Interfacial tension dynamic

So far, we have treated the interfacial tension as an eqnilibriutn property that can be determined in a system that is relaxed dnring the time of the measurement. However, if the interface is off-eqnilibrinm, that is, dnring the relaxation process toward the equilibrium state, the interfacial tension is time dependent. Such a nonequilibrium, time-dependent interfacial tension is referred to as dynamic interfacial tension. Interpretation of dynamic interfacial tensions is nsnally in terms of surface rearrangements, transport of snrface-active componnds to or from the interface, conformational and orientational changes of adsorbed molecnles, and so on. 

In a single-component system the time dependency of the interfacial tension is determined by the time needed for the molecnles in the interfacial region to attain their equilibrium distribution. Except for solids (as discussed earlier), this is a fast process typically on the order of milliseconds, so that essentially all measuring procedures yield the equilibrium interfacial tension. However, for solutions containing surface-active compound(s), adsorption and desorption processes usually determine the rate of relaxation of the interface. Depending on the system and the conditions, the time scale may be much longer, say, on the order of seconds up to hours. We return to this in Section 17.4. 

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The dynamic interfacial tension behavior of reacting acidic oil-alkaline solutions has been studied for both an artificially acidified synthetic oil and a real crude oil at various concentrations [131,132] with either a drop volume tensiometer or a spinning drop tensiometer. 

A study on a commonly used demulsifier, namely, a phenol-formaldehyde resin, elucidated how various parameters such as interfacial tension, interfacial shear viscosity, dynamic interfacial-tension gradient, dilatational elasticity, and demulsifier clustering affect the demulsification effectiveness [1275]. 

Our goal is to develop a property-performance relationship for different types of demulsifiers. The important interfacial properties governing water-in-oil emulsion stability are shear viscosity, dynamic tension and dilational elasticity. We have studied the relative importance of these parameters in demulsification. In this paper, some of the results of our study are presented. In particular, we have found that to be effective, a demulsifier must lower the dynamic interfacial tension gradient and its ability to do so depends on the rate of unclustering of the ethylene oxide groups at the oil-water interface. 

The oil-water dynamic interfacial tensions are measured by the pulsed drop (4) technique. The experimental equipment consists of a syringe pump to pump oil, with the demulsifier dissolved in it, through a capillary tip in a thermostated glass cell containing brine or water. The interfacial tension is calculated by measuring the pressure inside a small oil drop formed at the tip of the capillary. In this technique, the syringe pump is stopped at the maximum bubble pressure and the oil-water interface is allowed to expand rapidly till the oil comes out to form a small drop at the capillary tip. Because of the sudden expansion, the interface is initially at a nonequilibrium state. As it approaches equilibrium, the pressure, AP(t), inside the drop decays. The excess pressure is continuously measured by a sensitive pressure transducer. The dynamic tension at time t, is calculated from the Young-Laplace equation... 

Figure 3 Dynamic interfacial tension of the crude oil-brine...

Lee. S.. Kim, D.H., and Needham, D. Equilibrium and dynamic interfacial tension measm-ements of microscopic interfaces using a micropipet technique. 1. A new method for determination of interfacial tension, Lang/nurr. 17(18) 5537-5543,2001. 

V. Schroder, O. Behrend, and H. Schubert Effect of Dynamic Interfacial Tension on the Emulsification Process Using Microporous Ceramic Membranes. J. Colloid Interface Sci. 202, 334 (1998). 

Interface by the du Noiiy Ring Method Basic Protocol 2 Dynamic Interfacial Tension Determination by the Drop D3.6.5... 

Excellent reference to gain an enhanced understanding of the fundamentals of the dynamics of adsorption phenomena at liquid interfaces. An in-depth theoretical description of methods to determine dynamic interfacial tensions is included in the book. 

