Interfacial tension in microemulsions

D. Langevin In S.-H. Chen, J. S. Huang, and P. Tartaglia (eds), Low interfacial tensions in microemulsion systems. Structure and Dynamics of Strongly Interacting Colloids and Supramolecular Aggregates in Solution. 325. p. Kluwer, Dordrecht (1992). 

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]. 

As discussed in Chapter 4 (Section 8), DLS has also been used to determine phase transitions and criticality in microemulsion systems. It has also been used to measure interfacial tensions in microemulsion systems (Langevin, 1987). Measurements show that values are ultralow as required by theory (Chapter 4, Section 10). 

Langevin, D., Eow interfacial tensions in microemulsion systems, in Microemulsions Structure and Dynamics, Friberg, S.E. and Bothorel, R (eds.), CRC Press, Boca... 

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. 

This transition may j-.e. reducing the specific surface energy, f. The reduction of f to sufficiently small values was accounted for by Ruckenstein (15) in terms of the so called dilution effect". Accumulation of surfactant and cosurfactant at the interface not only causes significant reduction in the interfacial tension, but also results in reduction of the chemical potential of surfactant and cosurfactant in bulk solution. The latter reduction may exceed the positive free energy caused by the total interfacial tension and hence the overall Ag of the system may become negative. Further analysis by Ruckenstein and Krishnan (16) have showed that micelle formation encountered with water soluble surfactants reduces the dilution effect as a result of the association of the the surfactants molecules. However, if a cosurfactant is added, it can reduce the interfacial tension by further adsorption and introduces a dilution effect. The treatment of Ruckenstein and Krishnan (16) also highlighted the role of interfacial tension in the formation of microemulsions. When the contribution of surfactant and cosurfactant adsorption is taken into account, the entropy of the drops becomes negligible and the interfacial tension does not need to attain ultralow values before stable microemulsions form. 

Several theories have been proposed to account for the thermodynamic stability of microemulsions. The most recent theories showed that the driving force for microemulsion formation is the ultralow interfacial tension (in the region of 10 4-10 2 mN m 1). This means that the energy required for formation of the interface (the large number of small droplets) A Ay is compensated by the entropy of dispersion —TAS, which means that the free energy of formation of microemulsions AG is zero or negative. 

This expression was suggested previously by de Gennes and Taupin7 on the basis of scaling arguments. Experimentally, one can measure three different interfacial tensions in the three-phase system, namely, at the microemulsion—excess water phase boundary (yMw), at the microemulsion—excess oil phase boundary (yMo), and between the excess oil and excess water phases (yow). On the basis of intuitive arguments, it has been suggested8 that... 

Miller, C.A., Reinan, H., Benton, W.J., Tomlinson, F., 1977. A mechanism for ultralow interfacial tension in systems containing microemulsion— theoretical consideration and experiments with ultracentrifuge. J. Colloid Interface Sd. 61, 554. 

Jeng, J.-E. and Miller, C.A., Theory of microemulsions with spherical drops. I. Phase diagrams and interfacial tensions in gravity-free systems. Colloids Surf., 28, 247, 1987. 

Binks, B.P., Meunier, J., Abillon, O. and Langevin, D. (1989) Measurement of film rigidity and interfacial tensions in several ionic surfactant-oil-water microemulsions. Langmuir, 5,415-421. 

Figure 2 (a) Schematic diagram of volume fractions in a system of equal volumes of oil and water with, e.g., 5% of a PEO-type nonionic surfactant versus temperature, (b) Interfacial tensions in the same system. The line corresponding to the interfacial tension of the oil/microemulsion interface and the one corresponding to the microemulsion/water interface, are indicated. (After Ref. 6.)... 

The largest interfacial tension in the Winsor III systems (bicontinuous microemulsions in equilibrium with both excess oil and water) is also equal to the interfacial tension between oil and water in the presence of a saturated surfactant monolayer, i.e.. 

