Interfacial tension and phase behaviour

From the above, it is clear that a pre-requisite of low water/oil interfacial tensions is the complete saturation of the water-rich and oil-rich phases as well as the water/oil interface by surfactant molecules. Of course, this pre-requisite is fulfilled if one of the phases considered is a microemulsion. Furthermore, since the pioneering work of Lang and Widom [81] it is known that if a system is driven through phase inversion the interfacial tensions may become ultra-low. However, about 20 years ago, a number of experimental investigations were devoted to clarifying the origin of the ultra-low interfacial tensions [15, 17, 39, 71, 81-85]. In order to understand this correlation between phase behaviour and interfacial 

From the temperature dependence of the phase behaviour the qualitative shape of the three interfacial tension curves can be deduced. As the two phases (a) and (c) are identical at the critical tie line at T the interfacial tension aac has to start from zero and increases monotonically with increasing temperature. Whereas the interfacial tension ubc decreases (monotonically) with increasing temperature and vanishes at Tu, because the two phases (c) and (b) become identical at the critical tie line at Tu. This opposite temperature dependence of crac and Ubc results in a minimum if one considers the sum of the two, crac + CTbc- In order to assure the stability of the water/oil interface 

Furthermore, knowing crab, T and Tu, the relative location of the individual crac- and CTbe-curves is fixed. Near the critical endpoint temperatures T and Tu even a quantitative description of the interfacial tensions crac and Tbc can be obtained applying the scaling laws 

As was mentioned earlier, it is above all the water/oil interfacial crab that plays an important role in technical applications. Thus, much work has been carried out to obtain the variation of rab as a function of the respective tuning parameter, i.e. temperature T [17, 84, 87, 88], salinity [15, 89] and co-surfactant to surfactant ratio 8[16, 90]. In the following the variation of the water/oil interfacial as a function of temperature and composition of the amphiphilic film (see Section 1.2.3) is discussed by way of example. 

As can be seen independently of the parameter used to drive the system through the phase inversion the shape of the interfacial tension curves is similar. Because of the fundamental link of the interfacial tension and phase behaviour discussed above, both systems show 

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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]. 

Comelisse, P.M.W. (1997) The gradient theory applied, simultaneous modelling of interfacial tension and phase behaviour, PhD-thesis, Delft University of Thechnology, The Netherlands. 

Once formulated, exploitation of the special properties of microemulsions is facilitated by knowledge of the types of microstructure, characteristic sizes, and the dynamics of structure fluctuations. Unfortunately, determination of microemulsion microstructure and dynamics remains difficult, and thus is discussed elsewhere in this book (see Chapter 40). Here, the relationships between microstructure, interfacial tensions and phase behaviour are is discussed, and a qualitative description of the dynamic processes in microemulsions is given. For simple ethoxylated alcohol-water mixtures, the correlations below allow an estimation of the sizes and interfacial tensions in microemulsions without resort to any complex measurements. 

Dispersion behaviour in systems with liquid/liquid or liquid/gas interfaces (i.e. droplet or bubbles) has traditionally been described in terms of rheological properties, wetting properties, including contact angle and interfacial tensions, or phase behaviour and stability measurements. Direct force measurements provide a means to fundamentally probe the interactions between deformable interfaces that significantly impact the dispersion (or emulsion) behaviour. 

By virtue of their simple stnicture, some properties of continuum models can be solved analytically in a mean field approxunation. The phase behaviour interfacial properties and the wetting properties have been explored. The effect of fluctuations is hrvestigated in Monte Carlo simulations as well as non-equilibrium phenomena (e.g., phase separation kinetics). Extensions of this one-order-parameter model are described in the review by Gompper and Schick [76]. A very interesting feature of tiiese models is that effective quantities of the interface—like the interfacial tension and the bending moduli—can be expressed as a fiinctional of the order parameter profiles across an interface [78]. These quantities can then be used as input for an even more coarse-grained description. 

In these past 10 years, it has been demonstrated that the TR-QELS method is a versatile technique that can provide much information on interfacial molecular dynamics [1-11]. In this chapter, we intend to show interfacial behaviour of molecules elucidated by the TR-QELS method. In Section 3.2, we present the principle, the historical background and the experimental apparatus for TR-QELS. The dynamic collective behaviour of molecules at liquid/liquid interfaces was first obtained by improving the time resolution of the TR-QELS method. In Section 3.3, we present an application of the TR-QELS method to a phase transfer catalyst system and describe results on the scheme of the catalytic reactions. This is the first application of the TR-QELS method to a practical liquid/liquid interface system. In Section 3.4, we show chemical oscillations of interfacial tension and interfacial electric potential. In this way, the TR-QELS method allows us to analyze non-linear adsorption/desorption behaviour of surfactant molecules in the system. 

Phase Behaviour, Interfacial Tension and Microstructure of Microemulsions... 

Engelskirchen, S., Eisner, N., Sottmann, T. and Strey, R. (2007) Triacylglycerol microemulsions stabilized by alkyl ethoxylate surfactants - A basic study Phase behaviour, interfacial tensions and microstructure. /. Colloid Interface Sci., 312, 114-121. 

Whereas Winsor III systems exhibit ultra-low interfacial tensions between the three phases and also very high solubilisation capacity, Winsor I systems have higher interfacial tensions and much lower solubilising power. At the transition between the two types of microemulsion systems, an intermediate behaviour can be found which is called supersolubilisation [47,70]. The uptake of oils into surfactant aggregates is usually enhanced by one to two orders of magnitude compared to effective micellar systems, but interfacial tension reduction is still moderate. The transition point can be adjusted by varying the salinity or organic components. 

In these two equations r/ad, is the viscosity of the adsorbed polymer, >i2e the (non-equilibrium) excess interfacial tension and y,2 the (equilibrium) interfacia] tension, so that the quotient yi2j i2 describes the distance of the thermodynamic system from the equilibrium state. It is ea.sy to see that such behaviour is not at all in accordance with the idea of statistically distributed dispersed phases and non-interacting interfaces. 

The interfacial tension behaviour of carbon dioxide/decane mixtures appears to be very interesting due to the formation of the second liquid phase at low temperatures. To see the similarities between the interfacial tension behaviour and the phase behaviour at constant temperature, the interfacial tensions are being presented in a similar way as p-x,y sections in Figure 3. In Figure 4 the relations between pressure, interfacial tension and composition are shown schematically for a system in which a three-phase equilibrium is present. 

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