•?S?u'rface, and on an aged surface, of ':•[{the system under investigation. • •iSuch measurements lead to the [•ild•namic and the static surface ?•nsion respectively. The time taken •}iby a .{reshl• formed surface to reach :•{• ...... uilibrium is related to the rate of of the solute from the bulk to the surface layer. The mechanical properties of the Surface layer may be investigated the two-dimensional pendulum cometer. Plateau," Wilson and • and later on Clark', have used •his instrument for determining the !i:i•i:i•Sscosity, elasticity and plasticity }• i!øf the surface layer.. Interesting iii:i. results were obtained on aqueous iii?Sølutions' of saponins, Na oleate, Na ?Stearate, proteins, and proteins ?tanned with polyvalent metal'ions. ?'Xlthough this technique would :iSappear quite simple, the results are }i?'erratic and the method is not sensi- •i: tive enough to detect a change in the }11. mechanical properties following the ?.iadsorption of surface-active agents !:i which are known' to ca. use extensive .':.)•foaming. For example, Burcik • was :unable to detect any change in the :: pendulum viscometer behaviour, ,. using Na laurate, Na laurylsulphate aqueous solutions, from a clean :::: water surface. A NEW TECHNIQUE DEVELO1 ED A new technique has been devel- oped by the author' which enables us to follow a significant change in the pr.operties of such system. This technique has been applied to a wide range of amphipatic compounds, AND FOAM STABILITY anionic, cationic and non-ionic series. Saponins and proteins were also investigated. This technique originates from the work of Lenard • in 1924, who meas- ured the surface tension of liquids by means of a rectangular wire frame. When the frame is with- drawn from the bulk of the liquid and raised above the surface, a pull is being applied. This pull increases gradually and, after reaching a maximum, drops slightly. The value then obtained is a measure of the surface tension of the liquid. An interesting feature inherent. to the rectangular wire frame, as opposed to the ring, is the appearance for some systems of a region over which the frame is raised under a constant pull. When this is observed, a thin lamina forms in the plane of the wire frame. Using an aqueous solution of Na oleate, for example, this lamina is seen to display interference colours and drainage of the inter- lameIlar liquid similar to those observed in the foam columns. Before the rectangular wire frame technique could be applied for the study of foaming, several factors had to be controlled. It was found, for example, that the rate of withdrawal of the frame had a marked effect on the extension of the lamina. Moreover, the surface tension of the lamina was also found to vary with this rate. For this purpose an automatic recording apparatus was designed by the authorJ ensuring the with- drawal of the platinum wire frame under a constant rate. At the same 219

JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS •, T,f ?,. tit Y Figure 3. The three tyDes of curves (See adjacent column.) This apparatus permits the auio- matic recording of the force exerted on the frame by the liquid surface as a function of the height of the frame over the surface. Figure 1 shows the torsion head and the driving mechanism, while Figure 2 represents schematically the essential parts of the instrument used by the author. time the force acting on the frame was recorded on a photographic paper as a function of the displace- ment of the frame above the liquid surface. The traction curve was obtained during this part of the experiment. When the wire frame was returned to the liquid before breakage occurred with the surface, the variation of the force acting was then recorded on the retraction curve. The apparatus consists essentially of a torsion head measuring device, of an optical system and of a driving mechanism. The frame is connected to a well-balanced wheel, centred on a horizontal torsion wire. The movements of the wheel are damped by attaching to it an oil damper. The vessel containing the liquid under investigation is placed on a table, which may be moved vertically in both directions by connecting it to a motor, which drives at the same time a photographic paper. The table may'be moved at various speeds by a system of gears which connect it to the motor. A light source is placed in front of the wheel and a small mirror attached to it reflects the rays through a slit on a photographic paper. EXPERIMENTAL t•ESULTS I. Low Traction Speeds Using low speeds of traction, i.e., 1 cm. for 10 minutes, three types of curves may be obtained depending on the System • (see Figure 3). Type I. The frame breaks the liquid surface after a maximi•m pull has been applied in Q. This always,occurs with a clean water surface at any speed of traction. :' Type II. The force 'applied drops.:: ii slightly after the maximum ism: reached, from Q to Ri This'!11!•i ' occurs when the water surface covered by an adsorbed layer, :' :i:' leading to a lamina of short ? life-time. Type III. Along RS the force acting I on the frame remains constanL!.: This is obtained when the/i((. solution can give rise to lamina of prolonged stability !:,' Along OPQ a meniscus' is and as O (see Figure 4) decreases force rises to a maximum till 0 -- A lamina is obtained in R and this lamina increases in area,' RS is.'fi! recorded on the traction curve. The slight drop in the 220