Static and Dynamic Interfacial Tension Analysis

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. 

The choice between the static methods (Wilhelmy plate method and the du Noiiy ring method) should primarily be based on the properties of the system being studied, in particular, the surfactant. As mentioned in UNITD3.5, the transport of surfactant molecules from the bulk to the surface requires a finite amount of time. Since static interfacial tension measurements do not yield information about the true age of the interface, it is conceivable that the measured interfacial tension values may not correspond to equilibrium interfacial tension values (i.e., the exchange of molecules between the bulk and the interface has not yet reached full equilibrium and the interfacial tension values are therefore not static). If the surfactant used in the experiment adsorbs within a few seconds, which is the case for small-molecule surfactants, then both the Wilhelmy plate method and the du Noiiy ring method are adequate. If the adsorption of a surfactant requires more time to reach full equilibrium, then the measurement should not be conducted until the interfacial tension values have stabilized. Since interfacial tension values are continuously displayed with 

Method Liquid/liquid Liquid/gas Time range 

Maximum bubble pressure0 Possible Good 1 msec-100 sec 

The main advantage of the static methods is cost. The equipment needed to conduct the dynamic measurements is approximately five times as expensive as the equipment required for static measurements (- 25,000 for a drop shape and drop volume analyzer versus - 5,000 for du Noiiy and Wilhelmy instruments). This is due to the additional capability of the former instruments to determine not only interfacial tension values but also the corresponding age of the interface. For more information on equipment, costs, and suppliers, see Internet Resources. 

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