Adsorption

ADSORPTION. If a gas or solution is brought into contact with a very finely divided or porous material (e.g., charcoal, kao lin) the pressure of the gas or the concentration of the solution generally decreases, the gas or solute being concentrated on the surface of the solid. This phe nomenon is known as adsorption, and the finely divided or porous solid is called the adsorbent. The gas or solid can be easily re moved from the surface of the solid by exhaustion, heating or washing. The efficiency of an adsorbent depends largely on its specific surface (i.e., area of sur face per unit mass), but as we shall see later it also depends on other factors.

This phenomenon of adsorp tion is to be distinguished from absorption by the fact that the adsorbed gas or solute is concentrated on the surface of the adsorbent, while in absorption the matter taken up penetrates throughout the mass of the absorbing agent. (See ELECTRIC LAMP.) The Adsorption of Gases.—Wood charcoal is very porous; so that it posse'sses a large specific surface, and is very efficient as an adsorbent. On exposure to the air it adsorbs the atmospheric gases. In order to demonstrate its adsorbing properties it should first be heated to redness in order to remove the adsorbed gases from its surface. If then it is brought into a vessel containing a gas, an immediate diminution of pressure would be indicated by an attached manometer, reaching eventually a constant value.

This final equilibrium pressure depends on the temperature, the nature of the gas and adsorbent, and also on the specific surface of the latter. Adsorption can also be shown by filling a glass tube with ammonia with the open end of the tube dipping into a trough of mercury. A piece of charcoal (after being heated to redness and allowed to cool) is introduced on to the surface of the mercury in the tube. The mercury rises rapidly in the tube, indicating a rapid diminution in the volume of the gas. The ammonia can be recov ered from the charcoal by heat or by exhaustion. Hydrogen burns with a pale blue flame which is non-luminous; if the hydrogen is bubbled through benzene it burns with a smoky luminous flame due to the presence of benzene vapour. If the mixture of hydro gen and benzene vapour is passed over charcoal the benzene is adsorbed, as shown by the non-luminous character of the flame of the gas which passes through.

When an adsorbent remains for some time in contact with a gas, an adsorption equilibrium is established, the amount of gas adsorbed per unit mass of the adsorbent increasing with decreasing temperature and with increasing pressure. By plotting the pres sures p as abscissae, and the amount of gas adsorbed per unit mass of the adsorbent as ordinates, curves similar to those shown in fig ure (Curve 3) are obtained. Such a curve, which represents the amount adsorbed at constant temperature, is termed an "adsorp tion isothermal." It is seen from the curves that the amount of gas adsorbed is not proportional to the gas pressure, but that it increases much more rapidly at low pressures than at high press ures. The adsorption isothermal can be represented by the equa tion: where x is the amount of gas adsorbed by m grams of the adsorb ent at gas pressure p; a and n are arbitrary constants. The value of a depends on the units of measurement, and n is characteristic of the adsorbent and of the gas. Thus, for the adsorption of CO, NH, or SO, on charcoal, the values of i/n are 0-437 and 0.324, respectively, whereas for the same three gases on glass, the values are o.66, 0.53, 0.28. These constants also vary with the tem perature, i/n approaching unity at high temperatures. The tem perature at which the unity value is reached depends on the critical temperature of the gas. For hydrogen, which has a very low criti cal temperature, the value of i/n is unity at ordinary temperatures, but it is less than unity at the temperature of liquid air. In general the value of i/n becomes 1 at temperatures considerably above the critical temperature of the gas.

The logarithmic equation of the isotherm is: which is the equation of a straight line. On plotting experimental values, however, it is found that it only holds at low pressures. As the pressure increases the logarithmic curve ceases to be a straight line, and bends towards the pressure axis. The pressure at which this deflection occurs varies from gas to gas ; it is lower the more readily the gas is condensed. Below the critical temperature the equation does not hold for even low pressures. The adsorbability of gases runs more or less parallel with their condensability. The above equations, which are due to Freundlich, imply that an indefi nite increase in pressure produces an indefinitely large adsorption. Recent work by Langmuir and others indicates that on increasing the pressure surface saturation is reached. Arrhenius introduced this (s) into his equation for adsorption, viz., k.dx/dp=(s—x)/x.

The rate of adsorption is high, equilibrium being reached in a few minutes. The kinetic equation is dp/dt.-----k(pck —p), dp/dt being the rate of diminution of pressure ; p oo and p being the pressures at equilibrium and at time t respectively. Since adsorp tion is decreased by a rise in temperature, it follows that it is accompanied by evolution of heat. This can be readily shown by means of a thermometer immersed in the adsorbent when adsorp tion takes place. The heat of adsorption can be calculated from the temperature—coefficient of adsorption. If the adsorbent is in contact with several gases at the same time, the adsorption of each is diminished, and at equilibrium the amounts of the several gases adsorbed are in proportion to their respective adsor6abilities. Nothing is definitely known about the influence of the nature of the adsorbent. Wood or animal charcoal is by far the most effi cient. Silica gel is also a very powerful adsorbent. In the adsorp tion of gases on the surface of solids the adsorbed gas forms a layer one molecule thick on the adsorbent, but in the adsorption of vapours the layer may be several molecules thick.

Adsorption in Solution.

