Adsorption distinguishing from surface

On the other hand, as applied to the submonolayer region, the same comment can be made as for the localized model. That is, the two-dimensional non-ideal-gas equation of state is a perfectly acceptable concept, but one that, in practice, is remarkably difficult to distinguish from the localized adsorption picture. If there can be even a small amount of surface heterogeneity the distinction becomes virtually impossible (see Section XVll-14). Even the cases of phase change are susceptible to explanation on either basis. 

The lateral interactions in the adsorbate can enhance or diminish the interaction energy in the surface. If the adsorption sites at the boundary between reconstructed and unreconstructed areas of surface are further distinguished from those inside these patches, we can introduce more interactions such as... 

When a precipitate separates from a solution, it is not always perfectly pure it may contain varying amounts of impurities dependent upon the nature of the precipitate and the conditions of precipitation. The contamination of the precipitate by substances which are normally soluble in the mother liquor is termed co-precipitation. We must distinguish between two important types of co-precipitation. The first is concerned with adsorption at the surface of the particles exposed to the solution, and the second relates to the occlusion of foreign substances during the process of crystal growth from the primary particles. 

Sposito, G. Distinguish adsorption from surface precipitation. In Geochemical Processes at Mineral Surfaces. J.A. Davis, K.F. Hayes, eds. Washington D.C. Americal Chemical Society Symposium Series No. 323,1986. 

Although valence band spectra probe those electrons that are involved in chemical bond formation, they are rarely used in studying catalysts. One reason is that all elements have valence electrons, which makes valence band spectra of multi-component systems difficult to sort out. A second reason is that the mean free path of photoelectrons from the valence band is at its maximum, implying that the chemical effects of for example chemisorption, which are limited to the outer surface layer, can hardly be distinguished from the dominating substrate signal. In this respect UPS, discussed later in this chapter, is much more surface sensitive and therefore better suited for adsorption studies. 

Sol id Sol utions. The aqueous concentrations of trace elements in natural waters are frequently much lower than would be expected on the basis of equilibrium solubility calculations or of supply to the water from various sources. It is often assumed that adsorption of the element on mineral surfaces is the cause for the depleted aqueous concentration of the trace element (97). However, Sposito (Chapter 11) shows that the methods commonly used to distinguish between solubility or adsorption controls are conceptually flawed. One of the important problems illustrated in Chapter 11 is the evaluation of the state of saturation of natural waters with respect to solid phases. Generally, the conclusion that a trace element is undersaturated is based on a comparison of ion activity products with known pure solid phases that contain the trace element. If a solid phase is pure, then its activity is equal to one by thermodynamic convention. However, when a trace cation is coprecipitated with another cation, the activity of the solid phase end member containing the trace cation in the coprecipitate wil 1 be less than one. If the aqueous phase is at equil ibrium with the coprecipitate, then the ion activity product wi 1 1 be 1 ess than the sol ubi 1 ity constant of the pure sol id phase containing the trace element. This condition could then lead to the conclusion that a natural water was undersaturated with respect to the pure solid phase and that the aqueous concentration of the trace cation was controlled by adsorption on mineral surfaces. While this might be true, Sposito points out that the ion activity product comparison with the solubility product does not provide any conclusive evidence as to whether an adsorption or coprecipitation process controls the aqueous concentration. 

A central problem in the chemistry of natural water systems is the establishment of experimental methods with which to distinguish adsorption from surface precipitation (1-3). Corey ( 2) has written a comprehensive review of this problem which should be read as an introduction to the present essay, particularly for his set of six conclusions that set out general conditions likely to result in adsorption or precipitation. The discussion to follow is not a comprehensive review, but instead focuses on three popular approaches to the adsorption/surface precipitation dichotomy. The emphasis here is on the conceptual relationship of each approach to the defining statements made above To what extent is an approach capable of distinguishing adsorption from surface precipitation ... 

SPOSITO Distinguishing Adsorption from Surface Precipitation... 

Solubility and kinetics methods for distinguishing adsorption from surface precipitation have the common features of being essentially macroscopic in nature and of not utilizing a direct examination of sorbed material. The essential difference between an adsorbate and a surface precipitate lies with molecular structure, however, and it is inevitable that methodologies not equipped to explore that structure directly will produce ambiguous results requiring ad hoc assumptions in order to interpret them. The principal technique for... 

Solubility and kinetics methods for distinguishing adsorption from surface precipitation suffer from the fundamental weakness of being macroscopic approaches that do not involve a direct examination of the solid phase. Information about the composition of an aqueous solution phase is not sufficient to permit a clear inference of a sorption mechanism because the aqueous solution phase does not determine uniquely the nature of its contiguous solid phases, even at equilibrium (49). Perhaps more important is the fact that adsorption and surface precipitation are essentially molecular concepts on which strictly macroscopic approaches can provide no unambiguous data (12, 21). Molecular concepts can be studied only by molecular methods. 

Abiotic, for example by adsorption of reactants onto mineral surfaces, distinguished from biotic catalysis by the absence of a temperature optimum. 

The property of adsorption from solutions of a particular solute is in general, apart from the fact that both solvent and solute are adsorbed (see p. 181), complicated by the fact that the adsorbing surface presented to the liquid is not uniform but broken up into a series of fissures or capillaries as is the case with solids such as charcoal and pumice or gels such as those of silica and alumina with the result that true equilibrium between solution and adsorbent may not result until after long periods of time, necessary for the intradiffusion of the solution into the absorbent during which period secondary chemical action may take place. For comparative purposes adsorption as distinguished from absorption or sorption (J. W. McBain, Phil. Mag. xvili. 6,1909) is considered to take place rapidly in solutions as well as in gases (see p. 123). 

Sposito, G. 1986. Distinguishing adsorption from surface precipitation. In Davis, J. A. Hayes, K. F. (eds) Geochemical Processes at Mineral Surfaces. American Chemical Society, Washington, DC, 217-228. 

Adsorption on solids is an important step in the industrially important process of heterogeneous catalysis. Adsorption, which takes place on the surface (including that of the pores) of the solid, should be distinguished from absorption, which occurs throughout its bulk. The latter is illustrated by the taking up of water by anhydrous calcium chloride. 

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