This unit will introduce two fundamental protocols—the Wilhelmy plate method (see Basic Protocol 1 and Alternate Protocol 1) and the du Noiiy ring method (see Alternate Protocol 2)—that can be used to determine static interfacial tension (Dukhin et al., 1995). Since the two methods use the same experimental setup, they will be discussed together. Two advanced protocols that have the capability to determine dynamic interfacial tension—the drop volume technique (see Basic Protocol 2) and the drop shape method (see Alternate Protocol 3)—will also be presented. The basic principles of each of these techniques will be briefly outlined in the Background Information. Critical Parameters as well as Time Considerations for the different tests will be discussed. References and Internet Resources are listed to provide a more in-depth understanding of each of these techniques and allow the reader to contact commercial vendors to obtain information about costs and availability of surface science instrumentation. 

Table D3.6.1 Suitability of Different Techniques for Determining Static and Dynamic Interfacial Tension...

DYNAMIC INTERFACIAL TENSION DETERMINATION BY THE DROP VOLUME TECHNIQUE... 

While the quasistatic method is quite accurate, it requires a long time to determine a complete adsorption kinetics curve. This is because a new drop has to be formed at the tip of the capillary to determine one single measurement point. For example, if ten dynamic interfacial tension values are to be determined over a period of 30 min, -180 min will be required to conduct the entire measurement. On the other hand, the constant drop formation method is often limited because a large number of droplets have to be formed without interruption, which may rapidly empty the syringe. Furthermore, the critical volume required to cause a detachment of droplets depends on the density difference between the phases. If the density difference decreases, the critical volume will subsequently increase, which may exacerbate the problem of not having enough sample liquid for a complete run. 

The theoretical foundation of the drop volume technique (DVT) was developed by Lohnstein (1908, 1913). Originally, this method was only intended to determine static interfacial tension values. Over the past 20 years, the technique has received increasing attention because of its extended ability to determine dynamic interfacial tension. DVT is suitable for both liquid/liquid and liquid/gas systems. Adsorption kinetics of surface-active substances at liquid/liquid or liquid/gas interfaces can be determined between 0.1 sec and several hours (see Fig. D3.6.5). 

Figure 3. Dynamic interfacial tension of soybean oil/water systems. Soybean oil drop expanding in water, pH=7, flow rate 3.10"4 mm3/s, Rc=0.141 mm.

Several generic kinds of results are pertinent to the properties of dispersions. The surfactant solutions formulated to stabilize microemulsions, and some kinds of macroemulsions, can exhibit marked dynamic interfacial tension behaviour. Figure 3.14 shows an example in which a series of commerical surfactant additions are made to a system containing crude oil and a base. Under alkaline conditions the interfacial tension is already dynamic due to the saponification of natural surfac-... 

Figure 3.14 Example of some dynamic interfacial tensions in a system containing crude oil, 1 mass% Na2C03, 0.5 mass% NaCI, and varying concentrations of a commercial surfactant (Neodol 25-3S). From Taylor and Schramm [142],...

Dynamic interfacial tension, therefore the emulsifier used, and related adsorption kinetics influence the emulsification process. In general, the faster an emulsifier adsorbs to the newly formed interface, the lower the interfacial tension the smaller the droplet produced. Figure 21.8 shows a linear behavior between the Dd/Dp ratio and interfacial tension. 

Figure 15.14. The droplet size depends on the concentration of surfactant the more surfactant, the lower the interfacial tension will be, and therefore the smaller the droplets will become. As the pressure over the membrane is increased, the flow of the dispersed phase (in this case oil) is increased the droplets grow faster, and the surfactant has to dilfuse faster to the interface. This results in a higher dynamic interfacial tension, and larger droplets. (After van der Graaf Schoen et al. 2004.)...

Schroder, V., Behrend, O., and Schubert, H. (1998a). Effect of dynamic interfacial tension on the emulsification process using microporous, ceramic membranes. J. Coll. Interf. Sci. 202, 334-340. 

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