Microemulsions spinning drop method

For emulsions, the interfacial tension is usually of most interest. Here, the du Noiiy ring, Wilhelmy plate, drop volume, pendant, or sessile drop methods are the most commonly used. The spinning drop or captive drop techniques are applicable to the very low interfacial tensions encountered in the enhanced oil recovery and microemulsion fields. The maximum droplet pressure technique can be used when there is little or no density contrast between the phases, such as in bitumen-water systems at elevated temperature. 

Many methods for the measurement of surface and interfacial tensions, details of the experimental techniques, and their limitations are described in several good reviews (27-29). Some methods that are used most in emulsion work are the du Nouy ring, drop weight or volume, pendant drop, and the spinning drop. The spinning drop technique is applicable to the very low interfacial tensions encountered in the enhanced oil recovery and microemulsion fields (30). In all cases, when solutions rather than pure liquids are involved, appreciable changes can take place with time at the surfaces and interfaces. 

Perhaps the most striking property of a microemulsion in equilibrium with an excess phase is the very low interfacial tension between the macroscopic phases. In the case where the microemulsion coexists simultaneously with a water-rich and an oil-rich excess phase, the interfacial tension between the latter two phases becomes ultra-low [70,71 ]. This striking phenomenon is related to the formation and properties of the amphiphilic film within the microemulsion. Within this internal amphiphilic film the surfactant molecules optimise the area occupied until lateral interaction and screening of the direct water-oil contact is minimised [2, 42, 72]. Needless to say that low interfacial tensions play a major role in the use of micro emulsions in technical applications [73] as, e.g. in enhanced oil recovery (see Section 10.2 in Chapter 10) and washing processes (see Section 10.3 in Chapter 10). Suitable methods to measure interfacial tensions as low as 10 3 mN m 1 are the sessile or pendent drop technique [74]. Ultra-low interfacial tensions (as low as 10 r> mN m-1) can be determined with the surface light scattering [75] and the spinning drop technique [76]. 

As the latter is comparatively simple to use it can be regarded as the most suitable method to measure low and ultra-low interfacial tensions. In the following the general features of interfacial tensions in microemulsion systems are presented. The dramatic decrease of the water/oil interfacial tension upon the addition of surfactant, the correlation of interfacial tension and phase behaviour, the variation of the water/oil interfacial tension with the respective tuning parameter and the scaling of the interfacial tension will be discussed in detail. All data presented have been determined using the spinning drop technique [17]. 

The coalescence behavior of isolated oil drops in microemulsion systems and of microemulsion drops in brine systems is studied, A new experimental method is presented for conducting such controlled coalescence tests in an inclined spinning drop apparatus. Observations on five oil-water-surfactant systems are presented, three involving iso-octane as the oil phase and two involving crude oils. Coalescence times were measured in terms of the applied coalescence force and the droplet-droplet contact radius. 

There are many methods available for the measurement of surface and interfacial tensions. Details of these experimental techniques and their limitations are available in several good reviews [101-104]. Table 5 shows some of the methods that are used in petroleum recovery process research. A particular requirement of reservoir oil recovery process research is that measurements be made under actual reservoir conditions of temperature and pressure. The pendant and sessile drop methods are the most commonly nsed where high temperatur pressure conditions are required. Examples are discussed by McCaffery [i05] and DePhUippis et al. [J06]. These standard techniques can be difficult to apply to the measurement of extremely low interfacial tensions (< 1 to 10 mN/m). For ultra-low tensions two approaches are being used. For moderate temperatures and low pressures the most common method is that of the spinning drop, especially for microemulsion research [107], For elevated temperatures and pressures a captive drop method has been developed by Schramm et al. [JOS], which can measure tensions as low as 0.001 mN/m at up to 200 °C and 10,000 psi. In aU surface and interfacial tension work it should be appreciated that when solutions, rather than pure liquids, are involved appreciable changes can occur with time at the surfaces and interfaces, so that techniques capable of dynamic measurements tend to be the most useful. 

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