Principles of General Physiology

S-x o is the proportion of the amount adsorbed at a given concentration to that adsorbed at saturation. A and K are constants. In the case of acetic acid and charcoal, this formula gives correct values for all concentrations of acid between 1 and 3,000. The reason for taking saturation into account is that a surface already completely covered cannot take up any more substance, since no change of surface energy would result. This fact is found to be in agreement with experimental data.

In Schmidt's equation the constant S expresses the maximum amount adsorbed in saturation, and A refers to the amount adsorbed at a particular concentration. Now, Arrhenius points out (1912, p. 31) that the product of S and A in Schmidt's experiments is, within the limits of experimental error, equal to the reciprocal of log, 10 or 0'4343. If this is so, Schmidt's equation amounts to the integral of the following differential equation : — -where c is the concentration. This represents, in a simple form, how the amount adsorbed in •different concentrations is inversely proportional to the amount already adsorbed (a), and xlirectly proportional to the distance from the point of saturation (S - a). Arrhenius finds that the phenomena of adsorption follow very closely this formula, except in the cases where the amount adsorbed is very small, on account of the large value of the heat of adsorption for the first quantities adsorbed (Arrhenius, 1912, p. 37). Titoff (1910, p. 659) finds for nitrogen the heat of adsorption per cubic centimetre of adsorbed gas, for the first small amounts, 0'373 gram-calorie, and when nearly saturated, 0'203 gram-calorie. For small values of «, in fact, the isotherms giving log a as a function of log p ( = concentration), instead of being straight lines, diverge until they cut the axes of co-ordinates at 45°, thus obeying the law of Henry.

If a process is found experimentally to be best expressed by parabolic formulae of the kind given above, the conclusion must not be drawn hastily that it is an adsorption. Other facts must be taken into consideration. For instance, suppose a substance is soluble in two immiscible solvents in contact with one another, but to a greater degree in one than in the other, it will be distributed in a certain ratio between the two, this ratio being known as the partition coefficient. If the dissolved substance is in single molecules in both solvents, as succinic acid in ether and water, a simple linear relationship holds, whatever the concentration. But, if the substance is associated in one of the solvents, so that the number of the molecules is halved or otherwise diminished, as in the case of benzoic acid, which is bitnolecular in benzene, the ratio is no longer a linear one, but an exponential one, e.g., in the case of benzoic acid in water and benzene, the concentration in water is equal to the square root of the concentration in benzene (Nernst, 1911, pp. 495-498). We see that the concentration of a substance in one phase may vary as a power of that in the other phase. If we find, then, that n in the Freundlich formula works out in a particular case to be a whole number, say 2, it might be a simple case of partition between two solvents, in one of which the substance is bimolecular. It is obvious that no difficulty arises when the exponent is such as to be an impossible one, except as an adsorption. Such is the case when it would imply the existence of fractions of molecules in one of the solvents. In the case of the adsorption of arsenious acid by freshly precipitated ferric hydroxide, as investigated by Biltz, the exponent is one-fifth. As Nernst points out (1911, p.

499), if this were a case of distribution between solvents, arsenious acid must have a molecular weight in ferric hydroxide onefifth of that which it has in water. But in water it is already in single molecules. Again, as is pointed out by Philip (1910, p. 227), the concentration of carbon dioxide on charcoal increases proportionally to the cube root of the pressure in the experiments of Travers (1907). If this were a case of solution in charcoal, the carbon dioxide must have a molecular weight in the charcoal one third of that in the gaseous state, which is not possible. The gas is evidently condensed on the surface.

Arrhenius ("Medd. k. Vetenskaps akad. Nobel institut," [2], No 7, 1910, quoted by Marc, 1913) has proposed a simple formula, to apply to the adsorption of gases by charcoal. It is pointed out that the compressibility of gases obeys the same formula ; adsorption is regarded, accordingly, as a purely molecular property of the adsorbed matter and not as a surface phenomenon. It appears, however, from the work of Marc (1913) that the formula of Arrhenius applies only to u very limited number of cases of adsorption, so that it is probable that the fact of the satisfactory application of this theory to certain cases is due to the connection of surface tension with molecular attraction, in arrnnlunce with the Yoaog-Laplaoe theory. In any case, as will be clear from what is said in other parts of the present chapu-i , we must admit that the actual process of adsorption in any particular case is a complex of several factors.

