SURFACE ENERGY EXPERIMENT* JOHN R. CALDWELL Ohio State University, Columbus, Ohio
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URFACE tension may be characterized as a force exerted a t the surface of a liquid tending to make the surface area as small as possible. In water, for example, this force is of such magnitude that it may be made to do an appreciable amount of work. Two familiar examples of such work are represented by capillary rise, and by the darting about of bits of camphor on water. This capacity of a surface film to perform work is admirably demonstrated during the process of spontaneous emulsification, and a t the same time, the important function of stabilizing agents or protective colloids is well illustrated. The method of preparing dilute, finegrained emulsions and suspensions is discussed by Holmes,' and its application to various technical problems is outlined by FlemingZand by W i l ~ o n . ~ The oil to be emulsified, together with an emulsifying agent such as soap, is dissolved &a solvent that is miscible with water. When the solution is poured into water, the oil is thrown out of solution in minute droplets and forms a fine suspension. During the process of droplet formation, a thin film of soap or other emulsifying agent is thrown around each drop and the emulsion is stabilized. Various substances may be added to the mixture to change the degree of dispersion, the stability and other properties of the suspension. To illustrate the process, a solution is prepared having the approximate composition: castor oil, 3 cc.; 95% alcohol, 25 cc.; soap, 0.10 g.; camphor, 0.10 g. The solution is drawn up in a glass tube having a 'At the author's request the date of acceptance of this manuscript--namely, Nov. 27, 1934--is here made known. ' HOLME~, H. N., "Laboratory manual of colloid chemistry," 2nd ed., John Wiley & Sons, Inc., New York City, 193, p. 81. 'FLEMING,W. E.. "A nonstratifying carbon disulfide emulsion." Ind. Eng. Chem.,17, 1087 (Oct., 1925). a WILSON,A. .L.,"Triethanolamine emulsions." aid., 22, 144 (Feb.,1930).
capillary tip about 0.5 rnm. in diameter and the tip is placed about 1 mm. below the surface of the water in a small beaker. As the alcohol solution is allowed to flow from the tip, the surface tension of the water is so reduced by the camphor and alcohol that oil droplets are thrown through the water with explosive force and soon cloud the surface with a milky layer. So much force is exerted that in a few moments the water in the beaker is brought into appreciable circulation and the emulsion is completely dispersed without the aid of mechanical stirring. These.emirlsions are quite stable and will last for several days. To show that the action is essentially a surface pbenomenon the capillary tip is lowered to a position 2 or 3 cm.below the surface. In this case, the alcoholic solution rises in a slender stream, practically unchanged, until it reaches the surface. Here it mushrooms suddenly, throwing the oil droplets forcibly out in the plane of the surface where they are soon carried downward by the circulation. If a fat-soluble dye such as Sudan I11 or Scarlet R is added to the alcoholic solution, the emulsion droplets will be colored and their rapid motion may be more readily followed. This point is illustrated in the accompanying photographs. In Figure 1 the colored oil solntion is seen issuing from the capillary tip at A and rising in an unbroken stream to the surface. At this point, it disintegrates to form a cloud of colored droplets. Figure 2 shows the condition existing a few moments later. A fine emulsion has formed and is being driven downward by the circulation so that it obscures the rising alcoholic solution at B. The experiment may be performed on a larger scale, using a liter beaker and a capillary of greater diameter, with equally satisfactory results. If soap is omitted from the solution, the emulsion is formed in the same manner but the droplets coalesce
in a few minutes and form large globules, thus illustrating the important function of stabilizing agents. Solids may be dispersed in the same way, and an extremely fine, stable suspension of rosin may be pre-
pared from a solution of the composition: rosin, 0.20 g.; alcohol, 50 cc.; camphor, 0.30 g.; soap, 0.05 g. When this mixture is allowed to flow into water, it behaves much as does the castor oil emulsion, and is soon distributed by a rapid circulation. When a drop of the solution is brought into contact with a drop of water on a glass slide, the action may be viewed with a low-power microscope. As alcohol d i u s e s into the water, minute particles of rosin are precipitated from solution a t the point where the two drops touch. At this boundary, the difference in interfacial tension is so great that as soon as the particles are formed they are shot like tiny bullets toward the interior of the water drop. A vigorous current is set up, and in a few seconds the two drops are completely mixed to form a cloudy suspension. Various other substances may be dispersed by the same method, and following the general formula outlined above, stable suspensions or emulsions of shellac, nitrobenzene, mineral oil, linseed oil, etc., may be prepared. I n some cases, a mixed solvent such as alcohol-ether must be used.
