Interfacial tension changes

Mechanisms involved in the demulsification by surfactants of petroleum W/O emulsions include adsorption of the surfactant at the oil-water interface and reduction of the interfacial tension, change in the nature of the interfacial film from a highly hydrophobic one to a less hydrophobic one (and, consequently, one more wettable by water), reduction of the viscosity of the interfacial film by penetration into it of the surfactant, and displacement of the original W/O emulsion stabilizers, particularly the asphaltenes, from the interface into the oil phase. 

The adsorption of a surfactant at an interface between CO2 and a second fluid, such as water, may be determined directly from measurement of the interfacial tension (change in Gibbs free energy with surface area), y, versus surfactant concentration. A novel tandem variable-volume pendant drop tensiometer has been developed to measure equilibrium and dynamic values of y as a function ofT.p and time (Figure 2.4-1) [21]. An organic [21] or aqueous phase [18] is preequilibrated with CO2 in the first variable-volume cell (drop-phase cell). A droplet of this liquid is injected into the second variable-volume cell, with two windows at 180° mounted on a diameter, containing either pure CO2 or CO2 and surfactant. 

Note that as a liquid spreads on a surface the interfacial tensions change, with the result that the spreading coefficient changes. For example, benzene spreads on a pure water surface, 9 x 10 N/m initially. When the water is saturated with benzene and the benzene saturated with water (o- )sat — 2 x 10 N/m and any additional benzene... 

The interfacial tension change with time at a growing drop as given by Joos and Van Uffelen (110) has the form ... 

In Fig. 17 some typieal interfacial tension changes are shown which have been obtained for a trapezoidal area change of an aqueous protein solution drop in tetradecane. The elasticity can be calculated from the initial jump of the y t) dependence immediately following an expansion or compression. 

Figure 15 Interfacial tension changes with pH, showing the maximum for various crudes. (From Ref. 184.)...

Chatzi and Kiparissides [19] studied the aqueous dispersion of -butylchloride in the presence of various PVA stabilizers and they observed two critical concentrations of PVA at which the interfacial tension changed (see Figure 5.3). At low PVA concentrations (e.g., less than 0.001 gL ), the interfacial tension was relatively independent of the PVA concentration for aU types of PVA studied. At higher concentrations, the interfacial tension decreased almost linearly with the PVA concentration on a semi-log scale. This convex behavior was... 

It was shown that the surfactant adsorption decreased the interfacial tension between the stationary and mobile phases. In the absence of micelles (ion-pair chromatography), the nonionic solute retention factor was affected by this interfacial tension change according to eq. 4.7 [32] ... 

A complete screening of an EOR surfactant must include determination of the kinetics of interfacial tension changes in addition to their equilibrium values. Considerable work remains to be done to characterize dynamic processes such as oil droplet mobilization, entrapment and oil bank formation. 

Monolayers seldom, if ever, show an entirely viscous behavior. They always present an elastic contribution. Interfacial dilation or compression causes a change in the interfacial tension which, after releasing the stress, relaxes with a characteristic time toward equilibrium. Thus, the interfacial tension change induced by changing the interfacial area is determined by an elastic and a viscous contribution that are likely to be additive ... 

During chlorophyll adsorption at the octane/water interface, the interfacial tension changes markedly. This fact was utilized in determining the surface excess of chlorophyll by the Gibbs equation. From the slope of the initial section of the isotherm,... 

The drop and bubble shape tensiometer allows one to perform oscillation experiments. However, in contrast to similar studies with small spherical drops it is limited to slow oscillations. Depending on the hydrodynamic conditions, mainly the rheological behaviour of the bulk phases, oscillation from a certain frequency onwards do not provide drops or bubbles with a Laplacian shape, so that the data analysis will fail and yield unrealistic data. Even for liquids of high grade of purity interfacial tension changes are simulated and hence misinterpretations can be the consequence. Thus, the use of high speed video technique is not really relevant for shape analysis tensiometry (although provided by several companies), as the hydrodynamic relaxation can take much time to yield Laplacian menisci. 

With a similar system, Saito and Shinoda showed that the solubilisation of oil into an aqueous micellar solution phase increased rapidly as the type 3 region is approached. Around the type 3 region, as the temperature rose the system moved from two phases (0 and D, where D = surfactant phase), to three phases (O, W and D) and then back to two phases (W and D). With further rise in temperature, water solubilised in the oleic micellar phase was released to the water phase. Saito and Shinoda also showed that the interfacial tension changed during the transition from type 1 to type 2 phase behaviour. In the three-phase region the interfacial tension is ultra low this has important consequences for the drop sizes and stability of emulsions of type 3 systems. 

Brooks and Richmond " studied the dynamics of transitional phase inversion for various combinations of nSOW systems. They found that drop sizes decreased as the transitional inversion point was approached. Figure 6.9 shows the change in drop diameter across the phase transition for the oil-water-NPE system. This change in drop diameter reflects the interfacial tension change through the phase transition. For all the individual mixtures that were studied hy Brooks and Richmond, the locus for the transitional inversion was found to he a straight line. It was also found that, contrary to what was suggested by other workers, when... 

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