The taking up of arsenious acid by ferric hydroxide introduces us to the study of an important class of substances called " adsorption compounds" or, by some, "colloidal complexes." Although we are still dealing with surface action, the surfaces in question are those of the minute particles of matter in the colloidal state, and the complexes formed behave in many ways like true chemical compounds. How the two are distinguished will be shown in the following section.

If we take a (colloidal) solution of the free acid of Congo-red, which has a blue colour, and add to it, quickly, a solution (also colloidal) of thorium hydroxide, a precipitate of a blue colour is formed. This precipitate can be filtered off, or better, centrifuged off, and resuspended in water. On allowing it to stand at room temperature, it slowly becomes red and part of it goes into solution : this change can be produced quickly by boiling. What is the explanation of this phenomenon?

The surfaces of the particles of the Congo-red acid have a negative charge, as can easily be shown by the behaviour to charged electrodes. The particles of the thorium hydroxide, on the other hand, have a positive charge. By aggregation together of these two substances the charges neutralise one another and free energy disappears, so that such a process will occur. But chemical combination only takes place very slowly, owing probably to very slight degree of ionisation of these two colloids. We have, in fact, free acid and free base in close apposition, but uncombined, as shown by the blue colour, which is that of the free acid When chemically combined, the salts have a red colour, such as appears on heating the adsorption compound, or slowly at ordinary temperature. There are certain precautions to be observed to ensure success in this experiment, for which the reader is referred to my paper (.1911, i. p. 83).

This peculiar type of compound is commonly met with where colloidal bodies are present, as in living organisms. It is rarely, however, that the nature of the complex is as clear as in the case given. Other properties must usually be taken into consideration. One of these is the absence of any quantitative, stoichiometric, relation between the constituents of the compound ; they may be present in any ratio whatever. The colloidal complex of ferric chloride and ferric hydroxide, present in dialysed solutions of ferric chloride, may contain any percentage of chlorine from 65'5 (that of the chloride itself) through all stages to 6'4 per cent.

It will perhaps assist the reader to realise the distinction between chemical combination and adsorption if a few actual cases are considered briefly. When a given quantity of charcoal is in equilibrium with solutions of acetic acid of varying concentrations, for each concentration there is a definite amount present in both phases, that is, there is always more or less acetic acid left in solution, however small the amount originally present. In seeking for a true chemical reaction to compare with this, it must be remembered that the acetic acid adsorbed on the surface of the charcoal is, for the time, fixed there ; it is not in solution. The adsorption compound is similar to a precipitate. Our chemical reaction must therefore result in the production of a precipitate. Take, then, silver nitrate, and add to it varying percentages of sodium chloride. What happens i> familiar to every one. At all concentrations of sodium chloride less than that equimolar with the silver present, all the chlorine is carried down and none is left in solution ; at all concentrations of sodium chloride greater than equimolar, the amount of precipitate is always the same, whatever the concentration of the sodium chloride. The graph, instead of being parabolic, like that of adsorption, consists of two straight lines at right angles to one another. The figure by Freundlich (1909, p. 287) shows this in the case of the combination between diphenylamine and picric acid as investigated by Appleyard and Walker (Journ. Chem. i'oc., 69,

1334, 1896). It can also be deduced theoretically from the law of mass action, as shown by Freundlich in the place referred to. Cases of combination between weak acids and bases which do not result in precipitation are not comparable. When charcoal adsorbs either bromine, benzoic acid, aniline, or phenol, the value of the exponent in the formula of Freundlich varies only between 0*5 and 0'2. It is difficult to believe that any process of a chemical nature can be in question here.