Smalldrops of a pure liquid, when free from external influences assume a spherical shape. This can be seen with drops of water on a waxy surface—if the drops are large then the effect of gravity is appreciable and the drops be come flattened. A liquid suspended in another liquid of the same density and in which it is insoluble also assumes a spherical shape. The second law of thermodynamics states that free energy always tends to become a minimum, and in the case of liquids the free surface energy therefore tends towards a minimum. The free sur face energy of a liquid is the product of its surface tension and the area of its surface, so that if there is a diminution in the surface tension or in the superficial area there is a diminution in the free surface energy. A pure liquid cannot change its sur face tension, but its surface area is reduced to a minimum if it assumes a spherical shape, for it is characteristic of a sphere that for a given volume it has the minimum surface. If we consider a liquid containing a substance in solution, and having a lower sur face tension than the pure liquid, e.g., a solution of one of the fatty acids in water, we find that the lowering of the surface tension results in a lowering of the free surface energy of the liquid; the effect is all the greater the more concentrated the solution. The free surface energy would decrease still further if there were an unequal distribution of the solute in the solution, i.e., if the con centration of the surface layer of the solution were greater than the concentration of the bulk of the solution. This is found to be the case in all solutions where the solute lowers the surface tension of the solvent, and we may state generally that those solutes which decrease the surface tension of the solvent are concentrated at the surface, and, conversely, those solutes which raise the surface tension of the solvent have a lower concentra tion in the surface layers than in the bulk of the solution. This generalization is known as Gibbs's adsorption theorem. The in crease in concentration of the solute at the surface is a case of adsorption of the solute at this surface. Solutions of low surface tension are apt to foam or froth. Conditions favourable to froth ing are low surface tension, low volatility and not too high mobility of the liquid. Pure liquids do not froth. A solution of soap in water froths very readily; if this froth is collected it is found to contain a much greater concentration of soap than the bulk of the liquid, i.e., the soap has been adsorbed on the air-liquid surface.

Adsorption of Solutes by Solids.

Theadsorption of solvents by solids is explained in exactly the same way as that of solutes at the gas-liquid interface. It is the application of Gibbs's ad sorption theorem to the distribution of the solute. Those solutes which lower the surface tension at the liquid-solid interface will be concentrated or adsorbed at that surface and vice versa. When the surface area of the solid is very large (porous or very finely divided solids), the free surface energy assumes very high values, and may be considerably reduced as a result of adsorption. In the case of the gas liquid interface, the surface tension, and consequently the free surface energy, can be readily determined, and thus the results of adsorption interpreted quantitatively. It does not follow that a decrease in the surface tension at the gas liquid interface involves a decrease in the surface tension at the liquid-solid interface. The measurement of the surface tension in the latter case is extremely difficult, and the results are not trustworthy. If charcoal is shaken with a solution of methylene blue in water, a decrease in the concentration of the solution is indicated by a decrease in the intensity of its colour. A similar result is obtained by shaking charcoal with a solution of the fatty acids, amines, phenols, etc., in water; the solutes are ad sorbed by the charcoal, since they lower the surface tension and therefore the free surface energy at the solution-charcoal inter face.

In the case of solutes which raise the surface tension of the solvent, the converse holds, i.e., the concentration of the surface layers will be less than that of the bulk of the solution, and the concentration of the solvent is relatively greater at the solid– liquid surface. We thus have two cases of adsorption; the first, resulting in a diminution in the concentration of the solution, is generally known as positive adsorption, and the second, re sulting in an increase in the concentration of the solution, is known as negative adsorption.

It has been suggested that liquids in contact with charcoal are actually compressed on the surface of the solid, and this pressure has been estimated at Io,000 to 6o,000 atmospheres. Positive ad sorption is utilized largely in industry. Bancroft has determined the efficiencies of the more important decolorizing agents.

Material Efficiency Material Efficiency Alumina . . . . zoo Bone charcoal . . . 17 Fuller's earth . . . 5o Ferric oxide . . . 3 Bauxite . . . . 4o Kieselguhr . . . . 3 The amount of adsorption generally decreases with increasing temperature, but the rate of adsorption increases with increasing temperature. In adsorption efficiency, specific surface plays a very important part—the higher the specific surface the more efficient the adsorbent. Freundlich's isotherm for adsorption from solution is : x/m = Kcvn x/m is the amount of solute adsorbed per unit mass of adsorbent ; c is the concentration of the solution at equilibrium ; K and n are constants, the values of which vary considerably (e.g., n varies between I and 5), and depend on the temperature, nature and specific surface of the adsorbent, and also on the solvent.

The adsorption isothermal defines the relationship between con centration in the solution and the quantity adsorbed, as with gases; at small concentrations relatively more is adsorbed than at higher concentrations. This relationship only holds for dilute solutions : at high concentrations a saturation value is reached— in fact, at very high concentrations the amount adsorbed seems to decrease, due to more of the solvent being adsorbed. The adsorption of a salt is an additive property of the cation and the anion ; thus, the adsorbability of a series of potassium salts is in the same order as that of sodium salts. The order of adsorbability of cations is as follows: Organic dyes (basic), H+, Ag+, Hg+, Cu", Al+++, Zn++, Mg++, Ca++, NH+,, K*, Na+; and for the anions: Organic dyes (acidic), Apart from the organic dyes, hydrogen and hydroxyl ions are the most readily adsorbed. In the case of the metals, the valency and the position of the element in the electrolytic potential series seem to be of importance. This preferential adsorption of H÷ and OW gives the adsorbent a positive or negative charge, and explains the origin of the electric charge on colloidal particles. The coagulation of colloidal solutions by electrolytes is also ex plained by the preferential adsorption on the surface of the col loidal particles of one ion which neutralizes its charge (see COL