Surface Tension

SURFACE TENSION, the force of tension with which each section of the surface film (the free surface of a liquid or any interface between two phases) acts on adjacent parts of the surface. Internal pressure and surface tension. The surface layer of a liquid behaves like an elastic stretched membrane. According to the concept developed mainly by Laplace, this property of liquid surfaces depends on molecular forces of attraction that rapidly decrease with distance. Inside a homogeneous liquid, the forces acting on each molecule from the surrounding molecules mutually balance out. But near the surface, the resultant of the molecular attraction forces is directed inward; it tends to pull surface molecules into the bulk of the liquid. As a result, the entire surface layer, like an elastic stretched film, exerts a very significant pressure on the internal mass of the liquid in a direction normal to the surface. According to calculations, this "internal pressure," under which the entire mass of the liquid is found, reaches several thousand atmospheres. It increases on a convex surface and decreases on a concave one. By virtue of the tendency of free energy toward a minimum, every liquid tends to assume a shape in which its surface—the place of action of surface forces—has the smallest possible magnitude. The larger the surface of the liquid, the greater the area its surface film occupies, the more significant the reserve of free surface energy that is released upon its contraction. The tension with which each section of the contracting surface film acts on adjacent parts (in a direction parallel to the free surface) is called surface tension. Unlike the elastic tension of an elastically stretched body, surface tension does not weaken as the surface film is compressed. If σ is the surface tension, o is the free surface of the liquid, then the surface energy is equal to e = σ·o. Surface tension is equal to the work that must be performed to increase the free surface of the liquid by one unit. Surface tension is observed at the boundary of a liquid with a gas (also with its own vapor), with another immiscible liquid, or with a solid body. Similarly, a solid body has surface tension at the boundary with gases and liquids. Unlike the surface tension that a liquid (or solid body) has on its free surface bordering a gaseous medium, the tension at the internal boundary of two liquid (or liquid and solid) phases is conveniently designated by a special term—the term "interfacial tension" (Grenzflächenspannung) adopted in German literature. If a substance that lowers its surface tension is dissolved in a liquid, the free energy decreases not only by reducing the magnitude of the interfacial surface but also through adsorption (see): a surface-active (or capillary-active) substance collects in increased concentration in the surface layer. Since adsorption requires some time, a fresh liquid surface has a different, higher value of surface tension than after the completion of adsorption; surface tension measured on a fresh, newly formed surface is called dynamic; measurement after the establishment of adsorption equilibrium between the surface and the solution gives a static value. Since even the most insignificant contamination can be sufficient for a noticeable change in the surface tension of a pure liquid, the precise determination of its magnitude often presents significant difficulties. Methods of measurement. The methods allowing for the measurement of the surface tension of liquids are extremely diverse; however, only a few of them have gained wide distribution. The capillary method consists of measuring the level of a liquid in a capillary. If a capillary is immersed vertically into a liquid that wets its walls, the liquid in the capillary rises. Equilibrium occurs when the weight of the raised column of liquid becomes equal to the force acting on it. The latter represents the product of the magnitude of surface tension and the circumference of the internal cross-section of the tube. Denoting the radius of the tube by r, and the acceleration of gravity by g, we find: 2πrσ = πr²hρg or σ = ¹/₂hrρg. If the capillary is not wetted by the liquid (for example, a glass tube in mercury), the same formula will express the lowering of the liquid level in the capillary. The meniscus of the liquid in the capillary will in this case be not concave, but convex. For a more precise calculation, it is necessary to introduce a correction for the contact angle (α), which the liquid in the meniscus forms with the wall of the capillary. In reality, the column of liquid in the capillary is held not by the entire force of surface tension, but only by its vertical component σ·cos α. Therefore, the right side of the formula given above should also be divided by cos α. Another method is the detachment of a solid contour. If a solid contour wetted by the liquid, for example, a thin platinum ring, is on the surface of the liquid, it is necessary to overcome the tension of the liquid film holding it to lift and detach it. The external force that must be applied for this is proportional to the surface tension of the liquid. To measure this force, one can use a torsion balance. The drop method, based on measuring the number of drops into which a given amount of liquid