Applications of Adsorption from Solution

No discussion of adsorption from solution is anywhere near complete unless it includes some indication of its enormous practical applicability. As a matter of fact, the examples we briefly consider —detergency and flotation —encompass a wide variety of concepts from almost all areas of surface and colloid chemistry. We have chosen to stress principles rather than applications, however, so these subjects will receive an amount of attention that belies their actual importance. Following these traditional applications, two examples of new applications that are envisioned for surfactant layers deposited on solid substrates are discussed. 

It is impossible to do justice to the complex phenomena of detergency and flotation in a few paragraphs. All we can do is point out some of the ways in which the principles of colloid and surface chemistry apply in these areas. There are several ways in which detergency and flotation phenomena resemble one another  

A third point of resemblance between detergency and flotation (perhaps redundant in view of what has already been said) is that both have developed largely by empirical research with (partially satisfactory) explanations trailing far behind the actual practice. 

The process of removing an oily drop from a solid substrate may be described in terms of the work of adhesion, given by Equation (6.57). Applying this idea to the separation of oil (O) from solid (S) gives 

19 Schematic illustration of several configurations of three phases useful in the discussion of detergency and flotation. The shaded region represents the soiled spot in detergency and 0, is the relevant contact angle the shaded region is an air bubble in flotation, and 02 is the appropriate contact angle. The arrows in (b) and (d) indicate flow in the adjacent phase. 

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The numerous technological applications of adsorption from solution include liquid purification, the stabilization of suspensions, ore flotation, soil science, adhesion, liquid chromatography, detergency, enhanced oil recovery, lubrication, and last but not least, applications in the life sciences (e.g. adsorption by cell membranes, blood vessels, bones, teeth, skin, eyes, and hair). 

Applications of adsorption from solution 157 Assessment of surface area and pore size 157 Adsorption (and displacement) mechanisms 157 References 160... 

Applications of adsorption from solution Include the clarification of sugar liquors by charcoal, removal of Impurities from petroleum oils and motor spirits by colloidal silica and recovery of dyes from dilute solutions in a number of solvents. 

It would be difficult to over-estimate the extent to which the BET method has contributed to the development of those branches of physical chemistry such as heterogeneous catalysis, adsorption or particle size estimation, which involve finely divided or porous solids in all of these fields the BET surface area is a household phrase. But it is perhaps the very breadth of its scope which has led to a somewhat uncritical application of the method as a kind of infallible yardstick, and to a lack of appreciation of the nature of its basic assumptions or of the circumstances under which it may, or may not, be expected to yield a reliable result. This is particularly true of those solids which contain very fine pores and give rise to Langmuir-type isotherms, for the BET procedure may then give quite erroneous values for the surface area. If the pores are rather larger—tens to hundreds of Angstroms in width—the pore size distribution may be calculated from the adsorption isotherm of a vapour with the aid of the Kelvin equation, and within recent years a number of detailed procedures for carrying out the calculation have been put forward but all too often the limitations on the validity of the results, and the difficulty of interpretation in terms of the actual solid, tend to be insufficiently stressed or even entirely overlooked. And in the time-honoured method for the estimation of surface area from measurements of adsorption from solution, the complications introduced by... 

Langmufr s work on gas adsorption and insoluble monolayers prepared the way for more progress to be made in the interpretation of adsorption from solution data. In the light of the Langmuir theory, it seemed logical to suppose that the plateau of a solute isotherm represented monolayer completion and that the monolayer capacity could be derived by application of the Langmuir equation. 

Now, a particular feature of adsorption from solution is the variety of molecules which can be used. Playing on their polarity or charge, it is then possible to define applications where the interest is not to determine the total surface area of the sample but, rather, to define the percentage of the surface, which can be considered as polar or nonpolar, hydrophilic or hydrophobic, acid or basic, etc. 

Adsorption from multicomponent solutions is a basis of a number of industrial processes, such as separation, purification, recovery of chemical compounds, and so forth. Numerous practical applications of adsorption from multicomponent solutions require the formulation of theoretical foundations for interpretation of the experimental data. However, the difficulties mentioned... 

Very few attempts have been made to investigate the influence of porosity on the course of adsorption from solution. Recent work in this laboratory has shown that in favourable cases it is possible to extend the application of the OCg-method for the analysis of solute isotherms. The analysis gave a clear indication of the effect of primary micropore filling in distorting the isotherm shape when applied to certain L-type isotherms (e.g. of iodine and salicylic acid adsorption by activated charcoals), but difficulties were encountered when the non-porous reference material gave H-shaped or S-shaped isotherms (i.e. either very high or very low affinity). 

The technological, environmental, and biological importance of adsorption from solution onto a solid surface can hardly be overestimated. The impact of such phenomena on our everyday lives is evident in such areas as foods and food science, agriculture, cosmetics, pharmaceuticals, mineral ore froth flotation, cleaning and detergency, the extraction of petroleum resources, lubrication, surface protection, and the use of paints and inks. Each of these applications, and many more, would be difficult if not impossible in the absence of the effects of adsorbed surfactants and stabilizers at the solid-liquid interface. 

