Interfacial measurements

The development of hydrodynamic techniques which allow the direct measurement of interfacial fluxes and interfacial concentrations is likely to be a key trend of future work in this area. Suitable detectors for local interfacial or near-interfacial measurements include spectroscopic probes, such as total internal reflection fluorometry [88-90], surface second-harmonic generation [91], probe beam deflection [92], and spatially resolved UV-visible absorption spectroscopy [93]. Additionally, building on the ideas in MEMED, submicrometer or nanometer scale electrodes may prove to be relatively noninvasive probes of interfacial concentrations in other hydrodynamic systems. The construction and application of electrodes of this size is now becoming more widespread and general [94-96]. 

Emulsion properties and stability can be measured by rheological studies and dielectric spectroscopy. Rheological studies include forced oscillation experi ments. The formation of stable emulsions is marked by a sharp increase in the elastic modulus. Water con tent is not a good indicator of emulsion characteristics other than that low water contents (<50%) indicate that an emulsion has not been formed and that the product is entrained water-in-oil. Interfacial measure ments are useful for measuring the film strength of... 

Liquid-liquid systems find applications in many areas of engineering, physics and chemistry, such as liquid-liquid extractions, phase transfer catalysis, nanoparticle synthesis, coating flows and interfacial measurements. The efficiency and stability of the separation depends on the fluid flow field, the thermodynamic equilibria and the mass transfer between phases. Although experimental studies on liquid-liquid flows exist in the literature, both the use of ionic liquids and the nuclear application make this research particularly novel and timely. 

Workers have also used a number of commercially available FEM packages to allow the simulation of immiscible liquid-liquid interfacial measurements [125-127] and approach curves for scanning electrochemical microscope applications [128]. Compton and coworkers have also used a commercial FEM package to model coupled heat and mass transport at a wire [129], and dissolution kinetics in flow-through devices [130]. 

Unfortunately, owing to experimental difficulties, interfacial measurements in molten salts often need to be made at solid metal substrates, and these give rather less reliable results.For example, the agreement between the reported values of the minimum capacitance for both solid and liquid metals tends to be quite poor. This may be ascribed to melt impurities, especially water and, at the higher temperatures, spurious parallel components arising from materials instabilities. However, progress has been achieved in a number of experimentally difficult situations, notably in the alumina-cryolite melt system which is so important in aluminum production. 

H. Takiguchi, T. Odake, M. Ozaki, T. Umemura, and K.I. Tsunoda, "Liquid/ liquid optical waveguides using sheath flow as a new tool for Uquid/Uquid interfacial measurements," Applied Spectroscopy, vol. 57, pp. 1039-1041,2003. 

The interfacial tension is usually expressed in mN/m or dynes/cm. It measures the tendency for a liquid to form an interface having the least area. The interfacial tension decreases as the temperature Increases. 

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]. 

Fig. 11-13. Apparatus for measuring the time dependence of interfacial tension (from Ref. 34). The air and aspirator connections allow for establishing the desired level of ftesh surface. IV denotes the Wilhelmy slide, suspended from a Cahn electrobalance with a recorder output.

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]. 

IHP) (the Helmholtz condenser formula is used in connection with it), located at the surface of the layer of Stem adsorbed ions, and an outer Helmholtz plane (OHP), located on the plane of centers of the next layer of ions marking the beginning of the diffuse layer. These planes, marked IHP and OHP in Fig. V-3 are merely planes of average electrical property the actual local potentials, if they could be measured, must vary wildly between locations where there is an adsorbed ion and places where only water resides on the surface. For liquid surfaces, discussed in Section V-7C, the interface will not be smooth due to thermal waves (Section IV-3). Sweeney and co-workers applied gradient theory (see Chapter III) to model the electric double layer and interfacial tension of a hydrocarbon-aqueous electrolyte interface [27]. 

Usually one varies the head of mercury or applied gas pressure so as to bring the meniscus to a fixed reference point [118], Grahame and co-workers [119], Hansen and co-workers [120] (see also Ref. 121), and Hills and Payne [122] have given more or less elaborate descriptions of the capillary electrometer apparatus. Nowadays, the capillary electrometer is customarily used in conjunction with capacitance measurements (see below). Vos and Vos [111] describe the use of sessile drop profiles (Section II-7B) for interfacial tension measurements, thus avoiding an assumption as to the solution-Hg-glass contact angle. 

The points in Fig. V-12 come from three types of experimental measurements. Explain clearly what the data are and what is done with the data, in each case, to get the w-versus- plot. What does the agreement between the three types of measurement confirm Explain whether it confirms that is indeed the correct absolute interfacial (Krtential difference. 

SHG measurements have been made on a large number of interfacial sys-... 

Values of interfacial tension of nucleus from turbulent jet measurements, by various equations [44]. 

Yaminsky and Yaminskaya [114] have used a Wilhelmy plate to directly measure the interfacial tension (and hence infer the contact angle) for a surfactant solution on... 

The extensive use of the Young equation (Eq. X-18) reflects its general acceptance. Curiously, however, the equation has never been verified experimentally since surface tensions of solids are rather difficult to measure. While Fowkes and Sawyer [140] claimed verification for liquids on a fluorocarbon polymer, it is not clear that their assumptions are valid. Nucleation studies indicate that the interfacial tension between a solid and its liquid is appreciable (see Section K-3) and may not be ignored. Indirect experimental tests involve comparing the variation of the contact angle with solute concentration with separate adsorption studies [173]. 

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