Principles of General Physiology

Since the surface tension is not altered by enlarging the surface, as in blowing a soap-bubble, it follows that the pressure inside a small bubble is greater than that inside a large bubble, contrary to what happens when an india rubber ball is blown out. The state of affairs in a soap-bubble is due to the greater curvature of the small bubble, so that the component producing internal pressure is greater in the smaller one. Fig. 33 shows this in a diagram. The two curved lines are of the same length. The vertical component, that is, the line drawn vertically perpendicular to the chord of the arc, is obviously greater in the arc with the greater curvature.

The fact just mentioned indicates the possibility of great pressure being produced in very minute spheres of liquids, such as we find in certain colloidal solutions. If the surface of a liquid is in a state of tension, it is clear that work may be done by it, when the tension is able to diminish. Surface tension, in fact, is the intensity factor of a kind of energy whose capacity factor is the area of surface, thus : — FIG. 33. DIAGRAM TO ILLUSTRATE THE EFFECT OF CURVATURE OF THE SURFACE ON THE PRESSURE INSIDE A SOAP BUBBLE, A DROP OF LIQUID, OR A SOLID PARTICLE.

The length of the arc, and therefore the total amount of surface tension, is the same in both figures. The internal component of the surface tension may be roughly represented by the length of the vertical line in each case. What is the source of this energy? There can be little doubt that it is ultimately chemical. The fact that it differs according to the chemical constitution of the liquid is sufficient to show this. Hardy (1912, p. 621) has recently made some important experiments on this question. Various liquids, insoluble in water, spread out in a thin film when dropped on its surface, owing to the fact that they lower the surface tension. Substances of great chemical stability, such as the heavy liquid hydrocarbons, refuse to spread at all, and only very slightly lower the surface tension. Esters, glycerides, for example, produce great fall of surface tension and spread widely. The suggestion is made that this effect is due to decomposition at the interface, causing contact difference of potential between film and water.

Hitherto we have confined our attention to the interface between various pure liquids and air. When two immiscible liquids are in contact, there is also a state of tension at the interface, but less than that when either is in contact with air. It can be measured by the drop method, the stalagmometer being filled with the heavier liquid, and having its orifice immersed in the lighter one. Of course, proper correction must be made for the effective weight of the drops in the liquid, compared with that in air.

Since the surface energy at the contact of two liquids is less than the sum of that betweon each of them and air, it follows that when two liquids, previously in contact with air, are brought into contact with each other, work is obtained. An important fact found by Hardy (1913) is that this work is greatest in the case of the most chemically active fluids, such as esters, alcohols, and acids ; smallest in the case of the saturated hydrocarbons. The merest trace of oleic acid, added to an inactive hydrocarbon, reduces its surface energy to an enormous extent.

Interfaces between liquids and between these and solids are met with in physiology more frequently than those between gases and liquids. As regards the surface tension at the interface between solid and liquid, we have, unfortunately, no direct method of determination, but Ostwald (1900, ii. p. 503) indicated an indirect one, depending on the greater solubility of small particles than of large ones. This fact is due to the action of molecular forces at the interface, causing the liquid component to have greater solvent power. The larger the total area of surface on the particles, the greater will their solubility appear to be. This is the reason why large crystals grow at the expense of small ones, since the solution which is saturated as regards the large particles or crystals is not saturated with respect to the smaller ones (see also the book by Freundlich, 1909, pp. 143-145).

W. J. Jones (1913) has made renewed measurements by the method referred to, and finds that the surface tension of barium sulphate, in contact with its saturated solution, is 1,300 dynes per centimetre. It. will be noted that this is a very high value compared with that between liquid and liquid, or liquid and gas. At the water -air interface, for example, the surface tension is only 75 dynes. The fact is of importance in connection with £he large degree of adsorption manifested by the surfaces of solids, such as charcoal, as will be seen later.

As a rule, substances in solution in liquids lower the surface tension at the interface between these liquids and air. Inorganic salts (such as sodium chloride) raise it, but not to any great extent. There are great differences between the actions of different substances in their action on surface tension. Home, bile salts, for example, have a very great effect. The same statement applies to the interface between liquid and liquid, except that it appears that all bodies in solution, even inorganic salts, lower the surface tension.

W. C. M'C. Lewis (1909, 1, p. 469) finds that inorganic salts lower the interfacial tension between a hydrocarbon oil and water. He also calls attention (1910, 1, p. 632) to the circumstance that if we take into account the curvature of the surface, and the densities of the two phases, we obtain a quantity, which may be called the "specific capillary constant," and that this constant is always lowered by dissolved substances, even when air is one of the phases.