breaks up while flowing out of a narrow vertical tube, has gained very wide distribution. A drop hanging at the lower end of a vertical tube detaches when its weight becomes equal to the product of the circumference constituting its base and the surface tension of the liquid. In reality, however, the value obtained in this way always turns out to be less than the true one, as a small part of the drop remains hanging on the tube upon its detachment. Nevertheless, with a specific shape of the lower opening of the tube, the weight of slowly flowing drops turns out to be, although not equal, then approximately proportional to the weight of drops hanging on the end of the tube at the moment of detachment, and consequently to the surface tension of the liquid. Having calibrated such a device (the so-called stalagmometer) with a liquid of known surface tension, e.g., water, one can then use it for the liquid under study. For this, it is sufficient to count the number of drops (n and n') into which a given volume of liquid breaks up while flowing out of the stalagmometer. Knowing the specific gravity of both liquids, we find: σ = σ' · n/n' · ρ/ρ'. The pressure method, based on measuring the pressure required for slowly forcing air bubbles through a liquid, possesses great advantages. If a tube with a thin capillary tip is immersed in a liquid, a certain pressure on the air in the tube is required to force it out of the opening. This pressure is proportional to the surface tension of the liquid, which is overcome during the formation of an air bubble with a new interfacial surface. Recently, this method was successfully improved by Rehbinder. It is also suitable for measuring interfacial tension if, instead of air bubbles from a sharp capillary tip immersed in one liquid, droplets of another are squeezed out. Surface tension of liquids. With the help of the described methods, the surface tension of various liquids was measured. For some of them, its value (expressed in absolute units—dyn/cm) is given in the following table [the second column of the table indicates whether the measurement was performed at the boundary with air (A) or with the vapor of the liquid under study (V)]. Substances that lower the surface tension of their solvent are called surface-active or capillary-active. Interfacial tension. The tension between non-aqueous phases, on the one hand, and water or aqueous solutions, on the other, has been studied significantly less than the surface tension of the latter. Meanwhile, it is precisely this interfacial tension that is of the greatest interest to the biologist, as it determines capillary phenomena at the boundary of protoplasm and the aqueous solution washing it. The following table shows the surface tension at the boundary between water and some organic liquids (at 30°). Non-aqueous phase: Amyl acetate 10.80, Amyl alcohol 4.86, Nitrobenzene 6.00, Paraldehyde 32.5, Carbon disulfide 4.28, Chloroform 24.10, Ethyl acetate 9.6, Ethyl ether 46.31. Like pure liquids, aqueous solutions of various substances can also differ very significantly from each other in the magnitude of their surface tension. The following table shows (according to measurements by I. Traube) the surface tension of 0.25 molar solutions of various substances.

Solution (Solution and Isoamyl Urea.... 71.6 29.9 Formic Acetamide.... 70.5 acid . . 70.0 63.6 Paraldehyde . . 50.1 Isobutyl 71-propyl 44.1 57.7 Isovaleric- Propyl formate 47.3 acid . . . 35.0 Propionic acid 60.1 Tartaric acid. . 71.5 Sucrose .... 72.1 Glucose .... 71.9 Acetic acid 66.8 n-butyric acid ,47.9 Sodium acetate Oxy-iso-butyric . . . 71.6 acid . . . 63.3 Oxalic acid 71.0 . Methyl acetate . i. 60.0 Ethyl acetate . . . 49.7 Methyl alcohol 69.2 Ethyl alcohol 66.0 Methyl-propio- Ethyl ether 53.2 .49.9 \ Malic acid 70.3 As the data presented show, many dissolved substances (especially higher homologs of monohydric alcohols, fatty acids, and esters) very strongly lower the surface tension of water. Others do not have a noticeable effect on surface tension. Measurements of interfacial tension clearly show that there is no correspondence between the surface tension of various pure liquids at the boundary with air and their tension at the boundary with any liquid, for example, water. From this it follows that in isocapillary solutions of various substances, the interfacial tension between protoplasm and the aqueous medium surrounding it can be very unequal. Temperature has a significant effect on surface tension, causing a fairly significant decrease in it upon heating. Examples are the following measurements of the surface tension of water: at 0° σ=75.97, at 20° σ=72.75, at 40° σ=69.55, at 60° σ=66.28, at 80° σ=62.75. For interfacial tension, such a dependence cannot be established, since, depending on the nature of the contacting liquids, it can change in different ways with an increase in temperature. Surface tension of blood. Measurements of the dynamic surface tension of blood plasma give values slightly exceeding the surface tension of pure water. Such figures are obtained, however, only with very clean preparation of the plasma. The most insignificant hemolysis of erythrocytes causes a drop in surface tension; the addition of 0.1% hemoglobin lowers it by 12-14 dynes. The dynamic surface tension of