Bicontinuous microemulsions, description

One of the major steps to enhancing the understanding of surfactant association structures is to investigate the phase equilibria of water-oil-surfactant mixtures. Since the pioneering contributions of P. Ekwall in Scandinavia and K. Shinoda in Japan, who analyzed ternary systems containing ionic and nonionic surfactants, respectively, extensive experimental work has been devoted to different kinds of microemulsion systems [7-20]. During recent decades, detailed descriptions of the phase behavior of many ternary (water-oil-surfactant), quaternary (water-oil-surfactant-alcohol), and even quinary (water-oil-surfactant-alcohol-salt) systems have been presented (for example, see Refs. 21-36). These studies have provided evidence for several new phases where the surfactant creates surfaces. Thus, in addition to bicontinuous microemulsions, one finds dilute lamellar phases (L ) [37-43] and liquid isotropic phases of randomly connected bilayers called... 

Recently an alternative approach for the description of the structure in systems with self-assembling molecules has been proposed in Ref. 68. In this approach no particular assumption about the nature of the internal interfaces or their bicontinuity is necessary. Therefore, within the same formahsm, localized, well-defined thin films and diffuse interfaces can be described both in the ordered phases and in the microemulsion. This method is based on the vector field describing the orientational ordering of surfactant, u, or rather on its curlless part s defined in Eq. (55). 

From the results of self-diffusion, Lindman et al. (71) have proposed the structure of microemulsions as either the systems have a bicontinuous (e.g. both oil and water continuous) structure or the aggregates present have interfaces which are easily deformable and flexible and open up on a very short time scale. This group has become more inclined to believe that the latter proposed structure of microemulsion is more realistic and close to the correct description. However, no doubt much more experimental and theoretical investigations are needed to understand the dynamic structure of these systems. 

However, even without structural studies, Friberg et al. [32], Shinoda [33], and others noted that the broad existence range with respect to the water/oil ratio could not be consistent with a micellar-only picture. Also, the rich polymorphism in general in surfactant systems made such a simplified picture unreasonable. It was natural to try to visualize microemulsions as disordered versions of the ordered liquid crystalline phases occurring under similar conditions, and the rods of hexagonal phases, the layered structure of lamellar phases, and the minimal surface structure of bicontinuous cubic phases formed a starting point. We now know that the minimal surfaces of zero or low mean curvature, as introduced in the field by Scriven [34], offer an excellent description of balanced microemulsions, i.e., microemulsions containing similar volumes of oil and water. 

It is worth noting that this description assumes implicitly that the = 1 case is associated with a zero-curvature C layer, which could be provided either by a lamellar liquid crystal structure with alternating O and W flat layers or a zero curvature surface of the Schwartz type or as a transient and fluctuating combination of Si and S2 structures. It is now well recognized that middle-phase microemulsions, which are in equilibrium with both oil and water excess phases, exhibit bicontinuous structures as shown in Fig. 7 [27] that are not far from the transient mixture of Si and S2 swollen micelles predicted by Winsor. 

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