Experimental cookery

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 in- cludes 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 tremen- dously, 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 con- centration. 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 INTERFACIAL TENSION OF LIQUIDS/LIQUIDS 15 froth. Freundllch 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 iew m/x. 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 sur- face 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 power- ful." 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 con- ditions 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.