Absorption sorption phenomena

Whilst charcoal possesses this property to a marked extent similar phenomena are to be noted at all solid surfaces. A gas brought into contact with a solid surface will be adsorbed into the surface to- an extent which is dependent on several factors, the nature of the gas and solid, the partial pressure of the gas and the characteristic structure of the exposed surface. This phenomenon of adsorption is, however, frequently complicated by solution of the gas in the solid to form solid solutions or compounds with the solid. The term sorption has been proposed by McBain to include the two phenomena of absorption and adsorption. Our attention will be limited to the characteristics of adsorption. 

Despite limitations, the most common sorption medium is activated charcoal — a form of carbon treated in such a way as to open a large number of pores. The surface energy of the material and the pores combine to produce a material that can first attract and then trap small organic molecules. The attraction is via adsorption rather than absorption. Adsorption applies to attachment to the surface absorption is a bulk effect. Extraction is a bulk phenomenon. Simply put, adsorption is a function of surface area while absorption is a mass effect. 

SORPTION. A generalized term for the many phenomena commonly included under the terms adsorption and absorption when the nature of the phenomenon involved in a particular case is unknown or indefinite. 

When a gas or vapour is brought into contact with a clean solid surface, some of it will become attached to the surface in the form of an adsorbed layer. The solid is generally referred to as the adsorbent, adsorbed gas or vapour as the adsorbate and non-adsorbed gas as the adsorptive. It is possible that uniform absorption into the bulk of the solid might also take place, and, since adsorption and absorption cannot always be distinguished experimentally, the generic term sorption is sometimes used to describe the general phenomenon of gas uptake by solids. 

As mentioned above, soil sorption can be regarded as essentially a partitioning phenomenon, at least so far as adsorption and absorption are concerned. (Chemisorption clearly additionally involves reactivity.) It is thus not surprising that many QSARs have employed partition coefficient to model soil sorption. However, Bintein and Devillers (1994) and Baker et al. (1997) have pointed out that very hydrophilic chemicals (log Kow < 1.7) may not fit such a model, as they may not bind... 

The influence of temperature on the occlusion by palladium has also been investigated by Firth.6 He employs the term adsorption to denote the surface phenomenon of rapid occlusion of hydrogen, diffusion not being a determinable factor limits absorption to slow occlusion in which the rate of diffusion or solution is a determinable factor and uses the collective term sorption to include both adsorption and absorption.7 Firth observed adsorption only below 0° C., between 0° and 150° C. he also observed absorption but above 150° C. found absorption only. 

We use here the term sorption as the retention of a compound on the surface of a solid particle that removes it from the aqueous medium. This phenomenon affects the composition of water by transferring the compound or ion from the aqueous medium to a solid (mainly a sediment in suspension or a colloid). Then, it may no longer be present in water, especially if the sediment settles. Sorption may be identified and associated with adsorption, surface precipitation, surface complexation, and/or ion exchange (or even absorption). 

Sorption is a surface phenomenon determined by the surface charges and those of the ions surrounding it. One or more of the following mechanisms can be involved in the removal of species by sorption (i) mechanical entrapment, (ii) absorption, (iii) physical sorption, or (iv) chemical sorption on the surface of the solid particle. Physical adsorption (which is weaker than its chemical counterpart) occurs through Van der Waals forces and it is generally reversible and instantaneous. On the other hand, chemical adsorption or chemisorption occurs through the formation of chemical bonds at specific sites. This is closely related to ion exchange processes and complexation. 

Adsorption is primarily a surface phenomenon, but it is often accompanied by a deeper penetration of the vapor or solute into the body of the solid adsorbent. This deeper penetration is akin to the formation of a solid solution and is termed absorption. Since it is seldom practicable to distinguish clearly between absorption and adsorption, McBain8 suggested the use of the broader term sorption to cover both phenomena. 

Figure 12.2 also illustrates this point where the moisture concentration in the hrst absorption appears to approach equilibrium, but then after a longer period the moisture sorption accelerates. Much work on the thermal spiking phenomenon (see Section 12.6) has conhrmed that the relaxation of the network occurs in the presence of water making more polar sites available which can bind additional water molecules. 

Adsorption Adherence of gas molecules or of ions or molecules in solutions to the surfaces of solids with which they are in contact. If the surfaces are thought of as being inside an object or objects, the phenomenon is often called absorption. The general term sorption refers to either. 

Sorption A physicochemical phenomenon which retards the movement of solutes due to adsorptive and absorptive partitioning on aquifer sediments. 

Sorption n. The process of one substance taking up and holding another by physical or chemical action. Usually the sorbed substance is mobile, a gas or vapor, and the sorbing phase is dense, a liquid or solid. However, components of liquids can also be sorbed by solids, as by a molecular sieve. Surface phenomenon which may be either absorption or adsorption or a combination of the two. Often used when the specific mechanism is not fully known. Wypych G (ed) (2001) Handbook of solvents. Chem-tec Publishing, New York. 

Adsorption of liquids and gases can also occur in solid substances. If the liquid or gas does not stay on the surface layer of the solid substance, but penetrates into the interior, the phenomenon is known as absorption (formation of solid solutions). Resolution of both processes (adsorption and absorption) is usually not possible, because it is often a combination of both processes. Therefore, the term often used is sorption. 

The sorption isotherms of foods are only valid at the particular temperature at which the results were obtained. Many materials (such as fibrous proteins, cellulose and most foods) have different water activity at the same water content, depending on whether the system absorbs or desorbs water. The corresponding isotherms then have a different shape, and therefore the absorption isotherms and desorption isotherms are recognised. The phenomenon is called a hysteresis (Figure 7.44). In general, the water activity is higher when the food absorbs water. If it loses water, then the water activity... 

That this phase transition from the dispersed to the flocculated structure of the chains is accompanied by the formation of cavities or voids between the carbon black particles was deduced from the results of the investigations of density change behavior. This aspect of the model also derives from the gas absorption experiments performed by Tanioka et al. [33] they found a sudden increase in the CO2 sorption of carbon-black-fdled compounds at the critical volume concentration. They did not, however, offer any explanation for this phenomenon. The new model, which includes explanations for the creation of the cavities during the flocculation process, can be used to explain the experimental results. 

When a vapor- (or liquid-) phase component concentrates on a solid by adhering to the solid surfaces, even though the surfaces may consist of the interior of submicroscopic pores, the phenomenon is known as adsorption. If, on the other hand, penetration of the solid or semisolid structure occurs and produces a solid solution or a chemical compound, the phenomenon is termed absorption. The general term sorption has been proposed to cover both cases. 

---