blood plasma undergoes more or less significant fluctuations in various diseases. Its most sharp decrease was observed in anaphylactic shock. In the mildest shock, this decrease was about 3 dynes, with lethal doses-5-8 and even 10 dynes. Blood serum has a lower dynamic surface tension than plasma. Stalagmometric determination gives for it in humans, according to some authors, 68-69, according to others- 67.5 dynes/cm. If the dynamic surface tension of blood can be of some interest, in some cases even as a diagnostic sign, it should still be remembered that for cell surfaces in contact with blood, its static interfacial tension must be of decisive importance. A known approximation to this value is static surface tension. The static surface tension of blood is significantly lower than the dynamic one. According to Brinkman, its values for oxalate plasma are on average 55.4-57.2 for men, 59.2-61.5 dynes/cm for women. As du Noüy showed, the difference between the static and dynamic values of surface tension, which is comparatively small in pure serum, increases sharply upon its dilution with physiological NaCl solution, reaching a maximum at a certain concentration of serum, usually equal to approximately 10^-4. At this concentration, the static surface tension does not yet exceed the initial value, while the dynamic one approaches the surface tension of a pure saline solution. As a result of this, a freshly prepared solution upon standing (for 2 hours) gives a maximum drop in surface tension, corresponding to the transition from dynamic tension to static. During immunization phenomena, du Noüy discovered characteristic changes in the magnitude of this maximum drop. After the introduction of antigens, simultaneously with the production of antibodies, an increase in this value was observed, thanks to which it turned out to be possible, with the help of a purely physical method, to distinguish immune serum from normal. The more capillary-active colloids are contained in the serum, the higher the dilution at which the described maximum drop in the tension value over time is obtained. This dilution can thus serve as a measure of the content of capillary-active substances in the serum. By lowering the surface tension of the solution, blood colloids can at the same time prevent its further reduction by other substances with greater capillary activity; the latter are adsorbed by blood colloids and extracted by them from the solution. For example, when a small amount of sodium oleate is added to blood, the surface tension drops only for a very short time and quickly (already after a few minutes) returns to its original value. In a similar way, blood can obviously neutralize the action of other capillary-active substances entering it, e.g., bile salts. Surface tension of protoplasm. To date, there are no sufficiently reliable methods for measuring the surface tension of the cell surface and the internal boundary surfaces of the cell. And yet, the surface tension of protoplasm is of very great importance as a factor that often has a decisive influence on the shape of the cell and on the dynamics of many cellular processes. First of all, the spherical, globular shape that many isolated cells take is explained by capillary forces, which tend to compress the free surface to a minimum. Every protoplasm, freed from solid shells or from directing forces acting on it, approaches the same shape. In a compact mass of closely spaced cells forming a homogeneous tissue, the tendency of the free surface to a minimum leads to other results. In this case, the cells, pressing against each other, take on a polyhedral shape, with their faces arranged at a certain angle to each other. A cross-section of such tissue shows that the cell walls meet three at one point, forming an angle of 120° with each other, similar to how this takes place in the structure of honeycombs. For soap foam, and for any foamy structure in general, such an arrangement of the liquid films forming it is characteristic. In both cases, the condition of a minimum free boundary surface is realized. It is characteristic that intracellular structures very often reduce to the two forms just considered. In protoplasm, a foamy structure is often observed (see Foam), first described by Bütschli; vacuoles, most nuclei, various granules, and other formations have a spherical shape. However, even incomparably more complex forms taken by many cells often depend on the action of capillary forces, and not on solid shells, which the researcher is inclined to assume in such cases. If an oil drop in a well-known experiment by Plateau was brought into contact with a solid (wettable by it) hoop of a larger diameter, it flattened out and took on a disk-like, lens-like shape. A similar shape of erythrocytes apparently depends on a bundle of thin elastic fibrils encircling their circumference. In a series of studies, Koltsov showed that such a combination of supporting skeletal fibers with liquid protoplasmic masses lies at the basis of a multitude of cellular forms (see State of aggregation). Their diversity can be the result of the interaction of the elastic forces of a solid body and the surface forces of a liquid. Surface tension and cellular processes. Of the various cellular processes, amoeboid movement is most often