Interfacial tension methods

Extreme cleanliness of all surfaces and interfaces is required in all surface and interfacial tension methods. This is also required to achieve the condition 0=0 that some of the methods require. For most of the methods the apparatus must be absolutely vibration-free. In all of the methods the temperature must be carefully controlled. In the methods where densities are needed, these must be accurately known. 

The interfacial tension methods are described in ISO 6889 [20], ISO 9101 [21] and ASTM D1331 -89 (2001) [ 18 ]. The method described in ISO 6889 is a simple method and applicable for the systems if the interfacial values arebetween 4 and 50 dyne cm-1, the immiscible liquids are water and organic liquids and the systems contain non-ionic or anionic surfactants but not cationic surfactants. The repeatability is within about 2 dyne cm-1. On the other hand, the drop volume method as described in ISO 9101 can be used for viscous liquids and liquids containing all types of surfactants. This method can measure the interfacial tension as low as 1 dyne cm-1 with 0.5 dyne cm-1 accuracy. If the interfacial tension is lower than 1 dyne cm-1, the spinning drop will be the suitable method. 

Eq. (2.18) is the exact definition of the experimental relationship for the determination of surface tension by measuring the corresponding pressure differences and radii of curvature. This relationship is the basis of many experimental surface and interfacial tension methods measuring for example the volume of detaching drops (Section 5.2), the pressure inside bubbles (Section 5.3) or drops (Section 5.5), and the shape of sessile or pendent drops (Section 5.4). 

Characteristics of dynamic surface and interfacial tension methods... 

Other Dynamic Surface and Interfacial Tension Methods... 

In the following a brief overview will be given of the most frequently used surface and interfacial tension methods, mostly available as commercial set-ups. 

Relatively new are the developments of microfluidic surface or interfacial tension methods. Such methods offer the potential for rapid, online measurements on small volume samples. Typically, use is made of a property change (such as droplet deformation or pressure drop) associated with fluid flow through some kind of constriction in a microchannel. The fundamental principles are usually the same as in the examples just given above, such as shape or pressure changes. If the device is used to generate bubbles, then the bubble formation frequency can be used as the basis for surface tension calculation. In a multiple-channel device, multiple bubbles or droplets are usually sensed simultaneously. These kinds of approaches have been applied to the determination of both surface and interfacial tensions [20-22]. 

There are surface or interfacial tension methods for liquid systems, which cover quite a wide range of time. In many cases, however, several methods are required in order to produce the complete time-dependence of surface tension of a surfactant system. The following sections will briefly present the most frequently used methods for measuring dynamic surface and interfacial tensions, while the physical principles and a typical experimental set-up, as well as some experimental examples, will also be given. 

The Parachor is a parameter used to determine the interfacial tension. It can be estimated by a simple method proposed by Quayle in 1953 ... 

A zero or near-zero contact angle is necessary otherwise results will be low. This was found to be the case with surfactant solutions where adsorption on the ring changed its wetting characteristics, and where liquid-liquid interfacial tensions were measured. In such cases a Teflon or polyethylene ring may be used [47]. When used to study monolayers, it may be necessary to know the increase in area at detachment, and some calculations of this are available [48]. Finally, an alternative method obtains y from the slope of the plot of W versus z, the elevation of the ring above the liquid surface [49]. 

Princen and co-workers have treated the more general case where w is too small or y too large to give a cylindrical profile [86] (see also Refs. 87 and 88). In such cases, however, a correction may be needed for buoyancy and Coriolis effects [89] it is best to work under conditions such that Eq. 11-35 applies. The method has been used successfully for the measurement of interfacial tensions of 0.001 dyn/cm or lower [90, 91]. 

Another oscillatory method makes use of a drop acoustically levitated in a liquid. The drop is made to oscillate in shape, and the interfacial tension can be calculated from the resonance frequency [113]. 

Molten naphthalene at its melting point of 82°C has the same density as does water at this temperature. Suggest two methods that might be used to determine the naphthalene-water interfacial tension. Discuss your suggestions sufficiently to show that the methods will be reasonably easy to cany out and should give results good to 1% or better. 

Critical Micelle Concentration. The rate at which the properties of surfactant solutions vary with concentration changes at the concentration where micelle formation starts. Surface and interfacial tension, equivalent conductance (50), dye solubilization (51), iodine solubilization (52), and refractive index (53) are properties commonly used as the basis for methods of CMC determination. 

One concerned with the measurement of gas-hquid interfacial tension should consult the useful reviews of methods prepared by Harkius [in Chap. 9 of Weissberger, Techniques of Organic Chemstry, 2d ed., vol. I, part 2, Interscience, New York, 1949), Schwartz and coauthors [Suiface Active Agents, vol. I, Interscience, New York, 1949, pp. 263-271 Suiface Active Agents and Detergents, vol. 2, Interscience, New York, 1958, pp. 389—391, 417—418], and by Adamson [Physical Chemistry of Suifaces, Interscience, New York, I960]. 

Phosphoric acid ester was used as a model for the estimation of concentration of a reagent in an adsorbed layer by optical measurements of the intensity of a beam reflecting externally from the liquid-liquid interface. The refractive index of an adsorbed layer between water and organic solution phases was measured through an external reflection method with a polarized incident laser beam to estimate the concentration of a surfactant at the interface. Variation of the interfacial concentration with the bulk concentration estimated on phosphoric acid ester in heptane and water system from the optical method agreed with the results determined from the interfacial tension measurements... 

The mapping (7) introduces the unknown interface shape explicitly into the equation set and fixes the boundary shapes. The shape function h(x,t) is viewed as an auxiliary function determined by an added condition at the melt/crystal interface. The Gibbs-Thomson condition is distinguished as this condition. This approach is similar to methods used for liquid/fluid interface problems that include interfacial tension (30) and preserves the inherent accuracy of the finite element approximation to the field equation (27)... 

The phase inversion temperature (PIT) method is helpful when ethoxylated nonionic surfactants are used to obtain an oil-and-water emulsion. Heating the emulsion inverts it to a water-and-oil emulsion at a critical temperature. When the droplet size and interfacial tension reach a minimum, and upon cooling while stirring, it turns to a stable oil-and-water microemulsion form. " ... 

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