Measuring Interfacial Tension

The spuming drop device has been widely used to measure very low inter-fadal tensions in systems where one phase is a microemulsion (see Chapter 4). Unhke the sessile and pendant drop schemes, no contact between the fluid interface and a sohd surface is required. Both the drop and the surrounding fluid layer can also be made rather thin, so that results can be obtained even when the surrounding fluid is somewhat turbid, a frequent occurrence in practical systems. Finally, interfadal tension can, as with the sessile and pendant drops, be followed as a function of time. 

Provided that the contact angle k measured through A is zero and that the tube radius R is sufficiently small, the meniscus has the shape of a hemisphere of radius R. Hence from the Young-Laplace equation (Equation 1.22) we have 

If the interface between A and B in the large region outside the tube is flat, we can again invoke the Young-Laplace equation at the height of this interface  

As in the sessile drop analysis discussed previously, the equations of hydrostatics also apply, so that 

Substituting these equations into Equation 1.61 and solving for y, we find 

One of the key variables governing droplet breakup is the interfacial tension between phases. The pendant drop apparatus can be used to measure either the surface tension between a liquid and air or the interfacial tension between two liquids. The pendant drop method should be used in preference to the du Niioy ring, as it is easier to use and gives more reliable results. 

The method works on the principle that the volume that the droplet reaches within another liquid is related to the interfacial tension between the two liquids by the following expression  

Great care must be taken when making these measurements to ensure that all equipment is scrupulously clean and that the temperature of the fluids and equipment is monitored and constant. The interfacial tension should be calculated based on the average droplet size of a large nnmber of droplets. If repeatable measurements are not obtainable and suitable care has been taken in the performance of the experiment, it is possible that some form of contamination has occurred. 

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These studies, carried out by measuring interfacial tensions, Yq , between aqueous and oil phases, by using the pendant drop method, show that this method is very useful for ternary and quaternary systems. In one system (A), NaDDS + H2O + n-butanol + Toluene... 

The integrated DLS device provides an example of a measurement tool tailored to nano-scale structure determination in fluids, e.g., polymers induced to form specific assemblies in selective solvents. There is, however, a critical need to understand the behavior of polymers and other interfacial modifiers at the interface of immiscible fluids, such as surfactants in oil-water mixtures. Typical measurement methods used to determine the interfacial tension in such mixtures tend to be time-consuming and had been described as a major barrier to systematic surveys of variable space in libraries of interfacial modifiers. Critical information relating to the behavior of such mixtures, for example, in the effective removal of soil from clothing, would be available simply by measuring interfacial tension (ILT ) for immiscible solutions with different droplet sizes, a variable not accessible by drop-volume or pendant drop techniques [107]. 

The choice between the static methods (Wilhelmy plate method and the du Noiiy ring method) should primarily be based on the properties of the system being studied, in particular, the surfactant. As mentioned in UNITD3.5, the transport of surfactant molecules from the bulk to the surface requires a finite amount of time. Since static interfacial tension measurements do not yield information about the true age of the interface, it is conceivable that the measured interfacial tension values may not correspond to equilibrium interfacial tension values (i.e., the exchange of molecules between the bulk and the interface has not yet reached full equilibrium and the interfacial tension values are therefore not static). If the surfactant used in the experiment adsorbs within a few seconds, which is the case for small-molecule surfactants, then both the Wilhelmy plate method and the du Noiiy ring method are adequate. If the adsorption of a surfactant requires more time to reach full equilibrium, then the measurement should not be conducted until the interfacial tension values have stabilized. Since interfacial tension values are continuously displayed with... 

The controlled drop tensiometer is a simple and very flexible method for measuring interfacial tension (IFI) in equilibrium as well as in various dynamic conditions. In this technique (Fig. 1), the capillary pressure, p of a drop, which is formed at the tip of a capillary and immersed into another immiscible phase (liquid or gas), is measured by a sensitive pressure transducer. The capillary pressure is related to the IFT and drop radius, R, through the Young-Laplace equation [2,3] ... 

The results are compared to those above for the CCI4/H2O interface. Several properties of alkane/water and CCU/water interfaces suggest that their interfacial characteristics should be similar. The measured interfacial tensions are 49.7 mN/m for hexane/water and 45 mN/m for CCU/water [73,74], with molecular dipole polarizabilities of 11.9 and 11.2 X 10 cm respectively [75]. However, IR experiments by Conrad and Strauss [76,77] show that water molecules dissolved in an alkane solvent are free to rotate while water dissolved in CCU is relatively constrained. It is the details of these molecular interactions that dominate interfacial structure and dynamics. 

