Loids

LOIDS).

If we consider the adsorption of methylene blue by kaolin, we find that the adsorbent is very efficient, much more efficient than in the adsorption of the fatty acids, etc. The solute in the case of methylene blue is highly ionized, and it comes into contact with a solid (calcium silicate) whose anion is extremely insoluble, but whose cation readily goes into solution. The calcium ions of the solid therefore change places with the methylene blue cation in solution. This kind of adsorption is known as exchange ad sorption and is polar in character.

Theories of Adsorption.

Threetheories of adsorption have been advanced: (I) Chemical theory; (2) Solution theory; (3) Surface theory.

Chemical Theory.

Ifone regards adsorption equilibria as cherriical equilibria, then the adsorption isotherm must be a spe cial case of the law of mass action (see CHEMICAL AcrioN). If one considers the adsorbent and the compound of the adsorbent and adsorbed substance as solid phases of constant composition, one must also assume a phase of variable composition (a solid solu tion) in order to apply the adsorption isotherm. Then the adsorp tion equilibrium can be regarded as a chemical equilibrium. The opponents of this theory claim that chemical equilibrium is very specific, the equilibrium and velocity coefficients vary considerably with the number of reacting molecules (in the adsorption isotherm this is i/n). Adsorption equilibrium, on the other hand, is but slightly specific ; the constant K (the equilibrium constant in the law of mass action) does not vary much under very different con ditions; the same applies to I/n. The chemical theory makes it difficult to understand the relation between the adsorbability of a gas and its condensability, or the influence of a solute on the sur face tension of a solvent and the readiness with which it is ad sorbed. Again, the chemically inactive gases helium and argon are adsorbed.

Solution Theory.

Adsorption as a solution phenomenon has but few adherents. In adsorption, equilibrium is rapidly estab lished, but diffusion in a solid is extremely slow. Again to bring the phenomenon of adsorption into line with Henry's law, one must assume the dissociation of the gas, which is highly improbable in most cases. Some suggest that the gas is liquefied on the surface of the solid ; this explains the relationship between adsorption and condensability of gases, but does not assist in the case of adsorp tion from solution.

Surface Theory.

Many regard adsorption merely as a con centration on the surface of the adsorbent in accordance with the second law of thermodynamics. Assuming the validity of the gas laws and van't Hoff's law for dilute solutions, the following equa tion can be deduced thermodynamically : u= —c/RT.daidc where u=excess of gas or solute in the surface layer; c=con centration of solution (pressure of gas) ; R=gas constant; and T=absolute temperature.

It is difficult to test this equation in the case of solid surfaces because of the difficulty of determining the value of the surface tension o. In the case of gas—liquid and liquid—liquid surfaces, the experimental results agree qualitatively, but quantitative results are difficult to obtain on account of the necessity of using very large surfaces.

The laws of chemical combination, solution and adsorption are represented by the curves in figure.

Curve 1: x= K (law of constant composition). Action is due to the attractive forces between the atoms or molecules.

Curve 2 : X/1) = K (Henry's law of solubility).

Curve 3: x/p = Kun. K is given an exponent between o and 1, the values found in the case of chemical com bination and solubility respectively. This phenomenon is due to molecular attraction and also to the kinetic motion of the molecules.

Modern research tends to bring together the extreme views (chemical and physical) of adsorption. At one time the phe nomenon was regarded as purely physical and non-specific ; adsorp tion is now shown to depend on a variety of factors, many of which are extremely specific and closely related to chemical phenomena. I. Langmuir and N. K. Adam studied the behaviour of oils spread in unimolecular layers on water, and found that the forces operating were not readily distinguishable from chemical forces or affinity.

Applications of Adsorption.

Cocoa-nut charcoal is gener ally employed at very low temperatures (temperature of liquid air). Adsorption facilitates the separation of the difficultly ad sorbed gases hydrogen and helium from the more easily adsorbed, such as nitrogen and oxygen. The effectiveness of gas masks de pends on the efficiency of the charcoal which they contain for adsorbing such poisonous gases as chlorine, phosgene, etc. A very active and porous grade of charcoal is also used in an important process for the adsorption of gasolene from natural gas.

Adsorption is of very great importance in chemical reactions, for many of them are accelerated on various solid surfaces, and this has found extensive application in industry. Mention need only be made of the contact process for the manufacture of sul phuric acid (q.v.), the synthesis of ammonia (q.v.), hydrogena tion (q.v.) of oils, etc. Catalytic poisoning is also due to adsorp tion. (See CATALYSIS.) In dyeing, adsorption plays a very prom inent part in the fixing of the dye on the material, although the formation of compounds may also play a part in some cases. In the photographic plate the silver bromide is in the colloidal form and adsorbs the sensitiser. In brewing, the gas content and taste of the beer depend largely on adsorption of the colloidal particles. The growth of crystals from solution or from a molten mass is fre quently influenced by the presence of other substances in solution; this rate of growth of the crystals in different directions de termines their shape.

Adsorption plays a very important part in the agglutination of bacteria, and in the neutralization of a toxin by an anti-toxin. In the field of public health we find adsorption of importance. Aluminium hydroxide and ferric hydroxide, which are present in the soil, are very pronounced adsorbents, and filtration through soil is therefore a very efficient means of freeing sewage and other waste water from colloidal organic particles. This is also of im portance to agriculture as the means whereby the soil is able to retain solvents from solutions which percolate through it.

BIBLIOGRAPHY.-H.

Freundlich, Colloid and Capillary Chemistry Bibliography.-H. Freundlich, Colloid and Capillary Chemistry (trans. by H. S. Hatfield, 1926) ; E. K. Rideal, Introduction to Surface Chemistry (1926) ; • Treatise on Physical Chemistry (ed. by H. S. Taylor, 1924) ; A. Findlay, Physical Chemistry for Students of Medicine (1924). Burrell's, The Recovery of Gasolene from Natural Gas (1925) . (W. Ts.)