The amount of any particular adsorption compound formed depends on the concentration, not the total mass of the adsorbed substance. Now, Brailsford Robertson (Roll. Zeita., 3, p. 54) argues that this is also found in cases of reversible reactions like that of the formation of acetic ester from ethyl alcohol and acetic acid. He forgets, however, that the ester formed, although varying in amount with the concentration of the acid present, has always the same composition, whereas an adsorption compound would contain more acetic acid the greater the concentration of it in the mixture.

Raehlmann (1906, p. 152) has described how the constituents of certain adsorption compounds can be seen to be merely in close apposition. One of his experiments is as follows- The extract of a yellow wood, used in dyeing, and known as fustic, shows itself under the ultramicroscope to be a suspension of , minute particles too small to be visible as separate dots. By the addition of alum, these " amicrons " can be caused to aggregate together to form larger ones, visible as such, and of a greenish colour. Serum albumin behaves similarly, and, under the influence of alum, forms yellow particles. The dye, " Congo fast blue," even without alum, consists of visible particles of a red colour by the reflected light of the ultra-microscope. Taking each separately, we have then green, yellow, and red particles. When the three solutions are mixed, an adsorption compound, which gives a green

ENTS, as seen by ultra-microscopic observation of a mixture of fustic (white in the figure), Congo -blue (grey), and albumin (black, outlined by white). The fustic particles actually were greenish in colour, those of the Congo-blue were red, and albumin yellow. solution, is formed. This solution, under the ultra-micioscope, is seen to consist of compound particles, each containing three of the simpler ones, one each of the red, green, and yellow ones. Fig. 34 is a diagrammatic representation of Eaehlmann's fig. 4, the original being in colours. If albumin, Congo-blue, and fustic are mixed, without alum, the particles do not run together. It appears that Congo-blue, and probably also the other colloids, have a negative charge, which must be neutralised by the trivalent aluminium ion before aggregation can occur. The meaning of this experiment will be appreciated better after Chapter IV., « On the Colloidal State," has been read.

Let us take, as the next case for consideration, a solid in mass immersed in a solution of some substance which lowers surface tension, and is, therefore, deposited on the surface of the solid. Further, let us suppose that this adsorbed substance is capable of entering into true chemical combination with the s It is clear that this reaction can only proceed at the surface, and it will depei upon the solubility of the products of the reaction whether the whole solid finally enters into combination, or whether there is merely a layer ot to products on its surface. In any case, it is plain that the reaction will not the law of mass action in its usual form, since the rate of the reaction wil depend, not on the mass of the solid, but on its surface.

Now, imagine the solid to be split up into smaller and smaller particles until they become molecules. At this point the ordinary law of mass action will be obeyed, since surface has no longer any existence. This example shows the justification of the view taken by B. Moore (1909, p. 520), that there is no hard and fast line to be drawn between what he calls " molecular ' compounds, which are the same as those called by others adsorption compounds, and true chemical compounds. In the same way, as we shall see in the next chapter, there are all stages of transition between colloids and crystalloids. This fact, however, does not alter the necessity of taking into consideration the surface energy of colloids and matter in mass. It appears that Moore desires to explain the phenomena of adsorption by chemical forces of an obscure and indefinite kind (see p. 534 of the article referred to), whereas it is known that there is present, and active, surface energy of various well-known forms, capable of satisfactorily explaining the characteristics of these phenomena without any further assumptions.

It seems to me that the well-known principle of logic called " William of Occam's Razor" may legitimately be applied to such a case as the one before us ; " entia non sunt multiplicandi praeter necessitatem. " Sir William Hamilton (1853, pp. 628-631) gives a more complete form in his "Law of Parsimony": thus " Neither more, nor more onerous, causes are to be assumed than are necessary to account for the phenomenon." As physiologists, we must take the chemical or physical explanation, according to which leads further, when both are available. Some chemists appear to resent any explanation of a phenomenon apart from a chemical one. As has been pointed out above, the ultimate source of animal energy is almost entirely chemical, but, in the transportation and utilisation of this energy, physical factors intervene, and these factors cannot be neglected without serious error. Indeed, the same thing may be said of many non-vital processes, such as those of the galvanic cell or those taking place in surface films.