A point to be remembered is that small amounts of dissolved substances produce, for equal amounts, a greater lowering of surface tension than larger amounts. The curve expressing the relationship is one of the family of parabolas (Freundlich, 1909, p. 65). The importance of this will be seen when we are discussing adsorption. When living protoplasm is in contact with any solution, there must be surface tension at the interface. Some experiments by Kisch ( 1 912, p. 152) are of interest here. Yeast and other fungi were found to be permanently injured as soon as the surface tension of the solution in which they were immersed became, by the addition of various substances, less than half of that between water and air. The actual concentrations required were : —

The cells of higher plants were found by Czapek ("Ueber eine Methode zur direkten Bestimmung der Oberfliichenspannung der Plasmahaut von Pflanzenzellen," Jena, 1911) to be more sensitive, being injured when the surface tension was reduced only to 0'68 of that between water and air. These results are difficult of interpretation, especially in view of the complex series of phenomena to be described presently under the head of adsorption. In addition to the surface tension produced by unbalanced molecular forces, there are various other ways in which the properties of substances at their boundaries with other phases differ from those in the main body of the substances. We have first to consider the electric charge. In any charged body, as we know from Faraday's researches, the charge is accumulated at the surface.

It is somewhat remarkable to find that the boundary surface between liquid and solid, or between immiscible liquids, is nearly always the seat of electrical forces. It has also been shown by Hardy and Harvey (1911, p. 220) that the interface between water and air is similarly the seat of an electric charge. The origin of this charge is not, in all cases, clear. Electrolytic dissociation at the surface will account for the existence and the sign of the charge in perhaps the majority of cases. In other cases, however, ionisation of this kind seems to be out of the question. Drops of petroleum in water have a negative charge, investigated by W. M'C. Lewis (1909, ii. p. 211), and those of aniline have also a negative charge (Ridsdale Ellis, 1912, p. 346). If the charge in this latter case were due to ionisation, it should be positive. Aniline, as a base, dissociates to a certain extent into OH' ions, which pass into the water, leaving the aniline ion with a positive charge. The same process must be supposed to occur at the surface of a drop suspended in water : the mobile OH' ions will travel off, leaving the heavy insoluble anions aggregated on the surface of the drops, which then behave as huge electro-positive ions. This explanation is quite satisfactory for particles such as those of aluminium hydroxide, which have a positive charge, but it does not hold for aniline. W. M'C. Lewis (1910, ii. p. 64) suggests an electronic origin for such cases, on the ground of the similar values (0'04 volt) found for very different chemical substances, suspensions, emulsions, and filter plugs. Burton (1906) has also shown that the same value is obtained for suspensions in methyl or ethyl alcohol or ethyl malonate. The question cannot as yet be regarded as completely solved. The work of Rudge (1914) on the electrification of dust is of interest in this connection.

The Helmholtz "double-layer" demands a word at this point, although an adequate treatment is impossible. Those interested should consult the paper in his "Gesammelte Abhandl.," i. p. 925; an account of the theory will be found in Freundlich's chapter v.. "Die kapillarelektrischen Erscheinungen " (1909, pp. 184-262). It is unnecessary to remind the reader that an electric charge of a particular sign cannot exist without the simultaneous presence in its proximity of an equal and opposite one. The charge on the surface of a solid in a liquid, therefore, implies the existence of an equal and opposite one on the liquid side of the interface. This fact adds complexity to the interpretation of the phenomena now under consideration, but cannot be left out of account.

In later pages we shall see how the charge on a surface can be increased, diminished, annulled or reversed in sign by the presence of ions in the liquid which is in contact with it. The effect of an electric charge on the mechanical surface tension is to reduce it. The elements of the surface, when they have charges of the same sign, mutually repel one another, so that the area of the surface tends to increase, in opposition to the effect of the surface tension to decrease it. The bearing of this fact on the stability of emulsions will be seen in the following chapter.

The solubility of certain bodies is found to be different in the surface layer from what it is in the body of the liquid. For example, it was found by J. J. Thomson (1888, p. 254) that potassium sulphate is 60 per cent, more soluble in the surface film. In the same work it is shown dynamically that surface tension, will have a large effect in changing the degree of chemical combination (pp. 2:U i^7). Christoff (1912, p. 456) finds that the less is the surface tension of a liquid, the greater is the solubility of gases in the liquid. The values of the absorption coefficients (volume of gas dissolved by unit volume of liquid) at 0° of g of interest to the physiologist are as follows : —

The values for water are those of Winkler ; for alcohol, those of Bunsen ; and for ether, those of Christoff. The results obtained by Vernon (1907) are of interest here. He showed that oxygen is 4'5 times more soluble in oil and fat than in water, while nitrogen is 5'3 times more soluble. In a rough experiment which I made, it was found that carbon dioxide was rather more soluble in thick paraffin oil than in water. These various facts serve to show the futility of attempting to preserve solutions from the action of gases in the atmosphere by covering them with oil or hydrocarbons. They are also of importance in the results of exposure of animals to compressed air.