associated with the action of surface tension. Indeed, by causing a local decrease in surface tension in a drop of liquid, it is possible to obtain a visual model of amoeboid movement: pseudopodia protrude in those places where surface tension is lowered. However, a more detailed comparison of the movement of a living amoeba and its artificial model reveals significant differences between them, making it difficult to identify both processes despite their external similarity. But the reverse retraction of pseudopodia is in any case caused by the action of surface tension. The latter can also play the role of a limiting factor, with certain values of which the very possibility of amoeboid mobility is connected. This is indicated by experiments by Friedemann and Schönfeld, which showed that leukocytes transferred from blood to a pure saline solution lose their amoeboid mobility. If, however, any colloid that lowers surface tension is added to the latter, for example, gum arabic, the movement of the leukocytes resumes. Both ordinary blood serum proteins and colloids foreign to it (gelatin, dextrin) influence it in a similar way. These observations explain the absence of motile leukocytes in normal cerebrospinal fluid, which is poor in proteins. When its surface tension is lowered due to the pathological entry of protein substances, leukocytes freely penetrate into it and acquire the same mobility in it as in the blood. Another phenomenon based on the action of capillary forces is phagocytosis. If bare protoplasm comes into contact with a solid grain, then the observed phenomenon is determined by the ratio of the forces of attraction between the three contacting phases: protoplasm, water, and the solid body.

If the sum of the Surface Tension of protoplasm-water and protoplasm-solid body is less than the Surface Tension between the surface of the body and water, then the protoplasm covers the surface of the solid particle, similar to how a drop of oil spreads over the free surface of water. In this case, the protoplasm engulfs the encountered particle; otherwise, it expels it. Rhumbler illustrated this phenomenon with a visual model, taking a drop of chloroform instead of an amoeba or leukocyte, and a shellac thread as the engulfed particle. If a glass thread is covered with a thin layer of shellac, then after the shellac dissolves, the ratio of capillary forces changes, and the exposed glass thread is pushed out of the chloroform drop, thereby providing a simplified model not only of phagocytosis but also of the expulsion of undigested residues. Experiments by various researchers show that the extension of pseudopodia and the active movement of leukocytes do not play a significant role in their phagocytosis of bacteria. Completely immobile leukocytes that do not form pseudopodia possess a normal capacity for phagocytosis. Obviously, opsonins enhance the latter due to their action on the surface of the bacteria adsorbing them, which thereby acquire increased wettability by the protoplasm of the leukocytes. Cell division is also likely connected with changes in Surface Tension. As Bütschli has already pointed out, if one lowers the Surface Tension at the opposite poles of a liquid drop, it will constrict at the equator. Such division can be well observed in a drop of oil (containing oleic acid) if one touches its opposite ends with two small crystals of soda. Leaving aside the complex intracellular processes occurring during mitosis, one can see in the very constriction of the dividing cell the result of such a lowering of Surface Tension at the poles. In a drop of oil, the liquid on the surface moves from the poles to the equator, into the zone of increased Surface Tension. In the same way, in a living cell, peripheral currents of cytoplasm and the pigment granules contained within it are observed in the direction of the equator, to the place where the cleavage furrow is formed. Cell division is accompanied by an increase in the boundary surface between them and the surrounding liquid. Based on this, Bauer came to the conclusion that a lowering of the Surface Tension of tissue fluid, by reducing the obstacles to such an increase in boundary surfaces, should stimulate cell division. He therefore links the rapid growth of malignant tumors and the rapid multiplication of cancer cells with a lowering of the Surface Tension of tissue juice. A decrease in the capillary forces that tightly press tissue cells against each other should also facilitate their isolation, which is observed in malignant growths. Chemical carcinogens (coal tar, aniline) are also capillary-active substances. At the same time, Kagan found a lowered Surface Tension in extracts from malignant tumors compared to extracts from healthy parts of the same organs. However, the studies of Krodtovsky did not confirm Bauer's conclusions. They showed that the rapid growth of cells observed in tissue cultures is not connected with any decrease in Surface Tension. On the contrary, during the decay of both normal and cancerous tissues, a sharp decrease in Surface Tension occurs rapidly. A similar change in Surface Tension in extracts of malignant tumors likely represents a secondary phenomenon—not a factor stimulating their growth, but the result of necrotic processes occurring within them.