Interfaeial tension

Impurities usually eause a reduetion in the growth rates of erystalline materials due to bloeking of kink sites, thereby leading to smaller erystals than required. This is a eommon problem, whieh is often related to eontamination of the feed solution. In some other eases, however, impurities ean enhanee growth rates, thought to be due to a reduetion in interfaeial tension and lienee inerease in surfaee nueleation rates. 

HARRIOTT 25 suggested that, as a result of the effects of interfaeial tension, the layers of fluid in the immediate vicinity of the interface would frequently be unaffected by the mixing process postulated in the penetration theory. There would then be a thin laminar layer unaffected by the mixing process and offering a constant resistance to mass transfer. The overall resistance may be calculated in a manner similar to that used in the previous section where the total resistance to transfer was made up of two components—a Him resistance in one phase and a penetration model resistance in the other. It is necessary in equation 10.132 to put the Henry s law constant equal to unity and the diffusivity Df in the film equal to that in the remainder of the fluid D. The driving force is then CAi — CAo in place of C Ao — JPCAo, and the mass transfer rate at time t is given for a film thickness L by ... 

Hgure 15 13 Comparison between experimentally obtained droplet sizes and deseriptions by the model (data from Schrrider et al. 1998, 1999). The membrane used was a eeramic membrane with average pore size of 0.4 pm, and with transmembrane pressure of 3 bar sunflower oil was emulsified in water with different surfaetants. One only obtains a good quantitative fit with the data when one assumes that the interfaeial tensions are 18 mN/m with 0.05% Tween-20 11 mN/m with 0.5% Tween-20 and 1.7 mN.m with 2% SDS. These values are not physically realistic therefore, the model does not quantitatively deseribe the data. 

Centrifugal Contactors. These devices have large capacities per unit, short residence times, and small holdup. They can handle systems that emulsify easily or have small density differences or large interfaeial tensions or need large ratios of solvent to feed. Some types are employed as separators of mixtures made in other equipment, others as both mixers and settlers, and some as differential contactors. 

These data apply al a high interfaeial tension (30-40 dyn/cm), a viscosity similar to water, an inlet ratio of the phases of l.l pans by volume, and a density dilTerence of approximately 0.6 g/cm . 

The observation that mixed adsorption layers often provide an extreme reduction of the interfaeial tension and thereby enable easy or even spontaneous emulsification was studied systematically by Shinoda Friberg (1975) using the two surfactants CH3(CH2)g-(0CH2CH2)g50P(0H2) and CH3(CH2)j(0CH2CH2)30P(0H)2. By changing the composition of the mixed components in the system water/heptane conditions for the formation of liquid/crystalline phases and spontaneous emulsification were found. The results support the idea that the use of two suitably compatible emulsifiers is often much more efficient than single surfactant systems. 

SCH Schneider, A. and Wolf, B.A., Specific features of the interfaeial tension in the case of phase separated solutions of random copolymers. Polymer, 41,4089, 2000. 

In essence, the shape of a drop is determined by a eombi-nation of interfaeial tension and gravity effeets. Surface forces tend to make drops spherical whereas gravity tends to elongate a pendant drop or flatten a sessile drop. When gravitational and interfacial tension effects are comparable then, in principle, one can determine the interfaeial tension from an analysis of the shape of the drop. 

In many emulsion or microemulsion systems, the interfacial tension between the oil-rich phase and the aqueous solution is very low (or ultralow), which presents considerable difficulties for many experimental methodologies. The most commonly employed approach for measuring ultralow interfaeial tension is the spinning-drop technique (98). However, ADSA has also been used to study these systems and possesses a number of advantages over the... 

In brief, via the CCD camera (1) with objective (2) and the frame grabber (3), an image of the shape of a drop (9) is transferred to a computer, where by using the ADSA software the coordinates of this drop are determined and compared to profiles calculated from the Gauss-Laplace equation of capillarity. The only free parameter in this equation, die interfaeial tension Y, is obtained at optimum fitting of the drop-shape coordinates. The dosing system (7) allows one to change the drop volume and hence the drop surface area. This possibility is used in dilational relaxation experiments as outlined in Sec. VI. 

The intensive parameters, i.e., temperature T, interfaeial tension y, and interfaeial ehemieal potential are now defined from the fundamental equation as... 

Obviously lowering the value of y can decrease significantly the mechanical work required for a given cL4. It would be of interest to reduce y as much as possible if the aim is to minimize the siuface work. The question then arises as to how low the interfaeial tension can get. 

