Measurements of surface tension

If the second term is to have 1 % of the value of the first, then 0.01 = This 

In principle, by measuring the force needed to extend the film, the wire frame shown in Fig. 18.1 could be used to measure the surface tension. In practice, other devices are more convenient. The ring-pull device (called the duNouy tensiometer) shown in Fig. 18.2 is one of the simplest of these. We can calibrate the torsion wire by adding tiny masses to the end of the beam and determining the setting of the torsion scale required to keep the beam level. To make the measurement, we place the ring on the beam and raise the liquid to be 

The length is twice the circumference since the liquid is in contact with both the inside and the outside of the ring (Fig. 18.2b). This method requires an empirical correction factor,/, which accounts for the shape of the liquid pulled up and for the fact that the diameter of the wire itself, 2r, is not zero. Then Eq. (18.1) can be written as 

Extensive tables off as a function of R and r are available in the literature. The method is highly accurate if we use Eq. (18.1a) Eq. (18.1) is much too crude for accurate work. 

The Wilhelmy slide method is somewhat similar to the ring-pull method. A very thin plate, such as a microscope cover glass or a sheet of mica, is hung from one arm of a balance and allowed to dip in the solution (Fig. 18.3). If p is the perimeter of the slide, the downward pull on the slide due to surface tension is yp. If F and F are the forces acting downward when the slide is touching the surface and when it is suspended freely in air respectively, then 

One of the simplest measurements of surface tension is by means of capillary rise. A small-diameter glass tube is inserted in a bath of liquid see Fig. 17.4. The fluid is assumed to wet the surface of the tube perfectly, so that the contact angleis 0. For small-diameter tubes, the free surface of the liquid in the tube is practically a hemisphere, so the film pulls up uniformly around the perimeter, and the net surface force upward is 

This is opposed by the gravity force on the column of fluid, which is equal to the weight of the fluid which is above the free surface and which equals 

Here p, is the density of the liquid. Equating these surface tension, we find 

This equation omits the buoyant force due to the air, which is generally small compared with the fluid s weight. However, these omissions are not as serious as the difficulty of knowing the small diameter of the tube accurately and the difficulty of getting the inside of the tube very clean so that there will be perfect wetting and 6 will be zero. 

To solve these problems, we sometimes use the drop-weight method see Fig. 17.5. In this method we allow drops to fall slowly (one every 2 to 5 min) from the tip of a burette or hypodermic needle. The drops are caught and weighed. If the liquid wets the burette perfectly, then at the instant that the drop breaks away its weight must be exactly equal to the surface force holding it up, or 

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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. 

Bianco and Marmur [143] have developed a means to measure the surface elasticity of soap bubbles. Their results are well modeled by the von Szyszkowski equation (Eq. III-57) and Eq. Ill-118. They find that the elasticity increases with the size of the bubble for small bubbles but that it may go through a maximum for larger bubbles. Li and Neumann [144] have shown the effects of surface elasticity on wetting and capillary rise phenomena, with important implications for measurement of surface tension. 

Another approach to measurement of surface tension, density, and viscosity is the analysis of capillary waves or ripples whose properties are governed by surface tension rather than gravity. Space limitations prevent more than a summary presentation here readers are referred to several articles [123,124]. 

A direct measurement of surface tension is sometimes possible from the work of cleaving a crystal. Mica, in particular, has such a well-defined cleavage plane that it can be split into large sheets of fractional millimeter thickness. Orowan... 

The simplest technique introduced by Young as early as 1805 [18] is the measurement of the contact angle as a measure of surface tension and surface energy [1,19, 20,21], In many cases this gives an indication of surface composition and can be used to observe changes in composition, structure and/or roughness at the surface during a particular surface treatment. A quantitative description or distinction between different parameters is hardly possible in most cases. 

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. 

Is It Possible to Measure Surface Tension of Solid Metal and Solution Interfaces In contrast to measurement of surface tension for liquids, the direct measurement of surface tension for solids can be considered an impossible task. However, it is possible to apply indirect measurements to obtain electrocapillary curves of solid electrodes and therefore the information from these curves. 

The Lippmann equation gives the charge density of the electrode based on electrocapillary measurements. This equation can be approximate as (Ay/AV)const comp= —qM. Measurements of surface tension of Hg in contact with 1.0AHC1 gave the following data ... 

Now we are better equipped to consider measurements of surface tension in detail. Section 6.8 describes the use of shapes of menisci, drops, and bubbles for such measurements, and Section 6.9 considers the practically important case of contact of liquids with porous solids and powders. 

