Principles of General Physiology

Since the gas is condensed on the surface, according to some observers even liquefied in certain cases, and when gases are compressed, heat is evolved, it is not surprising to find that adsorption is attended by production of heat. Titoff (1910, p. 658, etc.) has determined this in a number of instances, and his data will be made use of later in discussing the nature of oxy haemoglobin. It is scarcely necessary to remind the reader that this adsorption of gases by surfaces is not a chemical reaction. If oxygen combined with the charcoal used to make a vacuum, so that CO or CO2 were produced, it is obvious that no disappearance of gas would take place. Moreover, the adsorbed gas can be driven off again by heat.

The adsorption of water vapour on the surface of vessels which have been dried in a vacuum desiccator is a well-known source of trial to the chemist. When finely divided platinum is exposed to a mixture of oxygen and hydrogen, combination takes place between these gases, with the formation of water. Faraday (1844, p. 165) suggested as an explanation of this, phenomenon that a condensation of the gases took place on the surface of the platinum, so that the molecules were brought into close contact.

It is interesting to note the clear conception of surface condensation which Faraday had formed. On p. 180 of his "Experimental Researches on Electricity" (1839) he speaks of an "attractive force of bodies" causing association more or less close, without at the same time producing chemical combination, but "which occasionally leads, under very favourable circumstances, to the combination of bodies simultaneously subjected to this attraction.' On p. 181 he refers to "the attraction between glass and air, well known to barometer makers," and to the fact that they have no power of combination with each other. On p. 181, again, mention is made of the power of water vapour to condense upon, although not to combine with clay, charcoal, and turf, "assisted a little, perhaps, by a very slight solvent action" in the latter case. (See the present author's letter to Nature, vol. xciv. (1914) p. 253.)

The question will come up for further discussion in Chapter X. II. The Adsorption of Sugar. — With respect to the mechanism of the action of enzymes, it is of importance to know whether sugars and related substances are adsorbed. It appears that sugar does not lower the surface tension at the interface between water and another phase to any great extent. It has been shown, however, by Michaelis and Rona (1909, p. 492), and by Parkin (1911, p. 16), that adsorption does occur. The diminution of surface energy must, therefore, concern one or more of the other forms of surface energy which we have referred to. Michaelis and Rona, in fact, suggest that the adsorption may be due to the change of compressibility or of solubility at the interface (see also Wiegner, 1911, p. 126).

III. Salts. — Inorganic salts, although raising surface tension at the airwater interface, lower it at a water-hydrocarbon interface, as Lewis has shown (1909, i. p. 469). Theoretically, then, there is a possibility of adsorption at such an interface. The actual fact can be demonstrated experimentally. J. J. Thomson (1888, p. 192) describes an experiment by Dr Monckman and himself, in which a deep coloured solution of potassium permanganate emerged almost colourless after trickling through finely divided silica. Samec (1911, p. 155) quotes an investigation by Kugel, in which it was found that the apparent solubility of the more insoluble salts might be as much as one thousand times more in starch solutions than in water, owing to adsorption by the starch granules.

IV. The Nature of Dyeing and Staining. — The first stage of this process is almost certainly one of adsorption. How far other processes, such as solid solution and chemical reaction, play a part in later stages, will be discussed presently. The adsorption of electrolytes and of dyes is a more complex process than that of mere reduction of surface tension, .since electrical forces come into play. In the experiments of Lewis, caffeine and aniline, and in those of Donnan and Barker, nonylic acid and saponin, obey the Gibbs formula. These, it will be noted, are all practically non-electrolytes. Lewis found, on the contrary, that bile salts and dyestuffs, such as methyl orange and Congo-red, were taken up in much larger amount than the Gibbs formula would indicate. These latter compounds, however, are electrolytes, i.e., they are dissociated in water, with the formation of electrically charged products. The non-dissociated part, moreover, tends to form aggregates of a colloidal nature, which carry charges.

