsorption of one component on to a solid surface Removal of water vapour by silica gel or molecular sieves. Odour removal... 

The use of molecularly smooth mica surfaces is a conceptual strength of the SFA. At the same time, it is a severe limitation since it is the only surface that can be directly studied. There have heen several approaches to overcome this limitation and allow the use of different surfaces. Evaporation of thin metal layers onto mica leads to increased roughness of the surfaces even when template stripping [192-194] is used. Most simple, but less controlled, is surface modification by adsorption, preferentially, of molecules that self-assemhle into monomolecular layers. Alternatively, chemical modification of the mica surfaces hy silanes can be achieved. Finally, mica can be... 

A detailed study by Grieser and co-workers [169] of the forces between a gold-coated colloidal silica sphere and a gold surface reveals the preferential adsorption of citrate ions over chloride to alter the electrostatic interaction. 

After reviewing various earlier explanations for an adsorption maximum, Trogus, Schechter, and Wade [244] proposed perhaps the most satisfactory one so far (see also Ref. 243). Qualitatively, an adsorption maximum can occur if the surfactant consists of at least two species (which can be closely related) what is necessary is that species 2 (say) preferentially forms micelles (has a lower CMC) relative to species 1 and also adsorbs more strongly. The adsorbed state may also consist of aggregates or hemi-micelles, and even for a pure component the situation can be complex (see Section XI-6 for recent AFM evidence of surface micelle formation and [246] for polymeric surface micelles). Similar adsorption maxima found in adsorption of nonionic surfactants can be attributed to polydispersity in the surfactant chain lengths [247], Surface-active impuri-... 

Preferential adsorption of one of the components may be used for the same purpose. Charcoal or silica gel may be employed to adsorb one of the constituents of an azeotrope in preference to the other. If the adsorbate is readily recoverable, the process will have practical applications. 

Adsorption Processes. Adsorption represents the second and newer method for separating and producing high purity PX. In this process, adsorbents such as molecular sieves are used to produce high purity PX by preferentially removing PX from mixed xylene streams. Separation is accomphshed by exploiting the differences in affinity of the adsorbent for PX, relative to the other Cg isomers. The adsorbed PX is subsequendy removed... 

After the second extraction/stripping cycle, the plutonium is concentrated by evaporation or by preferential adsorption (qv) on ion-exchange resins. As in the case for uranium, the newer faciHties, such as THORP, use only a single purification step. 

The adsorbed layer at G—L or S—L surfaces ia practical surfactant systems may have a complex composition. The adsorbed molecules or ions may be close-packed forming almost a condensed film with solvent molecules virtually excluded from the surface, or widely spaced and behave somewhat like a two-dimensional gas. The adsorbed film may be multilayer rather than monolayer. Counterions are sometimes present with the surfactant ia the adsorbed layer. Mixed moaolayers are known that iavolve molecular complexes, eg, oae-to-oae complexes of fatty alcohol sulfates with fatty alcohols (10), as well as complexes betweea fatty acids and fatty acid soaps (11). Competitive or preferential adsorption between multiple solutes at G—L and L—L iaterfaces is an important effect ia foaming, foam stabiLizatioa, and defoaming (see Defoamers). 

Gas chromatography, depending on the stationary phase, can be either gas—Hquid chromatography (glc) or gas—soHd chromatography (gsc). The former is the most commonly used. Separation in a gas—Hquid chromatograph arises from differential partitioning of the sample s components between the stationary Hquid phase adsorbed on a porous soHd, and the gas phase. Separation in a gas—soHd chromatograph is the result of preferential adsorption on the soHd or exclusion of materials by size. 

Preferential adsorption with probe gases Electi okinetics... 

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