Mixtures: adsorption from solutes

A mixture can often be separated into its components by utilising their selective adsorption from solution by a suitable substance, such as active alumina the separation can be readily followed if the components are coloured. 

Equation 10.27 is generally known as Freundlich equation. Equation 10.27 with concentration replaced by pressure was also used to describe the adsorption isotherms of gases on solids, suggesting the incorrect idea that adsorption from solution by a solid could be paralleled with gas or vapor adsorption on the same adsorbents. Whereas in some cases the restriction to dilute solutions was imposed by the solubility of solids (e.g., benzoic acid in water or stearic acid in benzene) it was not imposed on the investigation of mixtures of completely miscible liquids, e.g., acetic acid in water. 

Figure 15.7. Adsorption of liquid mixtures on charcoal. Chloroform + acetone and benzene + ethanol. The ordinate gives the amount of each individual substance that is adsorbed, the abscissa the mol fraction of chloroform (mixed with acetone) or the mol fraction of benzene (mixed with ethanol). (Data gathered by Kipling. Adsorption from Solutions of Non-Electrolytes, 1965).

A solution is t3q>lcally a system of more than one component. In actual cases, there are at least two substances that can adsorb. For a binary fluid mixture, including dilute solutions, adsorption of one type of molecule (say A) involves replacement of the other (B). Thus, adsorption from solution is essentially an exchange process. If one molecule of A replaces r molecules of B at the Interface, the adsorption equilibrium can be written as... 

Figures 2.21 and 2.22 refer to the adsorption of low molecular weight aliphatic alcohols from alcohol + benzene mixtures on montmorillonite. This adsorbent Is a so-called swelling clay mineral, meaning that it consists of packages of thin (aluminosilicate) layers that, under certain conditions, swell to give ultimately a dispersion of the individual sheets. Upon this swelling the specific surface area increases dramatically, it can readily reach several hundreds of m g" On adsorption from solution the swelling is determined by the extent to which one or both of the component(s) penetrate(s) between these sheets. In other words, we are dealing here with a non-inert adsorbent. The gas adsorption equivalent has been illustrated in fig. 1.30.

Adsorption from Solution at the Solid/Liquid Interface, G.D. Parfitt, C.H. Rochester, Eds., Academic Press (1983). (Contains chapters on adsorption of smEill molecules (G.D. Parfitt and C.H. Rochester), adsorption from mixtures of miscible liquids (J.E. Lane) and adsorption of non-ionic surfactants (J.S. Clunle,... 

Adsorption from liquid mixtures on solids underlies a number of extremely important processes and plays a significant role in many fields of the natural sciences. Studies of this phenomenon were reported in several monographies and reviews e.g. [1-3]. The formulation of a unified theory of adsorption from solutions is difficult because there are liquid mixtures with extremely different physicochemical properties. Usually, existing theoretical descriptions are limited to the following cases ... 

The most studies of adsorption from solution have been concerned with the adsorption from two-component mixture, for example [1,2], Practical use of adsorption however deals with the adsorption from multicomponent systems. In liquid chromatography in a many cases for the separation of mixture of solutes the multicomponent eluents are used. The most difficulties in the investigation of adsorption from multicomponent systems arise at the determination of some component concentration at once in equilibrium solution over the adsorbent. Moreover for the determination of adsorption isotherm in this case large experimental data are needed. 

The variant of hydrolysis PET calcium hydroxide interesting in that plan that both components in reactionary mixture are present in the manner of independent hard phases, but fluid phase needs for ensuring the mobilities reactionary mixture, as well as for carrying alkali on surface PET way to adsorptions from solution. Chemical reaction occurs on surfaces PET itself. 

The kinetics of adsorption from solutions of surfactant mixtures are described on the basis of a generalised Langmuir isotherm. The simultaneous adsorption leads to the replacement of less surface active compounds by those of higher surface activity, which are usually present in the bulk at much lower concentration. More general descriptions of the process are possible on the basis of the Frumkin and Frumkin-Damaskin isotherms, which include specific interfacial properties of the individual surface active species. Quantitative studies of such very complex models can be performed only numerically. 

No data on solutions are included either. Although there is considerable information in the literature on certain polymers, it is dependent on the particular choice of solvent and not amenable to systematic tabulation. The same is true of the wealth of adsorption from solution onto solids and spread film Langmuir trough data. Equations are available for calculating the surface tension of simple liquid mixtures that could be applied to polymers [14] and for calculating polymer solvent solution [15] surface tensions. 

A distinctive feature of adsorption from solution is that the surface is always completely covered and the sorption mechanism involves competition between the components of the liquid phase. Detailed theoretical interpretation of isotherms from liquid mixtures is most often based on binary liquid systems. The isotherms are measured by immersing the solid sample into the liquid phase. The concentration is monitored by UV, refractive index (RI), or other analytical methods. 

The analogy with adsorption from solution is readily demonstrated by considering as a first approximation Langmuir s theory applied to the adsorption of a gas component 1 from a mixture of gases (1 and 2) at a total pressure p chosen so that one of the components (say 2), when adsorbed from the pure... 

Although extensive data are available on vapour adsorption by zeolites, relatively little information is available on adsorption from solution. Among earlier papers dealing with this problem are those by Peterson and Redlich " who report somewhat fragmentary data on heptane + decane mixtures, with 5 A molecular sieve, and Sundstrom and Krautz who studied adsorption by 5 A molecular sieve of mixtures of lower alkanes (C7, Cw, C12, Cm), the alkane of lower mol. wt. being preferentially adsorbed. 

The molecular sieve properties of AC (Section 8.2.3) enable components of gas mixtures to be separated, the process utilizing differences in pore dimensions or differences in rates of adsorption (kinetic effects). The same general phenomenon of separation (the detail being different) is possible by adsorptions from solutions. Kononova et al. (2001) purified solutions of manganese sulfate from the contaminants of Zn(II), Cu(Il), Fe(lll) using AC from anthracite and brown coal (activated in steam 830-850 °C, reacted with air at 400-410 °C), and other cation exchangers. 

Studying adsorption from solution of polymer mixtimes is of great interest for the theory of PCM because many binders for composites are two-and more-component systems. The presence of two components determines the specificity of the properties of the boundary layers formed by two different polymeric molecules. From another point of view, as the large majority of polymer pairs is thermodynamically immiscible,there may arise interphase layers between two components in the border layer at the interface. The selectivity of adsorption of various components, which is a typical feature of adsorption from mixture, leads to the change in composition of the border layer as compared with composition in the equilibrium solution. This fact, in turn, determines the non-homogeneity in distribution of components in the direction normal to the solid surface, i.e., creates some compositional profile. As compared with stud3ung adsorption from solution of individual polymers, adsorption from mixture is studied insufficiently. The first investigations in this field were done " for immiscible pair PS-PMMA on silica surface, in conditions remote from the phase separation. It... 

Figure 1.10. Dependence ofPDMS adsorption from solution on C/C ratio (1), and from its mixtures with PS in ethyl acetate on C/C ratio at PS concentrations of 0.5 (2), 1.0 (3), and 2.5 g/100 ml (4). [Adapted by permission from Y. Lipatov, T. Todosijchuk, and V. Chornaya, J. Coll. Interface ScL, 155, 283 (1993)]...

Introduction of fillers into the reaction system may have an accelerating action during formation of both linear and crosslinked polymers because of redistribution of intra- and intermolecular interactions. This means that filler affects the very structure of the reaction mixture (solution or melt) in the same way as the formation of some aggregates or the development of entanglements affect the adsorption from solutions. These effects may be responsible for change in the reaction kinetics and changing the properties of the filled polymers. 

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