We have already seen how most insoluble surfaces immersed in water have a negative charge, some few a positive one. The origin of this charge does not concern us here, and will be treated in future pages. The point to be noted is that it gives rise to a considerable amount of free energy on the surface. If, therefore, the deposition of any substance, from solution in the water, upon such a surface would reduce the electrical potential there, it will, by the Gibbs principle, tend to take place. Suppose that the surface is that of charcoal, which has in water a negative charge, and that to the water we add substances with positive charges, such as colloidal ferric hydroxide, or a salt which dissociates with production of positive and negative ions. The colloid or the cation of the salt will be deposited on the surface, so that its charge is neutralised.

Perrin (1905, p. 100) was the first to suggest that electrical forces might play a part in the phenomena of dyeing, and V. Henri and Larguier des Bancels (1905) called in the aid of such forces to explain the fact that aniline blue, an electro negative colloid, is taken up by gelatine, itself an electro-negative colloid, in very small amount, because of the mutual repulsion of their charges. If, however, barium nitrate, which dissociates with formation of positively charged barium ions, be added, these ions discharge the dye particles (from Perrin's work, it is more probable that it is the surface of the gelatine that is discharged), so that there is no longer repulsion, and the gelatine becomes deeply stained. The first systematic investigation of this electrical adsorption was made by myself (1906). I found that the adsorption of various electrically charged bodies by electrically charged surfaces depended on the sign and the amount of the respective charges. An electro negative surface, say that of filter paper, will take up large quantities, of an electro-positive substance, such as night-blue, but only a trace of a negative dye, such as Congo-red. The amount adsorbed also depends on the amount of the charge, as is indicated by its connection with the dielectric constants of the constituents of the system ; for example, more Congo-red is taken up from dilute alcohol than from water. The charge of paper is proportional to the difference between its dielectric constant and that of the liquid in which it is immersed. Paper itself has a dielectric constant of 2-82, water one of about 80, pure alcohol one of 26 (see the article by Graetz in Winkelmann's " Physik," 2te Aufl., Bd. IV., pp. 112, 144, and 137). Hence the negative charge of paper is lower in alcohol, and a negative dye is more readily adsorbed.

I found further, that when neutral salts, having no chemical action on the materials concerned, such as sodium chloride in the cases of Congo-red and nightblue, are added, the effect is to increase the adsorption of negative dyes and todiminish that of positive dyes. The explanation will be obvious ; the positive ion (Na) of the salt diminishes the negative charge of the paper, in accordance with the Gibbs principle, and consequently the adsorption of a similarly charged body is facilitated while that of an oppositely charged one is retarded. The adsorption of colloidal arsenious sulphide (electro-negative) was found to be affected in the same way as that of Congo-red. Addition of a trace of gelatine or albumin to the solution prevents the effect of electrolytes, a phenomenon whose explanation will be found when we come to discuss the colloidal state.

Similar theories with regard to dyes, but less complete, since the actions of added salts and of dielectric constants were not included, were subsequently put forward by Pelet-Jolivet (1910), Michaelis (1908), and by Gee and Harrison (1910). That the electric charge on surfaces is in reality diminished, neutralised, or even reversed by tons with charges of opposite sign, has been shown experimentally by Pen-in (1904). The method used was to determine the rate at which water passes through diaphragms of paper or other substance, which had been exposed to the action of various electrolytes, in obedience to the attraction or repulsion of charged electrodes at opposite sides of the diaphragm. If this latter, for example, is negatively charged, the water in contact with it will be positively charged, and therefore attracted by the negatively charged electrode (anode).

Emil Baur (1913) describes a method of demonstrating and measuring the change of potential at a lipoid-water interface when anion or cation is adsorbed thereon. A model, on this principle, of the electrical organ of the fish is also described. The change of electromotive force produced in this manner is permanent and always of the sign predicted by the hypothesis, so that the effect appears to be actually due to adsorption (see Chapter XXII.). Acids and alkalies are very active in this power of conferring electric charges on surfaces, no doubt owing to the great mobility of H* and OH' ions, responsible for the effect. Graphite, for example, can in this way be made positive. Lachs and Michaelis have shown (1911, p. 5) that when such electro-positive graphite is immersed in a solution of potassium chloride, the negative ion (Cl') is adsorbed, while electro-negative graphite adsorbs, in preference, the positive ion (K*).