When gases are taken up by charcoal, it is clear that a large amount of compression must occur ; some observers hold that, in the case of certain gases, there must be actual liquefaction. Heat must be evolved in this process, a fact whose meaning will be apparent later. Some chemical reactions are accelerated at interfaces, others retarded. Thus, Freundlich (1906, p. 85) found an acceleration of the following reactions on the surface of charcoal : oxidation of formic, citric and mandelic acids, and of glycerol, hydrolysis of chlorine, esterification of alcohol with organic acids, decomposition of phenyl-thio-urea. Perman and Greaves (1908, p. 366) found that the rate of decomposition of ozone by heat depends on the extent of surface to which the gas is exposed and that in all probability the reaction takes place only there.

An interesting case of retardation of a reaction by surface forces is that called by its discoverer, Liebreich (1886), the "dead space." This observer noticed that if a molar solution of sodium carbonate be mixed with a half-molar solution of chloral hydrate in a test tube, the turbidity, which gradually forms by the production and separation of chloroform, is absent from the surface layer of the fluid, and he was able to show that this clear space was really due to the reaction not having taken place therein. This retardation can be accounted for, thermodynamically, if the reaction resulted in an increase of surface energy, since all processes which lead to an increase of free energy are opposed. It is interesting to find, therefore, that it was found by Dr Monckman in the Cavendish Laboratory at Cambridge, that the surface tension increased considerably as the reaction went on (.1. J. Thomson, 1888, p. 237). This effect is, no doubt, due to the comparative insolubility of chloroform and the disappearance of the chloral hydrate, from which it is formed.

Any substance dissolved in water lowers th^surface tension at the interface between the solution and a solid, or immiscible liquid. With the exception of certain inorganic salts, this is also the case at the interface between the solution and a gas. Further, at these interfaces there is a local accumulation of free surface energy, which can be altered in amount by the deposition of substances at the interface. It follows, then, from the second law of energetics,

that dissolved substances which lower surface tension will be concentrated in this situation, on account of the fact that free energy will be lessened thereby. This result is of fundamental importance, and was arrived at by Willard Gibbs (1906, i. p. 56) from thermodynamic considerations in 1878, and by J. J. Thomson in 1888 (1888, pp. 191, 192) from the dynamical point of view. It will be referred to in subsequent pages as the "Gibbs" or " Gibbs-Thomson " principle. It is really a particular application of the general doctrine of decrease of free energy, as shown by the headlines chosen by Gibbs himself for his work on " Heterogeneous Equilibrium," viz., the formulation by Clausius of the two laws of energetics, as given on p. 28 of the present volume. As applied to surface energy, the Gibbs principle has a wider application than may appear from the above statement of it as referring to surface tension. It may be expressed thus : Any process that diminishes the free energy at an interface will tend to take place, whatever be the nature of the energy concerned, whether mechanical, electrical, chemical, or other. If the surface has an electric charge, a process diminishing it will be favoured. If it possesses chemical energy, a reaction reducing this energy will take place, if possible ; and so on.

Accordingly, any substance in solution in a liquid, in contact with the v / surface of another phase, will be concentrated on that surface, if, by doing so, y\ the free energy present there is decreased. This process is called "adsorption," Its characteristic is the relation to surfaces of contact. Whatever further process may follow it, chemical reaction, or diffusion into the body of the other phase, the first thing to take place is the local concentration. The rate at which subsequent events happen will naturally depend, by mass-action, on the amount of this condensation. Given the diminution of surface energy, the adsorption process is thermodynamically bound to take place, and any other explanation of the phenomenon is superfluous.

As an example, the well-known effect of charcoal in decolorising or clarifying a solution may be given. If a dilute solution of a dye, such as " night-blue," be mixed with charcoal, it can be almost completely decolorised. That the dye is not destroyed, or chemically combined with the carbon, can easily be shown by filtering off the latter, and extracting it with alcohol, which will be found to become of a deep blue colour. The process is, in fact, reversible.

In the case of the ordinary form of surface energy, Gibbs has given a formula by which the amount of dissolved substance concentrated at the interface can be calculated. Thus : Let F be the excess of solute in the surface layer above that in the body of the solution, C the concentration of the solute, R the gas constant, T the absolute temperature, and o- the surface tension at the interface. Then -^ represents the change of surface tension with change of concentration of solute, which can be measured, and

The way in which this equation is obtained is beyond the scope of this work. The appearance of R and T is due to the assumption that dilute solutions obey the gas law, so that the formula cannot be of general application. This formula has been tested experimentally by W. C. M'C. Lewis and by Donnan and Barker, and found to give values in accordance with experiment in cases where the conditions arc such that no complication due to other forms of surface energy, especially electrical, intervene. Lewis (1909, i. p. 486) found in the case of caffeine on the surface of petroleum, and (1910, iii. p. 136) of aniline on the surface of mercury, satisfactory agreement with the values calculated from the Gibbs formula. Donnan and Barker (1911, p. 573) obtained similar results in the cases of nonylic (pelargonic) acid, and of saponin at the interface between water and air. Nonylic acid has an extraordinarily high capacity of lowering surface tension.