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 ... 

The material in this chapter is organized broadly in two segments. The topics on monolayers (e.g., basic definitions, experimental techniques for measurement of surface tension and sur-face-pressure-versus-area isotherms, phase equilibria and morphology of the monolayers, formulation of equation of state, interfacial viscosity, and some standard applications of mono-layers) are presented first in Sections 7.2-7.6. This is followed by the theories and experimental aspects of adsorption (adsorption from solution and Gibbs equation for the relation between... 

To provide a specific example, Imae and Ikeda (ref. 479) state that amine oxide is very hydrophilic and can constitute a good polar head group for nonionic surfactants at neutral pH. Dimethyl-dodecylamine oxide was first prepared by Hoh et al. (ref. 494), and its surface-active properties in aqueous solutions were investigated by measurements of surface tension (ref. 495), light scattering (ref. 496-498), and hydrodynamic properties (ref. 499). It was found that dimethyldodecylamine oxide can form only spherical micelles in water and aqueous NaCl solutions, when the micelle concentration is dilute (ref. 496,498). Similarly, the homolog dimethyltetradecylamine oxide forms only spherical micelles in water (ref. 496). 

The available data from emulsion polymerization systems have been obtained almost exclusively through manual, off-line analysis of monomer conversion, emulsifier concentration, particle size, molecular weight, etc. For batch systems this results in a large expenditure of time in order to sample with sufficient frequency to accurately observe the system kinetics. In continuous systems a large number of samples are required to observe interesting system dynamics such as multiple steady states or limit cycles. In addition, feedback control of any process variable other than temperature or pressure is impossible without specialized on-line sensors. This note describes the initial stages of development of two such sensors, (one for the monitoring of reactor conversion and the other for the continuous measurement of surface tension), and their implementation as part of a computer data acquisition system for the emulsion polymerization of methyl methacrylate. 

The independent measurements of surface tension were obtained by the tedious Wilhelmy plate method. Figure 3 illustrates such a calibration curve for one set of orifices and for five types of test fluids (methanol-water, ethanol-water, acetone-water, sodium lauryl sulfate in water saturated with methyl methacrylate, and polymethylmethacrylate latices). This is a "universal" calibration curve independent of the fluid being monitored. For the 63 data points shown in Figure 3, the least squares regression line is given by... 

Figure 7. Wilhelmy plate measurements of surface tension for SLS solutions (A) no MM A (B) saturated with MM A...

Measurement of Surface Tension in Surface and Colloid Science, Vol. 1, Matijevic, E. (Ed.), Wiley-Interscience New York, 1969, pp. 101-149. 

Examination of the relevant theory indicates that the adjuvant effect of surface-active agents on herbicide action is maximized when the quantity FI = yL cos 0, or the film pressure at the liquid/solid interface, has a maximum value. Measurement of surface tension of 1.0% aqueous solutions and of contact angle on a number of substrates (Teflon, paraffin) and plant-leaf surfaces (soybean, com) as a function of hydrophile-lipophile balance show at least one maximum, and these values are in good agreement with earlier experimental data on herbicidal activity. 

The proportionality constant between the applied potential and the charge due to the species ordering in the solution interfacial region is the double layer capacity. The study of the double layer capacity at different applied potentials can be done by various methods. One much used is the impedance technique, which is applicable to any type of electrode, solid or liquid, and is described in Chapter 11. Another method uses electrocapillary measurements. It was developed for the mercury electrode, being only applicable to liquid electrodes, and is based on measurement of surface tension. 

Fig. 3.2 The experimental arrangement for measurement of surface tension of mercury by Lippmann s method.

Thermal effects of increasing the surface. Total surface energy. The results of measurements of surface tension show that, almost invariably,1 surface tension decreases with rising temperature. Kelvin2 showed that it follows that there is an absorption of heat when the surface of a liquid is extended. Let q8 be the heat absorbed during an extension of one square centimetre. 

Solutions of soaps and other long-chain Colloidal electrolytes. The surface tension of soaps has been very extensively studied,1 but for the most part the results in the literature are discordant far beyond the usual error of measurement of surface tension. In general the surface tension diminishes rapidly with increasing concentration, reaching a steady, or nearly steady, low value after a certain concentration is reached this concentration is naturally lower the longer the hydrocarbon chain. The variation between the results obtained by different experimenters, and even by the same experimenter under different conditions, may... 

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. 

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