It is, however, incorrect to say, as these authors do, that the Gibbs principle fails in such cases. If the statement of this principle is understood to refer only to mechanical surface energy, it is true that electrical energy is left out of consideration ; but this is clearly not the intention of Gibbs himself, who would make it apply to all forms of surface energy. In fact, it is really a deduction from the principle of Carnot and Clausius, which controls all forms of energy whatever.

From the point of view of energetics, we may formulate the main fact of electrical adsorption as follows. Any process that will reduce the electrical energy at a surface will tend to take place. Hence, for example, if a surface has a negative charge, positively charged bodies will be concentrated upon it, so as to annul its charge. These bodies may be positive ions (cations) or colloidal aggregates. It is not clear, however, from this point of view alone, why the charge is, in many cases, not merely reduced to zero but actually reversed in sign (Perrin, 1904, p. 640). According to Harrison (1911, p. 20) the negative electrical charge on " diamine-blue " is annulled by aluminium sulphate in low concentration, but, in greater concentration, converted into a positive one. It is probable that, although the electrical energy at the surface, in such cases of reversal of sign, is greater than it is at the stage in which the original charge is abolished, other forms of surface energy, such as the mechanical one due to surface tension, may be decreased. The question of adsorption of ions, which decrease surface tension, has been considered by Freundlich (1909, p. 245), Elissafoff (1912), and Ishizaka (1913). The last observer finds that, in the precipitation of aluminium hydroxide, a strongly adsorbed (organic) anion, such as that of salicylic acid, is more powerful than one which is weakly adsorbed, such as a univalent inorganic anion, or that of sulphanilic acid. We see thus the possibility of a charge being increased, if the ion conferring the charge is one that is strongly adsorbed, owing to its effect in diminishing the mechanical surface energy. Similarly, Freundlich and Schucht (1913, p.

646) find that, in the precipitation of a negative colloid by cations, those of the heavy metals and of organic bases are more active than would be expected from their valency, and that this is to be accounted for by the fact of their great mechanical adsorption. The doctrine of energetics, as applied to chemical reactions, teaches us that such reactions will be favoured at interfaces if they lower the chemical potei there. The condition required for such cases is, of course, that the chemict nature of the phase regarded as that one at whose surface the reaction occurs such as to be capable of reaction with the substance in solution, between such surface phenomena and reactions in true solution is th*t i latter case the law of mass action is strictly obeyed, the total mass prese equivalent to the number of molecules concerned ; whereas, in the i

the extent of surface, or the number of molecules situated there, is the controlling factor, corresponding to the active mass. The surface of the same quantity of matter may vary enormously, according to the degree of subdivision, as already pointed out. The subdivision may indeed be carried out in imagination so far that molecular dimensions are reached, in which case ordinary chemical action is being dealt with. This possibility of the existence of every intermediate stage is apparent, and it is, perhaps, this fact that has led to many of the loose statements made by some writers on adsorption. Although theoretically, chemical adsorption, as defined above, should be included under the general name, it is usually understood that the more physical forms, due to changes in surface tension or electrical charge, are meant when adsorption is spoken of as distinct from chemical combination.

In cases spoken of as " specific " adsorption, where a particular kind of surface takes up preferentially a particular substance, it appears that the chemical configuration of the surface must be taken into account. At the same time, when we remember the manifold possibilities of differences in surface tension, electric charge, etc., it seems unlikely that recourse need be had to chemical phenomena, except in rare cases (see van Bemmelen, 1910, pp. 423-430 ; and Freundlich, 1909, pp. 153-162 ; also Barger and W. W. Starling, 1915).