That there is, as Moore points out, a kind of stoichiometric relation between the constituents of adsorption compounds is not to be wondered at, if we remember the fact of adsorption saturation, that is, when the whole of the adsorbing surface is covered with the adsorbed substance. This relationship is between the extent of surface and the amount of compound formed and is not stoichiometric in the proper sense of the word. The amount adsorbed depends, not on the mass of the adsorbent, but on its state of subdivision, or its shape.

The constituents of living cells consist largely of substances in the colloidal state, so that it is not surprising to find that adsorption compounds are frequently to be met with amongst those extracted from these cells. Specially interesting are those in which lecithin is one of the components. When yolk of egg is extracted with ether, a compound of lecithin with vitellin goes into solution, although vitellin alone is insoluble in ether. Jecorin, again, a complex of glucose with lecithin and albumin, also appears to be an adsorption compound. It has been prepared by A. Mayer and Terroine (1907) by mixing solutions of acid albumin, lecithin and glucose all in dilute alcohol. The mixture is evaporated to dryness, extracted with ether, and precipitated from solution by absolute alcohol, just in the same way as Drechsel's original preparation from the liver. The other properties of this artificial jecorin are exactly those of the natural one. The fact that shows it to be an adsorption compound is that its composition varies with the relative proportion of the constituents of the mixture from which it is made. It has been claimed that jecorin can, by repeated precipitation and redissolving, be obtained of constant composition. It must be remembered, however, that this fact does not exclude adsorption. For one thing, if the whole of the constituents are precipitated by absolute alcohol, it is obvious that the precipitate will always have the same composition. Suppose further that we take electro-negative paper and allow it to adsorb night-blue, which is electropositive. We find that, even from a moderately concentrated solution of the dye, practically the whole is taken up; so little is left that it would escape detection by analysis.

Suppose that we dissolve this stained paper and reprecipitate it; in the second precipitation, practically the whole of the dye would go down again with the precipitate. Another instructive case is the artificial laccase (an oxidising enzyme) prepared by Dony-H^nault (1908) by alcoholic precipitation of a solution containing gum arabic, manganese formate, and sodium bicarbonate. This precipitate can be redissolved in water and reprecipitated by alcohol. It is undoubtedly an adsorption compound of gum with colloidal manganese hydroxide. When the

gum, which acts as a protective colloid, ensuring fine subdivision of the manganese, in the way to be described in the next chapter, is thrown down by alcohol, it carries with it, in a state of adsorption, the manganese. The inorganic salts, usually associated with proteins, are probably adsorbed. The law expressing the way in which they are removed by water shows that they are not merely admixed, while the fact that they are so removed shows that chemical combination is not in question (see my investigation of gelatine, Bayliss, 1906, pp. 179-185).

Several other compounds in which adsorption plays a part will be discussed in later pages. It appears to be held by some observers that many of these adsorption compounds, especially those in which lecithin occurs, are more of the nature of solid solutions. The ratio in which their constituents stand to their concentration in the reacting mixture points rather to surface condensation, although solid solution cannot be entirely excluded. Loewe (1912, pp. 216-218) finds that the substances known as "lipoids," of which lecithin is an example, take up dyes, hypnotics, and tetanus toxin in a way which is not compatible with the solid solution views but with an adsorption process. The exponents of the equations, expressing the relation of the amount taken up to the concentration of the solutions, are not of such values as to admit of the interpretation of distribution between phases in unequal proportion. Moreover, when nicotine or methylene blue in solution is allowed to remain for a long time in contact with lipoid matter, no diffusion is found to take place into the interior of the lipoid. It appears, therefore, that the action is a surface one.

That adsorption does not preclude subsequent true chemical combination is obvious. So far is this the case that, in many cases, chemical change seems to necessitate preliminary adsorption. One at least of the constituents of an adsorption compound possesses, of course, a surface, either the visible one of such materials as paper, cell granules, and various fabrics or tissues, or the ultramicroscopic surfaces of colloidal particles. Substances in such states of aggregation are naturally inert, as far as chemical activity is concerned, so that when the chemical reaction is between the components of the phase possessing the surface and the adsorbed substance, it is to be expected that it will proceed very slowly.