A recent design of the maximum bubble pressure instrument for measurement of dynamic surface tension allows resolution in the millisecond time frame [119, 120]. This was accomplished by increasing the system volume relative to that of the bubble and by using electric and acoustic sensors to track the bubble formation frequency. Miller and co-workers also assessed the hydrodynamic effects arising at short bubble formation times with experiments on very viscous liquids [121]. They proposed a correction procedure to improve reliability at short times. This technique is applicable to the study of surfactant and polymer adsorption from solution [101, 120]. 

In writing the present book our aim has been to give a critical exposition of the use of adsorption data for the evaluation of the surface area and the pore size distribution of finely divided and porous solids. The major part of the book is devoted to the Brunauer-Emmett-Teller (BET) method for the determination of specific surface, and the use of the Kelvin equation for the calculation of pore size distribution but due attention has also been given to other well known methods for the estimation of surface area from adsorption measurements, viz. those based on adsorption from solution, on heat of immersion, on chemisorption, and on the application of the Gibbs adsorption equation to gaseous adsorption. 

Adsorption — An important physico-chemical phenomenon used in treatment of hazardous wastes or in predicting the behavior of hazardous materials in natural systems is adsorption. Adsorption is the concentration or accumulation of substances at a surface or interface between media. Hazardous materials are often removed from water or air by adsorption onto activated carbon. Adsorption of organic hazardous materials onto soils or sediments is an important factor affecting their mobility in the environment. Adsorption may be predicted by use of a number of equations most commonly relating the concentration of a chemical at the surface or interface to the concentration in air or in solution, at equilibrium. These equations may be solved graphically using laboratory data to plot "isotherms." The most common application of adsorption is for the removal of organic compounds from water by activated carbon. 

Adsorption phenomena from solutions onto sohd surfaces have been one of the important subjects in colloid and surface chemistry. Sophisticated application of adsorption has been demonstrated recently in the formation of self-assembhng monolayers and multilayers on various substrates [4,7], However, only a limited number of researchers have been devoted to the study of adsorption in binary hquid systems. The adsorption isotherm and colloidal stabihty measmement have been the main tools for these studies. The molecular level of characterization is needed to elucidate the phenomenon. We have employed the combination of smface forces measmement and Fomier transform infrared spectroscopy in attenuated total reflection (FTIR-ATR) to study the preferential (selective) adsorption of alcohol (methanol, ethanol, and propanol) onto glass surfaces from their binary mixtures with cyclohexane. Om studies have demonstrated the cluster formation of alcohol adsorbed on the surfaces and the long-range attraction associated with such adsorption. We may call these clusters macroclusters, because the thickness of the adsorbed alcohol layer is about 15 mn, which is quite large compared to the size of the alcohol. The following describes the results for the ethanol-cycohexane mixtures [10],... 

The rate of adsorption from dilute aqueous solutions by solid adsorbents (zeolites) is a highly significant factor for applications of this process for water quality control. 

These early applications of adsorption were based on intuition and not on a systematic study. It was in 1773 that Scheele made the first quantitative observations in connection with adsorption, whereas F. Fontana in 1777 reported his experiments on the uptake of gases from charcoal and clays. However, the modern application of adsorption is attributed to Lowitz. Lowitz used charcoal for the decolorization of tartaric acid solutions in 1788. The next systematic studies were published by Saussure in 1814. He concluded that all types of gases can be taken up by a number of porous substances and this process is accompanied by the evolution of heat (Dabrowski, 2001). 

The material in this chapter is organized broadly in two segments. The topics on monolayers (e.g., basic definitions, experimental techniques for measurement of surface tension and sur-face-pressure-versus-area isotherms, phase equilibria and morphology of the monolayers, formulation of equation of state, interfacial viscosity, and some standard applications of mono-layers) are presented first in Sections 7.2-7.6. This is followed by the theories and experimental aspects of adsorption (adsorption from solution and Gibbs equation for the relation between... 

One isotherm that is both easy to understand theoretically and widely applicable to experimental data is due to Langmuir and is known as the Langmuir isotherm. In Chapter 9, we see that the same function often describes the adsorption of gases at low pressures, with pressure substituted for concentration as the independent variable. We discuss the derivation of Langmuir s equation again in Chapter 9 specifically as it applies to gas adsorption. Now, however, adsorption from solution is our concern. In this section we consider only adsorption from dilute solutions. In Section 7.9c.4 adsorption over the full range of binary solution concentrations is also mentioned. 