Some examples may be given : — Wohler and Pliiddemann (1908, p. 664) found that carbon and red oxide of iron adsorb benzoic acid ten times as strongly as they do acetic acid. Chromium oxide adsorbs both acids equally ; while platinum black adsorbs acetic acid slightly more than benzoic acid, but neither to any great extent. These apparently specific adsorptions can scarcely be of a chemical nature. Another case which may have a bearing on the question of specific adsorption is given by Marc (1013, p. 692). Crystalline substances, such as barium carbonate, only adsorb crystalloids when these are isomorphous, or crystallise in a similar form to that of barium carbonate. They are supposed to be able to form a solid solution on the surface of the adsorbent. Thus, potassium nitrate is adsorbed, since it, like barium carbonate, belongs to the rhombic system. Sodium nitrate, of the hexagonal system, is not notably adsorbed. Calcium carbonate, of the hexagonal system, on the other hand, adsorbs sodium nitrate, but not potassium nitrate. Since calcium carbonate can be obtained also in crystals of the rhombic system, it seems possible to test the hypothesis ; these crystals should adsorb potassium nitrate but not sodium nitrate. It must be remembered also that potassium nitrate can crystallise in the hexagonal system, isomorphous with sodium nitrate. This deposition of a salt on an isomorphous crystal might be supposed to be merely the ordinary growth of a crystal in a solution of an isomorphous salt, say, calc-spar increasing in size by the addition of layers of sodium nitrate ; but, as it appears to follow a complex parabolic law, surface concentration, according to the laws of adsorption, needs taking account of.

Freundlich (1909, p. 514) points out that gelatine only adsorbs sugar after having been treated with formaldehyde. We have seen above that there is a considerable difference in the structure of gelatine after the action of formaldehyde, as shown by Hardy (1899, i. p. 165). But we must also remember that formaldehyde combines chemically with proteins, so that the interpretation of this fact is not quite simple. Drury's work (1914) shows that the condensation of a solute on to a surface is markedly influenced by the previous treat meiit of, or by the gas condensed on, that surface.

The physical configuration of the surface may also play a part when both adsorbent and body adsorbed have surfaces of a definite structure. A rough illustration may explain what is meant. A flat surface and one covered with projecting points cannot get into close contact, whereas two flat surfaces can do so. This idea is at present, however, purely hypothetical. The problem of specific adsorption has not yet received adequate investigation. The various forms of surface energy may be present at the same time on the same surface, and it is of some interest to know how they affect one another. The action of an electrical charge on mechanical surface tension is to diminish it, as may be seen from the following consideration. Surface tension is due to the mutual attraction of the elements of the surface ; when these elements receive an electric charge, they repel one another, being of the same sign, an'l thus a force is present in an opposite direction to that of surface tension.

It appears that electrical adsorption exceeds in amount that due to diminution of surface tension, so far as the cases at present known indicate. We see in the experiments of Lewis with sodium oleate (1909, p. 494) that the amount adsorbed by a water-oil interface was one hundred times greater than that calculated from the Gibbs formula to be due to diminution of surface tension. There is every reason to suppose that, when a substance reaches the surface at which it is adsorbed, the actual process of attachment itself is of very great rapidity. The difference between static and dynamic surface tension, referred to on p. 56 above, shows, however, that the rate of surface concentration is not absolutely instantaneous. Although this is the case, it is clear that, when an obvious interval of time is observed to elapse in an adsorption experiment before equilibrium is attained, in many cases several hours, what is being measured is the time taken for the substance to diffuse from the more distant parts of the solution to the adsorbing surface. As would be expected, it is found that the time taken for attainment of equilibrium is shortened by shaking.

The effect of temperature on the rate of adsorption, in accordance with the previous paragraph, is found to be of the same order as that which it has on diffusion processes. In the case of Congo-red and filter paper, my experiments (1906, p. 188) showed the coefficient to be 1'36 for 10° C. Brunner (1904, p. 62) 'found that for the diffusion of benzoic acid to be 1'5. Although the rate of adsorption is increased by rise of temperature, in agreement with the usual rule, the amount adsorbed when equilibrium is reached is diminished. Heat dissociates an adsorption compound. This fact is familiar in the case of charcoal, where the gas adsorbed at a low temperature is given off again on heating. In the case of Congo-red, my experiments showed that the amount taken up was in inverse linear proportion to the temperature (1906, p. 190). When the temperature was raised to 100° C., the dye was fixed in the paper and could not be removed by washing. Chemical combination appears to take place, and also goes on very slowly at ordinary temperatures. This fact will be referred to again below.

