Reduced surface excess amount

The meaning of relative and reduced surface excess amounts.145... 

Another function invariant with respect to the position of the GDS and named the reduced surface excess amount was derived at by Defay (1941) in a similar way. Starting again from Equation (5.46), we can write the following two equations ... 

Here again the right-hand side does not depend on anything other than experimentally measurable quantities, and therefore we conclude that the right-hand side does not depend on the position of the GDS. This function is defined as the reduced surface excess amount of 2 ... 

The meaning of relative and reduced surface excess amounts Although the relative and the reduced surface excess amounts (or masses) do not depend on the position of the GDS, there is a special position of the GDS which cancels the last term of the defining equations ((5.48) and (5.55)) and gives a useful idea of the meaning of these two quantities. For Equation (5.48), this special position of the GDS is the one for which the surface excess amount n of component 1 is zero. We then get ... 

Adsorption isotherms expressed in reduced surface excess amounts... 

The principle of the null procedure (Nunn and Everett, 1983) is to restore, by injection of an appropriate dose of initial solution, the concentration prior to adsorption. This is done at each adsorption step. A refinement is to arrange that the sample cell is bypassed during the injection of a new dose of solution until the determination of a new amount concentration d2. The flow through is re-started and, as the concentration decreases, it is restored to its initial value d2 by addition of a volume A Va of a solution of amount concentration c2. The determination of the increase in reduced surface excess amount of 2 simply requires a knowledge of the void volume of the bypassed section and the concentration d 1 of the former adsorption equilibrium. Thus,... 

Both surface excess amounts must also remain unchanged this necessarily holds when using reduced surface excess amounts by the application of Equation (5.75), since nfn) = -nf"] (see Equation (5.10)) so that it is enough to maintain a constant nfn). This does not hold, however, when relative surface excess amounts or simple surface excess amounts are used. 

In Figure 5.17, for a given total amount of solute n2 introduced into the microcalorimetric cell, the line I gives the dependence of the reduced surface excess amount n2w on the equilibrium molality b2. In order to plot line I we first consider the extreme case of b2 = 0, that is all the solute is considered to be adsorbed so that... 

Just as for gas adsorption, we can define a partial differential enthalpy of adsorption of a component, A ads/i(, which would correspond to the adsorption, from a solution of molality b, of an infinitesimal amount of component i, dn, on a solid surface already covered with solute at a reduced surface excess concentration J (n) ... 

The quantity n° is the total amount of components 1 and 2 (n° = i + 2. in moles) offered per gram of adsorbent, and Axi indicates the change of the molar fraction of component 1 by adsorption. The specific reduced surface excess = /( i) represents the composite isotherm from which the true... 

When solid particles are dispersed in liquid medium, solid-liquid interfacial interactions will cause the formation of an adsorption layer, the so-called lyosphere, on their surface. The material content of the adsorption layer is the adsorption capacity of the solid particle, which may be determined in binary mixtures if the adsorption excess isotherm is known [45-50], Due to adsorption, the initial composition of the liquid mixture, x°, changes to the equilibrium concentration Xi. This change, x - Xi = Axi, can be determined by simple analytical methods. The relationship between the reduced adsorption excess amount calculated from... 

Scale formation Controlled scale deposition by the Langelier approach or by the proper use of polyphosphates or silicates is a useful method of corrosion control, but uncontrolled scale deposition is a disadvantage as it will screen the metal surfaces from contact with the inhibitor, lead to loss of inhibitor by its incorporation into the scale and also reduce heat transfer in cooling systems. Apart from scale formation arising from constituents naturally present in waters, scaling can also occur by reaction of inhibitors with these constituents. Notable examples are the deposition of excess amounts of phosphates and silicates by reaction with calcium ions. The problem can be largely overcome by suitable pH control and also by the additional use of scale-controlling chemicals. 

Adsorption of e.g. rosin (abietic acid) at the pigment surface may - depending on the concentration of the rosin - reduce or accelerate the crystal growth. The presence of an excess amount of rosin during the production of diarylide yellow pigments of the Pigment Yellow 13 type affords an additional crystal modification, which can be identified by X-ray diffraction spectroscopy [4],... 

The Mossbauer spectrum of ferrous Y-zeolite is somewhat similar to that of the reduced silica gel samples (103). The spectrum consists of two overlapping and partially resolved doublets with the inner doublet, 3 = 0.89 mm sec-1 and A = 0.62 mm sec-1, being attributed to the ferrous ion on the surface. In both the Y-zeolite and the reduced iron oxide on silica samples, the inner doublets representing surface ferrous states are the first to be affected by adsorption of polar molecules, but in the case of Y-zeolite the addition of excess amounts of water or ammonia causes the disappearance of the spectrum, and this has been interpreted in terms of "solvation of the ferrous ions by absorbate causing weakening of the bonding to the crystalline lattice. It is also possible that the spectrum is a composite representing a multiplicity of parameters. 

Since the reduced and relative surface excess isotherms convey composite information on the adsorption of the two components, there is a strong incentive to determine the individual (or separate ) isotherms, i.e. the adsorbed amount n (or ) versus concentration, mole fraction or mass fraction. It will be recalled that this implies some assumptions about the thickness, composition and structure of the adsorbed layer, and therefore is not to be recommended for reporting adsorption from solution data in a standard form. Indeed, this second step is already part of the theoretical interpretation of the adsorption mechanisms. 

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