Early researchers turned to foamability or foaminess" measurements to screen surfactants for flow experiments (51). In one variation of this test, a long, vertical glass cylinder with a frit at the bottom was filled with the test solution, and gas was forced through the frit. The height of foam formed in the column was then measured, or, the foam was collected and the amount of liquid in the foam determined (51). In his screening of some 200 materials, Raza measured interfacial tensions of aqueous solutions with respect to air and to oil, and the foamability and foam stability in the absence and presence of oil. The latter experiment consisted simply of shaking the solution in a test tube and measuring the volume of foam at various times (60). 

Flow of the blend at melt temperatures generally stretches the discontinuous phase from its initial shape. Interfacial tension between the immiscible components will oppose this process and attempt to drive the system to a low-energy spherical-morphology state. Studies of these phenomena in a rheometer permit the estimation of the time required for both processes and the interfacial tension [90-93]. Alternatively, one can estimate the time required for the latter process, and the interfacial tension, from the evolving shape of the discontinuous phase using either a fiber break-up [94,95] or fiber-retraction [33,96] experiment. Interfacial tension depends on the molecular weight of each component [96,97] and on temperature, so it is preferable to measure interfacial tension for the materials of interest at their fabrication temperature. 

Basically, all the methods for measuring interfacial tensions described so far have in common that the Helmholtz energy for extending an interface is determined. Upon this extension, the interfacial tension should not vary, otherwise the quantity y would become ill-defined. One of the changes that might be incurred could result from strong curving of the interface. In the present chapter this issue was avoided because we have only considered macroscopic interfaces with radii of curvatures above 0(10-100 nm). Already in sec. 1.2.23c we showed that y is then still independent of curvature. 

Recall that we already derived a similar expression from the van der Waals theory under a number of restrictive simplifications, see (2.5.44 and 45]. There the geometric mean was related to the same mean of Hamaker constants. This equation can be tested experimentally for liquids like water, in which a variety of forces are operative, y can be established by measuring interfacial tensions against organic liquids in which the interaction is dominated by the dispersion forces. This analysis can be illustrated with the data of table 2.3. In (2.11.19] a is an organic liquid (like a hydrocarbon, he) for which it was assumed that only dispersion forces determined the surface tension y = Consequently, y" is the only unknown. Its value appears to be invariant at about 22 mJ m", comprising 30% of the total tension. 

Between [3.6.23a and b] there is a difference of principle, in that the Identity 7 = is operational, whereas in the first equality F° and the r s depend on the choice of the Gibbs dividing plane. Their sum, the grand potential per unit area, is independent of this choice. Equations [3.6.23a and b] do not help to measure interfacial tensions, but serve in Interpreting them. 

Methods used to measure interfacial tension are reviewed by Drelich, Fang, and White [ Measurement of Interfacial Tension in Fluid-Fluid Systems, in Encyclopedia of Surface and Colloid Science (Dekker, 2003), pp. 3152-3156]. Also see Megias-Alguacil, Fischer, and Windhab, Chem. Eng. Sci., 61, pp. 1386-1394 (2006). One class of methods derives intertacial tension values from measurement of the shape, contact angle, or volume of a drop suspended in a second liquid. These metho(Js include the pendant drop method (a drop of heavy liquid hangs from a vertically mounted capillary tube immersed... 

Contact Angle and SAXS Studies. The contact angles which developed at the junction of flocculated oil droplets, illustrated in Figure 2, can be used together with the measured interfacial tensions (Table 1) in a vector diagram to demonstrate that the interfacial tensions on the outer surface of the oil drops (yqw) are appreciably greater than the interfacial tensions (YqW) In the plane of contact between drops ... 

Table II retabulates the data of Table I according to equation (4) and shows that the measured interfacial tension of the oil/water interface and the contact angle allow determination of Ymo an YMW ...