Condensation of substances at the interface between their solutions and air shows itself in an interesting way in the experiments of Ramsden (1904). Certain substances, of which a list will be found in the original paper, such as white of egg, saponin, and quinine, are actually deposited in a solid form, so that the surface film of the solution becomes rigid. One of the simplest ways to see this fact is to blow a bubble with a solution of saponin, say 1 per cent., as a soap-bubble would be blown. If air be then sucked back out of the bubble, or it be allowed to contract spontaneously, collapse is even and regular in the case of the soap-bubble, so that it remains spherical. In the case of saponin, on the contrary, the film has ceased to be elastic, and can only collapse by falling into folds. It is sometimes possible to see little rods of the solid in the film. A similar phenomenon is found to take place with egg albumin, and is said to be the reason why cooks find that a froth beaten up for meringues, if allowed to stand, cannot be made again into a froth ; the albumin, in fact, has gone out of solution by surface coagulation. This coagulation in surface films is also, no doubt, the cause of the inactivation of enzymes when shaken with air, as found hy Schmidt-Nielsen (1909 and 1910).

When surface tension is measured, as in the experiments of several workers, by means of vibrating drops or surface waves, the surface tension plays the part of elasticity in the ordinary form of wave motion in air, so that, when this surface tension changes, the rate of vibration changes also. When pure liquids are investigated by this dynamic method, in which' the surface is being continually renewed, the same values are obtained as by static methods, such as rise in a capillary tube or drop method, where time is allowed for the surface to attain a state of equilibrium. With solutions of substances which lower surface tension, on the other hand, and are therefore concentrated in the surface layer, it depends upon the rate at which this adsorption takes place whether the two kinds of method give identical values. Conversely, if the values are not identical, it is clear that the adsorption has not had sufficient time for completion before a new surface is formed in the dynamic method. To take a well marked instance : A '025 per cent, solution of sodium oleate has a static surface tension of 26 dynes, but a dynamic one of 79 dynes, practically the same as water, so that no adsorption has taken place in the time allowed before a new surface is formed. The fact is of interest in that it shows that the actual process of adsorption is not instantaneous, although it is extremely rapid.

The Gibbs principle implies, as will be obvious, that if a substance raises surface energy, its concentration at an interface will be lowered, giving rise to negative adsorption. Such a case has been described by Lagergren (1898). When sodium chloride solution is shaken with charcoal, its concentration is raised, owing to its being displaced from the interface and sent into the main body of the solution. This effect seems to depend on change in solubility with pressure, since the water film on the surface of the adsorbing powder is, probably, in a highly compressed state owing to molecular forces (Nernst, 1911, p. 124).

Although the principle of Carnot and Clausius shows that adsorption must take place if free energy is lowered thereby, we have, as yet, made no reference to the forces which produce this surface action. Titoff points out (1910, p. 674) that the quantity adsorbed in the case of gases increases with the well-known quantity a of the Van der Waals equation — The meaning of this equation will be discussed later (page 149), but it may be stated here that a expresses the mutual attraction of the molecules. Therefore, as Arrhenius puts it (1912, p. 40) : "The forces which produce adsorption are of the same order and of the same nature as those which cause the mutual attraction of molecules." This view is confirmed by the fact, shown by Freundlich (1909, p. 154), that the extent to which a series of different substances is adsorbed by charcoal follows the same order, although different in absolute amounts, when adsorbed by wool, silk, cotton, and so on.

Four special cases of adsorption are of interest to the physiologist, on account of the part they play in the phenomena with which he has to deal. These may be given here as illustrating the nature of the process. Other cases will appear in the course of this book. I. The Adsorption of Gases by Solids. — This is familiar to all chemists in the use of charcoal. It is characteristic of adsorption to be diminished by rise of temperature, and here it is of importance to remember that this statement refers only to the condition of equilibrium, and that the rate of adsorption is increased by

rise of temperature, in accordance with the general rule. At a temperature of liquid air, charcoal adsorbs gases to such a degree that it is used by Dewar to produce a high vacuum. According to Arrhenius (1912, p. 29) adsorption by charcoal of gases which liquefy with difficulty, such as hydrogen and helium, is directly proportional to their pressure at ordinary temperatures, and of all gases (as well as some dissolved substances) at high temperatures. The fact suggests that deviations may be due to something like liquefaction on the surface. Titoff (1910, p. 673), indeed, concludes that ammonia is partially liquefied, because of the rate of increase of heat of adsorption as 0° is approached.