T H E SCRFACE PROPERTIES
BY WALTER
c. PRESTON ASD
OF SOAP SOLUTIONS* A . S. RICHARDSON**
The foaming tendency of aqueous solutions of soap and various other substances has long been the subject of scientific study. Most workers in this field have agreed in focusing attention upon the peculiarities of the surface film of the solution and many important facts have been discovered regarding its composition, structure and physical properties. It was early foreseen that the formation of stable bubbles from soap solutions must be associated with a concentration of soap a t the surface exposed to air. Leidenfrost, in 1;56, attempted to explain the peculiar properties of the soap bubble by postulating a preferential concentration of the fatty material in the surface of the soap solution,’ thus strangely foreshadowing the idea-so attractive to students of soap solutions-of a surface layer of oriented molecules which, a century and a half later, was developed by Hardy, by Langmuir, and by Harkins. I n a general way, it may be accepted that solutes which impart foaming properties tend to concentrate in the surface layer of the solution and hence in the foam itself, but this fact, considered alone, is of no great value in distinguishing solutions which foam readily from those which do not, as will appear below. As shown both by thermodynamic reasoning and by actual experiment, the concentration of a solute in the surface layer is accompanied by a decrease in surface tension. The fact that soap lowers the surface tension of water has frequently been invoked to explain, at least partially, the foaming tendency of soap solutions. Since, however, there are aqueous solutions, such as acetic acid solution, which do not foam readily in spite of having low surface tension comparable with that of soap solutions which foam copiously, it is evident that neither low surface tension nor the concentration of solute in the surface is, of itself, a sufficient explanation of foaming. Recognizing the inadequacy of low surface tension to explain foaming tendency, some investigators have assumed and attempted to demonstrate the importance of high viscosity, especially surface viscosity, as a basic property underlying high foam power. The idea of a tough surface skin, a superficial viscosity as distinguished from that of the interior, is even an older concept, though less widely known, than the concept of surface tension, which was proposed by J. A. von Segner2 * Presented in part before the Seventh Regional hleeting of the American Chemical Society, Lexington, Ky., Oct. 26, 1928. ** Chemical Division, The Procter and Gamble Company, Ivorydale, Ohio. Leidenfrost : “De aquae communis nonnullis qualitatibus tractabus,” Duisburg, I 756. See article on Capillary Action in Encyclopedia Britannica. * v o n Segner: Comment. Soc. Reg. Gottingen 1, 301 (1751). See article on Capillary Action in Encyclopedia Britannica.
THE SCRFACE PROPERTIES O F SOAP SOLl!TIOSS
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in 17 j r . The basic idea of surface viscosity is generally attributed to Descartes’ and, in its more precise form, has been developed by Plateau* and a number of other investigators. In general, different investigators have been inclined to assign to these two properties, low surface tension and high surface viscosity, the chief role in accounting for the lathering of soap solutions. Some interesting modifications of this view have been urged, but these will best be considered after the experimental part of the present paper. Originally it was the hope of the writers that, following a simultaneous study of surface tension, surface viscosity, and foam power, there might appear among these properties some more precise relationship than has heretofore been observed. As a matter of fact, prolonged investigation failed to develop any such relationship and the majority of the experimental results, being thus of a negative character, are scarcely worth presenting in detail. However, it may be worth while to record a few fragments of the work which go farther than mere failure to establish a positive correlation and suggest that foam power cannot ever be explained in terms merely of surface tension and surface viscosity. Experimental Measurements of Surface Viscosity .An oscillating disk, suspended by a torsion wire, was used for measuring surface viscosity. Its construction followed so closely the description given by Stables and A . E. Kilson3 and by R. E. Wilson and Ries4 that detailed description in the present paper is unnecessary. aisimplified sketch of the apparatus is given in Fig. I . In operation, the monel metal disk was lowered into the surface of the liquid by means of a micrometer screw capable of controlling the depth of immersion to within 0.01 mm. and, starting with an initial torque of 2 I j o ,the amplitude of successive oscillations was observed with the aid of a pointer, rigid with the disk, moving in a horizontal plane over a fixed circular scale. The moment of inertia of the oscillating system, determined by the method of removable disks of known weight as described 3 . The temperature of the water bath was by IT‘ilson and Ries, was 1444 constant at 3o’C 0.1’.The authors are indebted to Dr. C. H. hIilligan for valuable aid in the construction of this instrument especially, also for aid in the prpparation and planning of soiiie of the other apparatus used in this study. Descartes: “Les Ll6moires.” 1638.
* Plateau: “Statique des Liquides”; Pogq. Ann., 141, 44
(1870); Luvini: Phil. Mag., (41, 40, 190 (18701; Neyer: Pogg. Ann., 113, j j , 193, 383 (1861);Oberbeck: W e d . .Inn., 11, 634 (1880); Narangoni: Pogg. .Inn. 143, 342 (1871;;Stables and Wlson: Phil. Mag., (51, 15,406 (1883);Quincke: Kied. Ann., 35, 592 (18891; Shutt, .+. Phgsik, (41, 13,71 ( 1 9 0 4 ~ ’ 1Ietcalf: 2. physik. Chem., 5 2 , I ( i g o j ~ Rohde: ; .Inn. Physik, (4) 19, g j j (1906); i h o r t e r ; Phil. Slag., (6) 11, 317 (1906); 17, 560 (1909). Lo?. cit. 4 Wilson and Ries: First .Imerican Colloid Symposium, 245 (1923).
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WALTER C. PRESTON A S D A . 8. RICHARDSOX
The yiscosity of a solution, as measured by the rotating disk method, is expressed by the formula of Stables and TTilson, v = Kd, t, where v = relative viscosity (with no absolute significance whatsoever), K = moment of inertia of the oscillating system, t = period of oscillation, and d = logarithmic decrement, i.e., the logarithni of the ratio of the amplitude of one oscillation t o the amplitude of the nest succeeding oscillation.1 S L I D I N G 8RRC#
