This section is from the book "Experimental Cookery From The Chemical And Physical Standpoint", by Belle Lowe. Also available from Amazon: Experimental cookery.
Because of the size of micelles, surface phenomena assume an important place in colloidal reactions. Surface tension, the formation of foams, inter-facial tension, adsorption, formation of surface skins, orientation of molecules, cohesion, and adhesion all have application in food preparation. Different authorities use a different terminology to designate the chemical and physico-chemical processes taking place at the interface between two phases. Kruyt calls them boundary phenomena, Freundlich designates them as capillary chemistry, and other authorities use other terms.
Total surface area increases in proportion to the increase in number and decrease in size of the micelles. Molecular systems have proportionally a greater surface area than colloidal ones, but, on account of the small size of the particles, other forces have a greater effect than surface ones in molecular systems.
Freundlich states that "The subject of capillary chemistry may be divided into natural subdivisions, according to the nature of the interfaces which separate the various possible pairs of phases. We can distinguish the following interfaces: liquid/gaseous, liquid/liquid, solid/gaseous, solid/liquid, solid/solid. Because of the complete rigidity of the interface between two solids, the section relating to this pair drops out."
Surface tension. Arbitrarily, surface tension refers to the tension of a liquid/gas interface. Liquids like gases possess kinetic energy, but unlike gases they have a surface or a boundary layer. This boundary layer gives a liquid certain properties that gases do not have. Surface tension is the result of the inherent property of a fluid to tend to form a minimum surface under all conditions. The minimum surface for a given volume is in the form of a sphere; hence, when free to do so liquids assume a spherical shape. Small drops of water falling on a dusty surface or a waxy leaf tend to form in drops. Large drops are flattened by gravity. When drops of water fall on a surface like clean glass, they spread and wetting occurs. The forces acting between the clean glass and the liquid prevent the liquid assuming a spherical shape.
The molecules in the interior of a homogeneous liquid do not exhibit any surface-tension phenomena in relation to one another since they are subjected to a balanced attraction. That is, they are equally attracted by other molecules on all sides. But the surface film is in a state of tension due to the unbalanced attractions of the molecules at the surface. The molecules are attracted only downward and sideways. Whereas the molecules in the interior of the liquid are evidently arranged at random, those in the surface are oriented or arranged in a definite and orderly manner. Freundlich states that it is only a step to conceive that this tendency to form a minimum surface resides in a membrane. Kruyt speaks of a boundary layer. The tension of this membrane is the so-called surface tension of the liquid. Surface tension is defined by Buchanan and Fulmer as "the amount of work required to produce a new surface of unit area at constant temperature." To enlarge the surface of the liquid requires work. The amount of work expended to enlarge a surface multiplied by the area increased is termed free surface energy. The amount of work to enlarge a surface is greater with increased surface tension. Just as the surface area of a liquid tends to assume a minimum surface through its inherent surface tension, so free energy tends to assume a minimum value. The free surface energy is decreased (1) by reducing the surface area or (2) by reducing the surface tension. Hence, small drops of liquid will unite with large drops if they are within the same space so that they are connected by their vapor, thus reducing the surface area.
Water has a high surface tension. Surface tension is measured in dynes per centimeter. The surface tension of water at 18°C. is 73.0; that of ethyl alcohol, methyl alcohol, and chloroform at 20°C. is 21.7, 23.0, and 26.7, respectively. The surface tension of mercury at 15°C. is 436.0. Surface tension decreases with increase in temperature, becoming zero at the critical temperature.
The surface tension of solutions. When a substance is dissolved in a pure liquid the surface tension of the solution may not be changed, it may be raised, or it may be lowered. The substances that scarcely change the surface tension or elevate it slightly include the aqueous solutions of most electrolytes and some organic compounds. Sugar increases the surface tension of water. Freundlich calls these substances capillary-inactive or surface-inactive. The group of substances that lower surface tension includes in aqueous solutions many organic compounds such as aldehydes, fatty acids, fats, acetone, amines, alcohols, tannins, saponins, and proteins. Freundlich calls this group of substances capillary-active or surface-active. If a substance lowers or increases the surface tension, the effect is always increased with its concentration. Surface tension can be lowered tremendously, but it can be raised to only a slight extent. If a substance lowers the surface tension its concentration is greater in the boundary layer than in the bulk of the liquid; and, conversely, if the substance raises the surface tension the concentration is less in the boundary layer.
Substances like the fatty acids, formic, acetic, propionic, and butyric, etc., that belong to a homologous series, show an increased lowering of the surface tension as the series is ascended, if they are kept at the same concentration. This regularity of increase with such a series is known as Traube's rule.
Formation of foams. Absorption at liquid/gaseous interfaces. A foam is a dispersed gaseous phase, the dispersing medium often being a liquid. The solutions of substances that lower surface tension are apt to 15 froth. Freundlich states that the formation of a foam is a complicated phenomenon. "Whilst in most other colloidal structures the particles of the disperse phase are of colloidal minuteness, this is by no means essential or even usual, in the case of foams. On the other hand the dispersion medium is often of colloidal fineness, that is, the gas bubbles are separated from one another by liquid films, having a thickness of only a few mu. Hence in a foam the surface of a liquid has been enormously extended, which is in opposition to the tendency of surface tension to make the surface a minimum. For this reason a liquid must fulfil a number of special conditions. In the first place the surface tension of the liquid must be small, for otherwise its tendency to reduce the surface would be too powerful." A second condition for the production of stable foams is that the vapor pressure shall be small, for substances with high vapor pressure evaporate rapidly. The surface films must not coalesce readily. These conditions are fulfilled by aqueous solutions of capillary-active substances, and especially by sols of many colloids, like soaps, saponins, tannins, and proteins. Freundlich states that in protein solutions a third influence plays a part, for they have the property of forming thin "pellicles" or surface skins on the boundary layer, which tend to prevent evaporation. Since the substance that lowers surface tension of the liquid is found in greater concentration in the foam, if the foam is continually removed as it is formed, the greater portion of the protein or other substance is removed. This is applied in the following and similar ways. In making sorghum molasses, in order to have a delicate-flavored product, one must have "a good boil" and remove the scum forming on the surface. In this way, tannins, which would increase the bitterness of the sorghum, proteins, and other substances are removed.