Variation in interfacial tension

While interfacial contaminants tend to reduce the mass transfer coefficients by causing the droplets to be stagnant rather than circulating, another surface effect may enhance mass transfer. This is the Marangoni effect, whereby local variations in interfacial tension due to the mass transfer process itself can create rapid motions (interfacial turbulence) at the interface. 

Electrowetting derives its roots from early observations of electrocapiflary phenomena by Gabriel Lippmann in 1875, who noted variations in interfacial tension as an electric potential is applied between an electrol34e solution in direct contact with a metal, in this case, mercury. This culminated in the classical Lippmann equation ... 

The relationship between hydrophobic group adherence and interfacial tension impinges on a wide spectrum of biological activities. Only the more important influences can be considered here and then only very briefly. It is convenient to discuss these in two parts, namely the influence of hydrophobic groups on water-structure, and the potential of variation in interfacial tension on the expression of biopolymer conformational change. 

The Potential of Variation in Interfacial Tension on the Expression of Biopolymer Conformational Change... 

In the case of oxides, the Gibbs adsorption equation (8.12) gives the variation in interfacial tension of particles in the presence of a basic solution XOH (or an acidic solution HY) and an electrolyte XY [39-41] (... 

Figure 7 Variation of interfacial tension between Athabasca bitumen and D20 containing Sun Tech IV (2 g/L) as a function of pH and temperature at constant ionic strength of 10 M. The dashed line represents data from reference T211 at 50 C in the absence of added surfactant or brine.

The second complicating factor is interfacial turbulence (1, 12), very similar to the surface turbulence discussed above. It is readily seen when a solution of 4% acetone dissolved in toluene is quietly placed in contact with water talc particles sprinkled on to the plane oil surface fall to the interface, where they undergo rapid, jerky movements. This effect is related to changes in interfacial tension during mass transfer, and depends quantitatively on the distribution coefficient of the solute (here acetone) between the oil and the water, on the concentration of the solute, and on the variation of the interfacial tension with this concentration. Such spontaneous interfacial turbulence can increase the mass-transfer rate by 10 times 38). 

The variation of interfacial tensions with temperature has been measured by Harkins in the case of a few organic liquids against mercury, and like surface tensions they diminish with rise of temperature. 

For the buffer solution Ph = 5 6 the variation of interfacial tension with the strength does not exceed the experimental error, but in the more alkaline solution we must conclude that either sodium ions or phosphate ions (or both) are positively adsorbed according to the equation... 

Movements in the plane of the interface result from local variations of interfacial tension during the course of mass transfer. These variations may be produced by local variations of any quantity which affects the interfacial tension. Interfaeial motions have been ascribed to variations in interfacial concentration (H6, P6, S33), temperature (A9, P6), and electrical properties (AlO, B19). In ternary systems, variations in concentration are the major factor causing interfacial motion in partially miscible binary systems, interfacial temperature variations due to heat of solution effects are usually the cause. 

The decrease in interfacial tension is related to the amount of extractant adsorbed at the interface through the Gibb s adsorption equation (46). The molecular areas of the extractant at the interface can thus be directly obtained from this equation. As an example, an area of 104 8 A2 is obtained for the. V,.V -dimc(hyl dibu-tyltetradecylmalonamide (DMDBTDMA) at the dodecane/water interface (4, 34). For classical surfactants, it should be noted that a nearly constant area per molecule with the addition of salt strongly suggests that anions and cations are adsorbed and extracted as pairs (47). Thus, the variation of the area per molecule with added salt can provide information on the mechanism of extraction. 

The effects of surface tension on sessile and pendent drops or lenses are but a simple manifestation of capillary hydrostatics. The field of capillarity can be far more extensive, principally when coupled with electromagnetic forces and also for liquid interfaces in motion, or in the motion in liquid interfaces that may result from local variations in surface tension as may be caused, for example, by local variations in temperature, or by the localized introduction of surfactants (interfacial tension modifiers), or by localized space-delimited chemical reactions. Wicking flows (as in heat pipes ) and flows in porous media (as in petroleum reservoir displacement) are a few of many other examples in which interfacial forces play a predominant role. ... 

Nevertheless, it is common to neglect the variations in surface tension at the interfaces, and the interfacial pressures are generally related by the Young-Laplace equation [244] [138] [54] ... 

In Table 3.22 are data of interfacial tension of the alcohol vs. water system analyzed. The variation of interfacial tension with a change in alkyl chain length for different organic liquids vs. water is given in Table 3.23. 

To obtain any thermodynamic information of such systems it is useful to consider the effect of temperature on the interfacial tension. The aUcane-water interfacial tension data have been analyzed (Eigure 3.10). These data show that the interfacial tension is lower for Cg (50.7 mN/m) than for the other higher chain length alkanes. The slopes (interfacial entropy -djIdT) are all almost the same, 0.09 mN/m per CH2 group. This means that water dominates the temperature effect, or that the surface entropy of the interfacial tension is determined predominantly by the water molecules. Eurther, as described earlier, the variation of surface tension of alkanes varies with chain length. This characteristic is not present in interfacial tension data however, it is worth noting that the slopes in the interfacial tension data are lower than those of both pure alkanes and water. The molecular description must be analyzed. 

In gel chromatographs of the first generation, the volume of each fraction collected is measured separately. In the instruments of the second generation, the retention volume is measured continuously, most frequently by means of a siphon. This device can easily be automated and utilized for controlling the injection system, and, eventually also the pumping system and fraction collector. The drawback of the siphon is its sensitivity to the variations of interfacial tension of the effluent and to the formation of the film of grease on its walls. Both effects are pronounced especially in the aqueous mobile phases. Smaller retention volumes can be measured by means of the drop counters or capillary volumeters. From the point of view of aqueous eluents, the drop counters exhibit the same limitations as siphons. The capillary volumeters are based upon counting how many times the effluent fills up the fixed volume of a capillary. The results of both, drop counters and capillary volumeters, depend on the flow rate and on the physical characteristics of the effluent. 

Fig. 4.12. Variation of interfacial tensions between the various phases O (oil), W (water) and M (microemulsion, containing surfactant), in the three-phase zone (P1P2), when a parameter governing chemical composition is varied (e.g., ionic strength of the aqueous solution)...

One of the simplest methods to study adsorption at the oil water interface is to measure the variation of interfacial tension as a function of concentration. If the polymer used for adsorption is monodisperse, then the Gibbs equation (51) may be used to estimate the surface excess. However, if the polymer is polydis-perse, this method will give erroneous values of the surface excess because the larger molecules will tend to adsorb preferentially, and the equation is imable to account for this adsorption behavior. As a result, most of the data available in the literature report the change in the interfacial tension as a function of concentration without attempting to convert it into an adsorbed amoimt. Apart from interfacial tension measurements, other techniques such as total internal reflection fluorescence microscopy (52) and scintillation measurements from radiolabeled polymers (53) have also been used to measure the adsorption at the liquid-liquid interface. 

However in both cases motion is due to variation of interfacial tensions related to concentration gradients. 

The role of interfacial deformation is considered in the stability analysis of fluid layers heated from below or above when there is an open interface to ambient air, and double diffusive transport of heat and solute thus leading to variations of interfacial tension that compete or cooperate with buoyancy phenomena. The onset of both oscillatory convection and steady patterns is described. 

Surface tension on the surface of a liquid at gas-Uquid or vapor-liquid interfaces can vary due to a variation in temperamre or species concentration. The components of the tangential stress, Tj Ty and are related to the corresponding gradients in interfacial tension, between the liquid phase, j = 1, and the gas/vapor phase, y = 2 by (Bird et al, 2002) ... 

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