Adsorption

Adsorption (from Lat. ad- and sorbeo-absorb), represents a change in the concentration of substances at the contact surfaces of two phases; as a phenomenon of accumulation of substance at the very surface (which is indicated by the prefix ad), A. differs essentially from other types of absorption (absorption), which are accompanied by dissolution, swelling, diffusion of the absorbed substance into the absorber, etc. Many porous absorbers act, mainly, by their highly developed surface, and the phenomena of absorption in these cases are determined almost completely by true A. The surface layers of all bodies always possess a certain reserve of excess energy, which is called "surface energy." When a number of substances accumulate in the surface layer (this includes, mainly, various organic substances, alcohols, fatty acids, some alkaloids, etc.), the amount of this surface energy, which determines the magnitude of surface tension (see), decreases. Since in nature processes always tend to occur which lead to a decrease in the amount of free energy, the above-mentioned substances tend to accumulate in the surface layer - they are energetically adsorbed on surfaces and are therefore surface-active. Electrolytes, for example, salts, are inactive, they increase surface energy and are therefore adsorbed negatively, i.e., the salt solution of the surface is less concentrated than away from it. The magnitude of A. is measured by the amount of substance adsorbed per unit surface. This value depends strongly on the concentration of the solution of the adsorbing substance, the dependence being non-linear (whereby with an increase in concentration the amount of adsorbed substance would increase proportionally), but more complex, whereby from diluted solutions relatively more is adsorbed than from concentrated ones. An approximate value for this dependence is given by Freundlich's equation (adsorption isotherm equation): x = k (a - c)¹/n, where x is the amount of adsorbed substance, a is the initial concentration, (a - x) is the amount of substance in the solution after the establishment of adsorption equilibrium, k is a certain constant, n is a number usually varying between 2 and 3. With an increase in temperature, A. usually decreases - it possesses a negative temperature coefficient. This is explained by the fact that A. is accompanied by the release of heat (heat of adsorption). True A. is practically reversible, i.e., with a decrease in the concentration of substance in the external medium, all absorbed substance can be completely extracted from the surface of the adsorbent. However, often A. is complicated by other processes, and reversibility is disturbed. Besides A., conditioned by a decrease in surface energy, great significance is attached to the so-called electrical A., i.e., the adsorption of electrically charged particles (ions or colloidal particles) on a surface having a charge of opposite sign. An example of such cases can serve for the adsorption of certain dyes by charged adsorbents; positively charged adsorbents adsorb only negatively charged dyes, and negatively charged adsorbents - only positively charged dyes. Such phenomena undoubtedly have considerable importance in dyeing processes, as well as in the animal organism, where adsorption proceeds, first of all, on the surfaces of biocolloids, the charge of which must play a large role in this case. The adsorption of ions by the surface of charged colloidal particles causes neutralization of their charge and, according to Freundlich, causes the coagulation of colloidal solutions (sol). The accumulation of substance due to adsorption extends only to a very small distance from the surface (of the order of the size of molecules), called the thickness of the adsorption layer. Langmuir showed that very often (in cases of smooth phase boundaries) the thickness of such a layer filled with the absorbed substance is equal to the diameter of the adsorbed molecule (monomolecular layers). The ability to be adsorbed depends on the properties of the adsorbing molecules and on the nature of the phases forming the phase boundary. Langmuir found that the adsorbing molecules must consist of two parts: polar - a (e.g., oxygen-containing groups - OH, - COOH) and nonpolar - b (e.g., hydrocarbon chain). If the substance ab is dissolved in water, then at the boundary of the aqueous solution with another phase (solid body, air, oil) an adsorption layer of ab molecules is formed, because the group b, which has no affinity for water, is "pushed out" by water molecules towards the 2nd phase, while the polar group a is somewhat drawn into the aqueous phase; thanks to this, the ab molecules orient themselves in the surface layer (a - towards the water, b - towards the 2nd phase). With an increase in the concentration of substance in the surrounding medium (or with an increase in the pressure of the adsorbing gas), A., all else being equal, increases, tending towards a maximum (limiting) value corresponding to complete saturation of the layer. With the saturation of the layer, the orientation of the adsorbed molecules also increases, so that in a saturated layer all ab molecules are fully oriented and form a kind of palisade. In the case of solid adsorbents with a highly developed surface, A. (from a solution or from a gaseous medium) is determined usually by direct measurements. The phenomena of A. are connected with the presence of certain phase boundaries. The greater this surface, the more pronounced are the phenomena of A. and the greater influence A. will have on the state of the system. The greatest development of surface is attained by colloids, and since the basic substance of every cell - protoplasm - and all juices and fluid of the organism consist of colloids, it becomes clear the extremely great significance which A. has for the most diverse biological processes. A. is the first stage of most processes occurring in heterogeneous systems, and organisms are such systems. Thus, the phenomena of heterogeneous catalysis (see Catalysis) begin with the accumulation (with A.) of reacting substances at the surface of the catalyst. Penetration of substances from one phase (e.g., aqueous) into another (e.g., lipid) through membranes present in organisms also begins with the adsorption of the active substance on the phase boundary. Thus, the permeability of membranes, osmotic phenomena (kinetics of osmosis and distribution of substances) are determined primarily by A. (Traube, Overton). The significance of A. for technology and medicine is also extremely great: for example, charcoal (carbon powder, activated charcoal) is applied in gas masks (see) for the absorption of poisonous substances from air, in industry - for decolorizing and purifying colored solutions (e.g., in sugar production), and in some cases - also as an antidote (for A., i.e., binding poisons that have entered the stomach).