Next, let us consider the application of Equation (21) to a particle migrating in an electric field. We recall from Chapter 4 that the layer of liquid immediately adjacent to a particle moves with the same velocity as the surface that is, whatever the relative velocity between the particle and the fluid may be some distance from the surface, it is zero at the surface. What is not clear is the actual distance from the surface at which the relative motion sets in between the immobilized layer and the mobile fluid. This boundary is known as the surface of shear. Although the precise location of the surface of shear is not known, it is presumably within a couple of molecular diameters of the actual particle surface for smooth particles. Ideas about adsorption from solution (e.g., Section 7.7) in general and about the Stern layer (Section 11.8) in particular give a molecular interpretation to the stationary layer and lend plausibility to the statement about its thickness. What is most important here is the realization that the surface of shear occurs well within the double layer, probably at a location roughly equivalent to the Stern surface. Rather than identify the Stern surface as the surface of shear, we define the potential at the surface of shear to be the zeta potential f. It is probably fairly close to the... 

A pore size distribution was not available for the coconut-shell carbon, but, again according to the manufacturer, approximately 55% of the intraparticle volume of the coal-base carbon was comprised of diameters between 15 and 20 A. (20). The coal-base carbon was designed primarily for adsorption from solution, while the coconut-shell carbon was intended primarily for application in gaseous systems (6). 

The area determinations by dye adsorption from solution discussed here are applicable to aqueous dispersions. Although saturation coverage of silver halides by Pseudocyanine remained unchanged in 40% methanol by volume, it is known that in organic solvents where ion-pairs may be adsorbed, the molecular cross section of the cyanine can vary with the dye s anion—cf. Reference 23 for discussion and literature citations. Recent determinations of Agl areas by adsorption of Pseudocyanine were reported to have been unrealistic and salt-dependent (van den Hul, H. J., Lyklema, J., J. Phys. Chem. 90, 3010 (1968)). A likely reason for this result is the circumstance that these investigators carried out their measurements in alcohol dispersions of the substrate where the cited solvent-dependent limitations would apply. 

The diversity of pore sizes coupled with the essentially cation-free framework make AlPOi, molecular sieves potentially applicable in separations of molecules and other adsorption applications. A broad screening program in air drying has shown many AlPOi, molecular sieves with performance characteristics similar to those of Linde Type A and X. The hydrophilic surface character has also been demonstrated in the selective removal of Hz0 from solutions of 2-butanone containing 4 vol.% HzO. The hydroxyl infrared region (4000-3400 cm-1) for several AlPO molecular sieves contains weak VqH absorptions at 3680 cm- attributed by Peri (14)... 

The assessment of surface area by adsorption from solution is still a major application, especially when the adsorbent cannot be outgassed or when the analytical techniques available in a laboratory are designed for the study of solutions. Various aspects of this application are examined in Chapter 6. 

Fructose—Dextrose Separation. Fructose—dextrose separation is an example of the application of adsorption to nonhydrocarbon systems. An aqueous solution of the isomeric monosaccharide sugars, C(5H120(5, fructose and dextrose (glucose), accompanied by minor quantities of polysaccharides, is produced commercially under the designation of "high11 fructose com syrup by the enzymatic conversion of cornstarch. Because fructose has about double the sweetness index of dextrose, the separation of fructose from this mixture and the recycling of dextrose for further enzymatic conversion to fructose is of commercial interest (see Sugar Sweeteners). 

The simple theoretical description of the adsorption from solutions on solids can be useful for characterizing sorption properties of inorganic sorbents. Such properties as the energetical and structural heterogeneities, surface phase capacity, specific surface area, pore size distribution curves and others are very important with regard to wide application of inorganic adsorbents on laboratory and industrial scales. 

The study of a particular adsorption process requires the knowledge of equilibrium data and adsorption kinetics [4]. Equilibrium data are obtained firom adsorption isotherms and are used to evaluate the capacity of activated carbons to adsorb a particular molecule. They constitute the first experimental information that is generally used as a tool to discriminate among different activated carbons and thereby choose the most appropriate one for a particular application. Statistically, adsorption from dilute solutions is simple because the solvent can be interpreted as primitive, that is to say as a structureless continuum [3]. Therefore, all equations derived firom monolayer gas adsorption remain vafid. Some of these equations, such as the Langmuir and Dubinin—Astakhov, are widely used to determine the adsorption capacity of activated carbons. Batch equilibrium tests are often complemented by kinetics studies, to determine the external mass transfer resistance and the effective diffusion coefficient, and by dynamic column studies. These column studies are used to determine system size requirements, contact time, and carbon usage rates. These parameters can be obtained from the breakthrough curves. In this chapter, I shall deal mainly with equilibrium data in the adsorption of organic solutes. 

Adsorption from solutions onto solid surfaces is important in many industrial practices, such as dye or organic contaminant removal, edible oil clarification by activated carbon, and ion exchange, where the adsorption of ions from electrolyte solutions is carried out. Adsorption from solution is also used in analytical chemistry in various chromatography applications. On the other hand, surfactant, polymer and biological material adsorption on solids, to modify the surface of solid particles in stabilizing dispersions, are also very important industrial fields. 

The process patterns found in liquid systems are more diverse and frequently much more complex than those in vapor-phase applications. In part, this arises from the greater number of factors that can influence adsorption from solution. The various permutations in which these factors can be joined confer a flexibility that makes liquid-phase adsorption adaptable to many diverse situations.1 2> 3... 

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