Surface tension of surfactant solutions

In order to measure the surface tension of solutions containing surfactants, the maximum bubble pressure, pendant drop and Wilhelmy plate (immersed at a constant depth) methods are suitable capillary rise, ring, mobile Wilhelmy plate, sessile drop and drop weight methods are not very suitable. These methods are not recommended because surfactants preferably adsorb onto the solid surfaces of capillaries, substrates, rings, or plates used during the measurement. In a liquid-liquid system, if an interfacially active surfactant is present, the freshly created interface is not generally in equilibrium with the two immiscible liquids it separates. This interface will achieve its equilibrium state after the redistribution of solute molecules in both phases. Only then can dynamic methods be applied to measure the interfacial tension of these freshly created interfaces. 

(1968). The Physics and Chemistry of Surfaces. Dover, New York. 

Aveyard, R. and Haydon, D.A. (1973). An Introduction to the Principles of Surface Chemistry. Cambridge University Press, Cambridge. 

Adamson, A.W. and Gast, A.P. (1997). Physical Chemistry of Surfaces (6th edn). Wiley, New York. 

Davies, J.T. and Rideal, E.K. (1963). Interfacial Phenomena (2nd edn). Academic Press, New York. 

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It was determined, for example, that the surface tension of water relaxes to its equilibrium value with a relaxation time of 0.6 msec [104]. The oscillating jet method has been useful in studying the surface tension of surfactant solutions. Figure 11-21 illustrates the usual observation that at small times the jet appears to have the surface tension of pure water. The slowness in attaining the equilibrium value may partly be due to the times required for surfactant to diffuse to the surface and partly due to chemical rate processes at the interface. See Ref. 105 for similar studies with heptanoic acid and Ref. 106 for some anomalous effects. 

Below, in Sections 5.2 and 5.3, we consider effects related to the surface tension of surfactant solution and capillarity. In Section 5.4 we present a review of the surface forces due to intermo-lecular interactions. In Section 5.5 we describe the hydrodynamic interparticle forces originating from the effects of bulk and surface viscosity and related to surfactant diffusion. Section 5.6 is devoted to the kinetics of coagulation in dispersions. Section 5.7 regards foams containing oil drops and solid particulates in relation to the antifoaming mechanisms and the exhaustion of antifoams. Finally, Sections 5.8 and 5.9 address the electrokinetic and optical properties of dispersions. 

SURFACE TENSION OF SURFACTANT SOLUTIONS 5.2.1 Static Surface Tension... 

The maximum bubble pressure technique is a classical method in interfacial science. Due to the fast development of new technique and the great interest in experiments at very small adsorption times in recent years, commercial set-ups were built to make the method available for a large number of researchers. Rehbinder (1924, 1927) was apparently the first who applied the maximum bubble pressure method for measurement of dynamic surface tension of surfactant solutions. Further developments of this method were described by several authors (Sugden 1924, Adam Shute 1935, 1938, Kuffiier 1961, Austin et al. 1967, Bendure 1971,... 

One of the oldest experimental methods for the measurement of dynamic surface tensions of surfactant solutions is the oscillating jet (OJ) method. The idea is based on the analysis of a stationary jet issuing from a capillary pipe into the atmosphere which oscillates about its... 

To get equilibrium surface tensions of surfactant solutions different procedures of extrapolation are used in the literature. For example, extrapolations of the following forms are used ... 

Effect of temperature on the surface tension of surfactant solutions... 

The data on the temperature dependence of surface tension of surfactant solutions are often used to estimate the thermodynamic characteristics of adsorption and micelle formation. One of such characteristics is the standard free energy of adsorption AG [83, 160, 178-191]. To derive the expression for AG , one can use the relations for the chemical potential in the surface layer and in the solution bulk. The chemical potentials p] depend on the composition of the surface layer and its surface tension y and are given by the relation (2.2), the potentials... 

The surface tension of surfactant solutions is the easiest accessible experimental quantity and hence the most frequently used method to study the adsorption process at liquid interfaces. As earlier shown the rate of adsorption is a function of surface activity and bulk concentration. This explains why a broad time interval has to be experimentally covered to study the large variety of surfactants. A single method cannot provide a sufficiently broad interval so that different complementary methods are needed. Some methods are particularly developed for the short adsorption times, such as the bubble pressure method providing data from less than 1 ms up to some minutes. On the contrary, so-called static methods like the Wilhelmy plate or drop and bubble shape methods give access to very large times, starting from few seconds and reaching up to hours and even days. Both techniques complement each other perfectly. 

The system volume Vs has even stronger effects on the dynamic surface tension of surfactant solutions. For a system volume Vs = 1.5 cm the error in the measured y values, as compared with the value for Vs = 20.5 cm, is 5 to 10%. The results of systematic studies presented elsewhere (V.B. Fainerman and R. Miller 2003) can serve as a guide for a rational choice of the measuring system volume which ensures precise measurement of the lifetime and surface tension. The optimum system volume, if all factors are taken into account, is that for which the ratio of system to bubble volume Vs/Vb is in the range between 2.000 and 5000. 

If the KEP-2 solubihty from the isotherms of surface tension of surfactant solutions in PTMG is compared with the data in [11], which reports that the KEP-2 fraction in PPG corresponding to the critical concentration of micelle formation (CCMF) is 2.5 x 10 , it is evident that KEP-2 solubility in these simple oligoglycols differs tenfold. Temperature dependences of the surface pressime of KEP-2 solutions in PTMG obtained from the experimental isotherms of surface tension at all the investigated mass fractions of KEP-2 are linear. 

The values of and are calculated from experimental data of equilibrium surface tension of surfactant solution upon its concentration according to (8.A.27), of Appendix 8.A. The values of r, F, and C are obtained from experimental conditions. And the values of and Ki are calculated using (8.B.12), if it is written as the following equation ... 

Lunkenheimer, K. and Wantke, K. D., On the applicability of the du Nouy (ring) tensiometer method for the determination of surface tensions of surfactant solutions, J. Colloid Interface ScL, 66, 579-581 (1978). 

The surface tension of surfactant solutions was measured as a function of bulk surfactant molal concentration (mol/kg H2O) with a Wilhelmy-plate-type tension meter (Kyowa CBVP). The accuracy of the surface tension measurements is +0.1 mN/m. The equilibrium surface tension was determined as the value obtained when the surface tension became constant within 0.1 mN/m for 10 min. Near CMC, about 3-5 h were usually required to reach the equilibrium. All measurements were carried out in a thermostated device maintained at a constant temperature of 25, 35, 40, and 45 °C. All glassware and Wilhelmy glass plate were washed in an aqueous solution of chromic acid and sulfuric acid, rinsed in triple distilled water and dried before each measurement. 

The most common methods used to measure surface tension of surfactant solutions using commercial instruments are the du Noiiy ring and Wilhelmy plate techniques (Fig. 4.7c and d). In the former, the force necessary to detach a ring or wire loop from a liquid surface is measured (for example using... 

Figure 4 Advancing contact angle versus surface tension of surfactant solutions (Zisman type plot) for SOM-coated planar quartz surfaces.

In spite of the apparent simplicity of surface tension measurement, correct and reproducible values are not always readily obtainable. In addition to the specific limitations of each technique, the time dependence of surface tension of surfactant solutions can be a major complication. Surface tension depends on the... 

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