0. C. M. Davis (1907) finds that charcoal takes up iodine with great rapidity up to a certain point of apparent equilibrium, but that, if the components are allowed to remain together for a longer time, a very slow further disappearance of iodine goes on. The first part of the process differs, as would be expected, according to the particular kind of charcoal used, since the surfaces would vary. The second process is the same for various kinds of charcoal and is interpreted by Davis himself as being a passage of iodine into the mass of the solid ; a solid solution, in fact. It is suggested by Freundlich (1909, p. 173) that chemical combination is more probable, since iodine is a very reactive substance. This suggestion explains why the second part of the process is irreversible and does not vary with the kind of charcoal used.

It is probable that the fixing of dyes on tissues by heat is due to chemical combination. When Congo-red is taken up by filter paper in the absence of electrolytes, it is readily washed out again. But if raised to 100° it becomes fixed. The same process goes on slowly at ordinary temperatures. There are two classes of reactions in which the rate of chemical combination is controlled by adsorption. The first is when the two reacting substances are condensed on the surface of a third and combine together there, leaving the adsorbing surface in the end unaltered. This process is one of those that we shall learn later to call " catalytic." Examples of such reactions are : —

(1) The production of sulphuric acid under the influence of platinum, in which it has been shown by Bodenstein and Fink (1907) that the rate of the reaction is governed by the adsorption of SO3 on the surface of the platinum. (2) The effect of platinum on the reduction of titanic sulphate by hydrogen (Denham, 1910). (3) The decomposition of ozone by heat takes place on the walls of the containing vessel, or other surface present (Perman and Greaves, 1908).

The second class of cases is typified by that of a colloidal hydroxide and colloidal acid, as described above. The reacting substances are first brought together by mutual adsorption, and chemical reaction then follows between the whole of the constituents of the system. A very similar case is described by van Bemmelen (1910; p. 486). If barium hydroxide solution be added to colloidal silica, a white precipitate falls, which is found to contain both barium hydroxide and silica, but not in chemical combination. On standing, barium silicate is slowly formed and crystallises. Another case is the action of tannin on leather. According to Freundlich (1909, p. 532), the amount of tannin taken up is conditioned by an adsorption process, which is then followed by true chemical reaction, which takes place slowly and results in the formation of insoluble bodies.

The fact to be insisted upon in these cases where chemical reaction follows adsorption is that the velocity of reaction, as affected by various conditions, does not follow the law of mass action in its usual form. The active mass here is the amount adsorbed on the surface, so that the reaction as a whole will be observed to follow the parabolic law of adsorption. The systems of chief interest to the physiologist are those of which colloids form part. Although these are heterogeneous systems, the internal or dispersed phase is so minutely divided and evonlv distributed that, in comparison with the cases investigated by Nernst, the rate of diffusion does not appear to play so important a part. We shall have to return to this aspect of the question when treating of enzymes.

This is perhaps the most appropriate place to refer to some cases of biological interest which illustrate the manner in which adsorption intervenes in a variety of processes. 1. The power of the soil in holding back soluble salts, so that valuable foods are not washed away by the rain. Shown by the experiment with sand and permanganate solution given by J. J. Thomson (1888, p. 192). 2. Dr Harriette Chick has shown (1906, p. 247) that the complex organic substances, which are detrimental to the nitrifying organisms in the filter process of sewage treatment, are kept back by adsorption in the upper layers of the filter bed.

3. The action of certain poisons on micro-organisms has been found to be proportional to the amount deposited on their surfaces (H. Morawitz, 1909, pp. 317-322). 4. Craw (1905) has shown that the combination between toxin and antitoxin follows more closely as to its laws the phenomena of adsorption than those of chemical combination. Perhaps the most striking fact in this connection is the explanation given of the puzzling phenomenon of Danysz (1902), who found that when a given quantity of diphtheria toxin was added in fractions to antitoxin, more toxin was neutralised than when the same quantity was added at one time.