THE MEASUREMENT OF SURFACE TENSION

There are static and dynamic methods. The static methods measure the tension of practically stationary surfaces which have been formed for an appreciable time, and depend on one of two principles. The most accurate depend on the pressure difference set up on the two sides of a curved surface possessing surface tension (Chap. I, 10), and are often only devices for the determination of hydrostatic pressure at a prescribed curvature of the liquid these include the capillary height method, with its numerous variants, the maximum bubble pressure method, the drop-weight method, and the method of sessile drops. The second principle, less accurate, but very often convenient because of its rapidity, is the formation of a film of the liquid and its extension by means of a support caused to adhere to the liquid temporarily methods in this class include the detachment of a ring or plate from the surface of any liquid, and the measurement of the tension of soap solutions by extending a film. 

The dynamic methods depend on the fact that certain vibrations of a liquid cause periodic extensions and contractions of its surface, which are resisted or assisted by the surface tension. Surface tension therefore forms an important part, or the whole, of the restoring force which is concerned in these vibrations, and may be calculated from observations of their periodicity. Dynamic methods include determination of the wave-length of ripples, of the oscillations of jets issuing from non-circular orifices, and of the oscillations of hanging drops. Dynamic methods may measure a different quantity from the static methods, in the case of solutions, as the surface is constantly being renewed in some of these methods, and may not be old enough for adsorption to have reached equilibrium. In the formation of ripples there is so little interchange of material between the surface and interior, and so little renewal of the surface, that the surface tension measured is the static tension ( 12.  

Unfortunately the dynamic methods do not give, m their present stage of development, a quantitative measure of the time which has elapsed since the formation of the surface before the measurement is effectively taken, and the extent to which adsorption has proceeded is therefore not ascertainable. In the case of solutions showing pronounced adsorption, therefore, the interpretation of the measurements made by dynamic methods can usually only be qualitative. 

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The topic of capillarity concerns interfaces that are sufficiently mobile to assume an equilibrium shape. The most common examples are meniscuses, thin films, and drops formed by liquids in air or in another liquid. Since it deals with equilibrium configurations, capillarity occupies a place in the general framework of thermodynamics in the context of the macroscopic and statistical behavior of interfaces rather than the details of their molectdar structure. In this chapter we describe the measurement of surface tension and present some fundamental results. In Chapter III we discuss the thermodynamics of liquid surfaces. 

Figure 1 gives the measurements of surface tension used for determining the CMCs of sulfonate/Genapol and nonylphenol 30 E.O. mixtures, with the last surfactant being called a desorbent (this term will be justified below). Minimum in surface tension was seen only for a few nonionic solutions (e.g. NP 50 E.O.). In this case, we used dyes that, once solubilized in the micelles, cause the solution to change color, which is another way of measuring the CMC. 

It is impossible to complete a discussion of the measurement of surface tension without saying something about the need for extreme cleanliness in any determination of 7. Any precision chemical measurement requires attention to this consideration, but surfaces are exceptionally sensitive to impurities. It is often noted that touching the surface of 100 cm2 of water with a fingertip deposits enough contamination on the water to introduce a 10% error in the value of 7. Not only must all pieces of equipment be clean, but also the experiments must be performed within enclosures or in very clean environments to prevent outside contamination. In addition, both surface tension and contact angle should be measured under constant temperature conditions. 

List a few methods for the measurement of surface tension and contact angle. Discuss the basic principles involved in each method. What are the experimental advantages and disadvantages in each case ... 

In this chapter attention will be directed to the conclusions which can be drawn as to the molecular structure of liquids, from the measurement of surface tension. No attempt will be made to tabulate the great amount of accurate data now available on surface tension this task has already been performed by Harkins and Young,1 by Bakker,2 and by many others. 

Harkins and his colleagues1 have extended these measurements of the work of adhesion to water, and also to mercury.2 The measurements of surface tension were made by the drop-weight method, using the corrections necessary for accurate results as three separate measurements of surface tension are required, considerable accuracy is desirable for trustworthy results in the work of adhesion. 

RESULTS OF THE MEASUREMENT OF SURFACE TENSION The following data, from Harkins, Clark and Roberts,1 are significant. 

Mar. 22,1874 Semily, then Austro-Hungarian Empire -Apr. 16, 1921, Prague, Czechoslovakia) Since 1912, Professor of experimental physics at Charles University, Prague. Kucera introduced the measurement of surface tension of polarized mercury by applying the dropping mercury electrode [i] rather than the Lippmann capillary electrometer, and he inspired thereby -> Heyrovsky, J. to introduce - polarography. 

The measurement of surface tension is an old trade in science. There are consequently many methods of determining this quantity. In chemistry, the surface tension is measured as a function of the solvent, the composition of the solution, and the nature of the two phases. In electrochemistry the potential is an added variable. The so-called electrocapillary curve is a plot of surface tension of a liquid metal (usually mercury) electrode versus potential, at a given composition of the solution. This type of measurement is then repeated in solutions of different composition, to obtain the surface excess of the appropriate species, as discussed earlier. 

J. F. Padday, Surface Tension, Part II. The Measurement of Surface Tension in Surface and Colloid Science, Vol. 1, E. Matijevic, Ed. Wiley-Interscience (1969), p. 101 (review of methods, contains some required tables) J.F. Padday, Surface Tension, Part III. Tables Relating the Size and Shape of Liquid Drops to the Surface Tension, ibid, p. 151. (Collation of Bashforth and Adams tables and extensions or modifications to make them more suitable for actual situations contains an introduction to explain the conversion of parameters in different geometries.)... 

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