Perhaps the most striking property of a microemulsion in equilibrium with an excess phase is the very low interfacial tension between the macroscopic phases. In the case where the microemulsion coexists simultaneously with a water-rich and an oil-rich excess phase, the interfacial tension between the latter two phases becomes ultra-low [70,71 ]. This striking phenomenon is related to the formation and properties of the amphiphilic film within the microemulsion. Within this internal amphiphilic film the surfactant molecules optimise the area occupied until lateral interaction and screening of the direct water-oil contact is minimised [2, 42, 72]. Needless to say that low interfacial tensions play a major role in the use of micro emulsions in technical applications [73] as, e.g. in enhanced oil recovery (see Section 10.2 in Chapter 10) and washing processes (see Section 10.3 in Chapter 10). Suitable methods to measure interfacial tensions as low as 10 3 mN m 1 are the sessile or pendent drop technique [74]. Ultra-low interfacial tensions (as low as 10 r> mN m-1) can be determined with the surface light scattering [75] and the spinning drop technique [76]. 

Equation (515) is known as Vonnegut s equation and it is valid on the assumption that the drop is in equilibrium and its length is larger than four times its diameter (/ > 4r ). The spinning drop tensiometer method is widely used for measuring liquid-liquid interfacial tension, and is especially successful for examination of ultra-low interfacial tensions down to l(T6mNnr1. In addition, it can also be used to measure interfacial tensions of high viscosity liquids when precise temperature control is maintained. 

The micropipette technique was developed in the 1990s to measure interfacial tensions of micrometer-sized droplets such as vesicles. This method is dependent on the pressure... 

It came to be realized that interfaces between low molecular weight liquids in immiscible liquid mixtures possess an interfacial tension. Experimental measurements of interfacial tension between low molecular weight liquids were first made in the nineteenth century (59,60). Various experimental techniques have been developed to measure interfacial tensions. A popular method has been the pendant drop measurement, which was originally developed by Andreas et al. (61)... 

Chen et al. [36] presented a conclusive comparistMi of the latex particle surface polarities (as determined by contact angle measuremoits) to the measured interfacial tensions of various polymer phases dissolved in the second-stage monomer against the aqueous phase. They prepared PS/PEMA composite particles using two different monodisperse PS seed latexes (produced by the Dow Chemical Co. as model colloids). The results of the interfacial tension measurements for each polymer phase, i.e. PS core particles and PEMA shell polymer dissolved in EMA monomer, showed that the interfacial tension first decreased and then remained constant as the polymer concentration was increased. This demonstrated that the polarity of the polymer-monomer solution interface with the aqueous phase increased until reaching a certain equilibrium point, which depended on the amount and nature of the polar components of the polymer. 

Protein first came into the model as a result of considerations of interfacial tension. Danielli and Davson (1934/5) pointed out that measured interfacial tension at lipid-water interfaces was much higher than that evident at cell surfaces. Therefore they put forward a new model in which the lipid bilayer was encased in a sandwich of protein, thereby avoiding this problem and accommodating the protein known to be present in membranes. Thirty years later, the basic premise upon which protein was added to the model was shown to be incorrect (Haydon and Taylor, 1963) phospholipids have interfacial... 

Wu selected molten polyethylene as a reference liquid. In Table 4, the dispersion and polar components for selected organic polymers are listed along with their measured interfacial tensions. 

Given that surface tension data are not always available at the temperature of interest, the expression derived by Guggenheim [35] can be used to take into account the variation of temperature [26]. It is also possible to experimentally measure interfacial tensions via the same techniques applicable for surface tension measurements [30]. 

This relationship among the two bulk phase pressures, the mean curvature, and the interfacial tension is fimdamental to the study of fluid interfaces. It is often referred to as the Laplace or the Young-Laplace equation and is the basis for several methods of measuring interfacial tension. Note that with the interface cnrved in the manner shown in Figure 1.2, H is negative and Equation 1.22 implies that Pa > Pb- Thus the pressure inside a drop or bubble always exceeds the pressure outside. 

A relatively simple way to measure interfacial tensions is the drop weight or drop volume technique. The size of a pendant drop (Figure 1.7) is slowly increased imtil the drop can no longer be prevented from falling and breaks off. The total weight or volume of a known number of drops is measured. 

FIGURE 1.11 Schematic diagram of the Wilhelmy plate method for measuring interfacial tension. 

As indicated in Section 7, one method of measuring interfacial tension between two fluids is the spitming drop device. A drop of the less dense fluid A is placed in the denser fluid B, and both fluids are rotated at a... 

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