The salinity at which the middle phase microemulsion contains equal volumes of oil and brine is defined as the optimal salinity. The oil recovery is found to be maximum at or near the optimal salinity (8,10). At optimal salinity, the phase separation time or coalescence time of emulsions and the apparent viscosity of these emulsions in porous media are found to be minimum (11,12). Therefore, it appears that upon increasing the salinity, the surfactant migrates from the lower phase to middle phase to upper phase in an oil/brine/surfactant/alcohol system. The -> m u transition can be achieved by also changing any of the following variables Temperature, Alcohol Chain Length, Oil/Brine Ratio, Surfactant Solution/Oil Ratio, Surfactant Concentration and Molecular Weight of Surfactant. The present paper summarizes our extensive studies on the low and high surfactant concentration systems and related phenomena necessary to achieve ultralow interfacial tension in oil/brine/surfactant/alcohol systems. 

The effect of surfactant concentration on interfacial tension in the TRS 10-410/IBA/Dodecane/Brine system is shown in Figure 1. It is evident that there are two regions in which ultralow interfacial tension is observed, i.e., in the low concentration region around 0.1% and the other around 4% surfactant concentration. In the low concentration region, the system forms two phases, namely, oil and brine whereas in the high concentration region the middle phase microemulsion is in equilibrium with excess oil and brine... 

The interfacial tensions in the two phase region are generally small (1-10" dynes/cm) and even smaller in the three phase region (10" -10 dynes/cm). These properties are expected to be related to the structure of the microemulsions in the bulk phases (3). 

In summary, as shown in Figure 9, the salinity shock design of mobility pol)niier solution can provide ultra low interfacial tension at microemulsion/polymer solution interface, reduce surfactant loss, and achieve high oil recovery efficiency. The poly-... 

This causes an increase in the microemulsion/brine interfacial tension. In a core flood this may lead to trapping of a microemulsion phase and thus to high surfactant losses. Polymer may also cause a drastic increase in the viscosity of the microemulsion (perhaps a non-equilibrium effect), which is an additional factor hampering the displacement efficiency and thus increasing surfactant loss. 

In microemulsions, oil and water mix over small length scales, and thus an extraordinarily large interfacial area spans the oil and water domains. In order to thermodynamically stabilize such fine structures, the surfactant must generate an ultra-low free energy per unit of interfacial area between oil and water microdomains within the microemulsion phase. Such low free energies result from a precise balancing of the hydrophilic-lipophilic nature of the surfactant. As a consequence of this precise balancing, the macroscopic interfacial tensions between microemulsion phases and excess oil phases are also ultra-low (of the order of 10-3 mN/m) (15, 16). 

The original microemulsion was first detected as a distinct entity by Hoar and Schulman in 1943 [24] and consisted of water, benzene, hexanol, and potassium oleate. Most of Schulman s work dealt with four-component systems a hydrocarbon, an ionic surfactant, a cosurfactant (i.e., four- to eight-carbon-chain aliphatic alcohol), and an aqueous phase. The microemulsion was formed only when the surfactant-cosurfactant blend formed a mixed film at the oil/water interface, resulting in interfacial pressure exceeding the initial positive interfacial tension (so-called negative interfacial tension). The microemulsion was therefore produced spontaneously. During years of research many important geometrical and compositional parameters of microemulsions were studied. 

Lattice models for bulk mixtures have mostly been designed to describe features which are characteristic of systems with low amphiphile content. In particular, models for ternary oil/water/amphiphile systems are challenged to reproduce the reduction of the interfacial tension between water and oil in the presence of amphiphiles, and the existence of a structured disordered phase (a microemulsion) which coexists with an oil-rich and a water-rich phase. We recall that a structured phase is one in which correlation functions show oscillating behavior. Ordered lamellar phases have also been studied, but they are much more influenced by lattice artefacts here than in the case of the chain models. 

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