The decrease of adsorption by rise of temperature is, no doubt, to be explained by the fact that surface energy itself is anomalous in having a negative temperature coefficient. The surface tension of a particular sample of tap water was found to be 73'8 dynes at 17° and 65 dynes at 60°. Lactic acid in 44 per cent, solution at 18° has a value of 50'5 dynes, and of 47 dynes at 67°. In accordance with these data, it was found that 2 grams of charcoal at 0° adsorbed 51 per cent, of the lactic acid from 20 c.c. of 0'71 per cent, solution, but only 42 per cent, at 40°. If we consider the surface tension at the interface between a liquid and its vapour, we see that it must vanish at the critical temperature, since the boundary surface disappears. Hence, we should expect that the surface tension would decrease as the temperature rises towards this critical point.

The physiological significance of the fact may be illustrated by the case of muscular contraction, whose strength is diminished by rise of temperature. Weizsilcker (1914) has shown experimentally that one of the components of the process has itself a negative temperature coefficient. The conclusion rnay be drawn that surface energy plays an important part in muscular contraction. Since adsorption is decreased by rise of temperature, the van't Hoff principle of mobile equilibrium implies that it takes place with evolution of heat. This is easy to detect in the case of gases, as we have seen ; in that of liquids and solids it is difficult to distinguish it from the heat of liquefaction or of dilution, etc.

One of the most characteristic properties of an adsorption process is that the amount taken up is not in direct linear relationship to the concentration of the adsorbed substance in the solution in equilibrium with the surface. Suppose a is the amount adsorbed from a certain solution, then the amount adsorbed from a solution of twice the concentration will not be a x 2, but a multiplied by nonir. root of 2, or less than twice; this root, expressed as exponent ( -\ usually lies between the values of 0*1 and 0'5. In the latter case

it is, of course, the simple relation of the square root, but, as we shall have many opportunities of seeing in succeeding pages, it is very rarely that it is precisely of this value. A table of values for a number of typical cases will be found on pp. 150 and 151 of Freundlich's work (1909). In other words, the more dilute the solution, the greater is the proportion of its contents that is adsorbed. The equation expressing this relationship is given by Freundlich (1909, p. 146) in the following form : —

m ~ where a; is the amount adsorbed by the surface m, from a solution whose final concentration is C, a and - being constants for a particular surface and solution. The temperature is supposed constant, so that the expression is that of the adsorption isotherm, a may be defined as the quantity adsorbed by unit surface from a solution which is of unit concentration when in equilibrium with the amount adsorbed by the surface. Its value varies considerably in different instances, according to surface tension, electric charge, and so on. The range

The relation of this formula to that correlating diminution of surface tension with concentration, as given on p 52 above, will be evident. If we consider the effect of successive deposits on a surface, it will be clear that the first one will cause greater diminution of surface energy than succeeding ones, and each of these less than its predecessor. Each successive deposit occurs on a surface whose energy is already lessened by the previous deposit. Finally, a state of saturation is reached.

The curve expressed by Freundlich's equation is usually, but incorrectly, called an "exponential" one. Properly speaking, an exponential curve is one whose equation has one of the variables as an exponent: y = a.e.tx. Our curve is one of the forms of the general equation to the family of parabolas : y — ax" ; when »t = 2, or - =0'5, the curve is the ordinary parabola, when n = 3, it is called a cubic parabola. In order to determine the values of - and a for a series of experimental results, the simplest

way is to plot out the values on logarithmic paper. Freundlich's formula may be written thus, by taking logarithms throughout : — This formula is that of a straight line inclined to the axes. If the values of log — be represented as ordinates, and those of log 0 as abscissa*, ., is the tangent of the angle made by the straight line joining the series of points with the axis of absciss*. This line cuts the axis of ordinates at a point above the origin ; the distance of this point from the origin is the value of log a.

Although this formula satisfies adsorption processes through a wide range, it has been shown by G. C. Schmidt (1911, p. 660) that a more complex one is needed to satisfy extremes of concentration, and he gives the following : — where x is the amount adsorbed, a the amount of substance originally present, and v the volume in which a was dissolved. a~ x js then the concentration of the solution in equilibrium. S is the amount at the maximum, i.e., the amount adsorbed when in equilibrium with a saturated solution, and, therefore,