Since is the internal energy of an interface, an interface must exist between two bulk phases, for a finite value of interfaeial tension. The consequence of Eq. (82) is that a stable interface requires a positive value of interfaeial tension, implying that energy must be increased if the interfa-... 

The models of adsorption from semi-infinite bulk phases predict in any case a monotonic relaxation of the interfaeial tension even in the presence of transfer of matter into the second phase. In partieular, if the initial bulk concentrations are at partition equilibrium the adsorption asymptotically reaches its equilibrium value, otherwise the system aehieves a stationary state. 

Owing to the limited amount of surfactant in the drop, the interfaeial tension passes through a minimum when the net number of moleeules adsorbing at the interfaee from the inner phase equates with the net number of moleeules desorbing in the external phase. It is important to notiee that, in ftiese dynamie eonditions, the interfaeial tension ean reaeh values whieh are well below the equilibrium values, whieh ean be relevant for some technologieal proeesses sueh as flie eontrol of droplet size or emulsifieation. 

Similar experiments have been run by forming a drop of hexane inside the eell filled with the surfaetant aqueous solution, in order to obtain a volume ratio Q = 1000 between flie supplying and reeipient phases. The measured dynamie interfaeial tensions for this kind of experiment are shown in Fig 16A—C for C13DMPO, C12DMPO, and CioDMPO, respeetively. In this eonfiguration the interfaeial tension minima disappear sinee the internal phase is rapidly saturated and a monotonie relaxation behavior is observed. 

Figures 32 and 33 show the fully eorreeted plots of shear stress versus shear rate. Taking aeeount of small differenees in the measured interfaeial tensions, all data eould be aeeu-rately represented by...

The presence of surfactants at the water/oil interface will lower the interfaeial tension between the two phases. The interfaeial pressure (n) is defined as... 

There are four rheologieal parameters which describe the response to imposed interfaeial stresses or deformation. For a Newtonian interfaee, the signifieant rheological properties that determine interfaeial motion are the interfacial shear viscosity, the interfaeial dilational viscosity, and the interfaeial tension gradient. The interfaeial shear elasticity, e and viscosity, describe the resistance of the... 

Mukerjee and Kushnick (167) showed that at low frequency the demulsifier behaves as a soluble mono-layer, and at high frequency as an insoluble monolayer. Variation in interfaeial tension from a local change in area is virtually instantaneous. This gradient is short circuited when the demulsifier moleeule moves to and from the surface to bulk or is sufficiently soluble in the bulk phase. 

Mukherjee and Kushnick s definitions were as follows the interfaeial tension increment, dy, per unit fractional area change, dA/A, is equated to the complex modulus, e (f) ... 

The equilibrium interfaeial tension between water and oil is a measure of the adsorption of surface-active components to the interface and can be related to siuface excess by the Gibbs equation (183). However, in crude oil systems the activity/concentrations of the surfaee-aetive components are not easily determined. Indirect measures are applied. In most process conditions with short resident times, it is the dynamic inter-facial tension gradient that is important. Interfaeial tension also tells whether or not the demulsifier is surfaee active, and as will be shown later, this is important for demulsification. The interfaeial tension gradient is the... 

Figure 5. Plots of vs.. 0 at constant tr values equal to -0,12. -O.OS. -0.04, 0, 0.02 C in (firom top ID bottom). Points are experimental data reprinted from J. Eleciroanal. Chfm.. 235, M, I.. F oresti et al. Adsorption ofTrietliylamine at the Mcrcury/Water Interphase from Charge and Interfaeial Tension Measurements, p. 267, Copyright I9E8, vrith permission from Elsevier Science. Curves were calculated from Eqs. (16), (21), and (23) using the parameters of Figure 4.

Use of a micrometCT syringe to feed the drop liquid to the tube makes determination of drop volume straightforward. The tip of the mbe should be carefully ground so that it is free from nicks and other irregularities. As with all other methods, evaporation must be prevented in liquid-gas systems to prevent cooling of the interfaee and a resulting change in interfacial tension. With these provisions, the use of the drop volume method is convenient, and it is widely used. It should be noted that, by its very nature, this method is unsuitable when diffusion or adsorption effeets dictate that considaable time is needed for the equilibrium value of interfaeial tension to be attained. 

Hamada, Y., Koga, K., and Tanaka, H. 2007. J. Chem. Phys. Phase equilibria and interfaeial tension of fluids confined in narrow pores. 127 084908. 

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