A Mechanical Analog of Surface Pressure When a surface-active agent (surfactant) is put into a solvent, the surface tension of the solvent is lowered. The difference between the surface tension of the pure solvent and that of the solution is commonly referred to as the surface pressure.' The effect of this "pressure" may be made visible if, before one adds the surfaetant, one lightly covers the liquid surface with a layer of talc. Then, when one adds a drop of surfaetant, the talc appears to be "pushed" aside by the surface pressure created by surfactant. Actually, the layer of tale is not pushed aside by the surfactant, but rather it is pulled aside by the layer of pure solvent surrounding the drop of surfaetant which is under greater tension than the solution. An analogous experiment involving a mechanical system illustrates the above point of view. If one takes a rubber membrane (made froma tov balloon) and stretches it over the mouth of adrinkinr elass. one has an analoe of a licluidsurface under ten& One can then cover the surface with "dust" bv makine ink marks bn the membrane witha om-If one then intact region whieh is under greater tension. The author feels that this analogy is a useful pedagogical tool for introducing students to the concepts of surface tension and surface pressure. 'See, for example, Salzberg, H. W., Morrow, J. I., Cohen, S. R., and Green, M. E., "Physical Chemistry Laboratory: Principles and Experiments," Maemillan Publishing Co., Inc., New York, 1978, p. 130.
Melvin P. Miller
Loyola College Baltimore, MD 21210
Volume 58
Number 2
February 1981
197
