Microemulsion structure

The shape, size, and structure of these dispersed droplets depend upon a multitude of variables including the surfactant type, ionic strength, the presence of cosurfactant(s), and the amount of Avater. Commonly used surfactants are of the five general categories anionic, cationic, nonionic, amphoteric and zwitterionic. The nature of amphoteric surfactants, i.e., whether or not they behave as anionic or cationic species, is dependent on the pH or ionic strength of the aqueous phase. 

Microemulsions have the ability to partition polar species into the aqueous core or nonpolar solutes into the continuous phase (See Fig. 1). They can therefore greatly increase the solvation of polar species in essentially a nonpolar medium. The surfactant interfacial region provides a dramatic transition from the highly polar aqueous core to the nonpolar continuous-phase solvent. This region represents a third type of solvent environment where amphiphilic solutes can reside. Such amphiphilic species will be strongly oriented in the interfacial film so that their polar ends are in the core of the microemulsion droplet and the nonpolar end is pointed towards or dissolved in the continuous phase solvent. When the continuous phase is a near-critical liquid (7)j = r/7 0.75) or supercritical fluid, additional parameters such as transport properties, and pressure (or density) manipulation become important aids in applying this technology to chemical processes. 

The amount of water added to a water-in-oil (w/o) microemulsion is defined by the molar water-to-surfactant ratio, W. Generally, the greater the W value, the larger the size of the nanometer-sized 

It should be noted that high concentrations of ionic species can alter the phase stability of microemulsions based upon ionic surfactant systems. Nonionic surfactant systems are much less susceptible to this effect. The curvature of the interfacial film of the microemulsion droplet is determined by a balance between the electrostatic interactions of the head groups and repulsive interactions of the surfactant tail group. Addition of ionic solutes can upset this delicate balance and induce phase separation. By changing the structure of the surfactant or through the addition of cosurfactants one can restore this balance and thus allow the dissolution of high concentrations of ionic species. 

A universal property of surfactant solutions is the existence of a critical micelle concentration (CMC) representing the minimum amount of surfactant required to form aggregates. The CMC also represents the solubility of the surfactant unimer in the oil or continuous phase solvent. At surfactant concentrations above the CMC, 

Finally, in the discussion of reverse microemulsion systems, mention should be made of one of the most widely studied systems. The surfactant, sodium bis(2-ethylhexyl) sulfosucdnate or Aerosol-OT (AOT), is one of the most thoroughly studied reverse micelle-forming surfactants since it readily forms reverse micelle and microemulsion phases in a multitude of different solvents without the addition of cosurfactants or other solvent modifiers. The phase behavior of AOT in liquid alkane/wata systems is already wdl documented. Indeed, the first report of the existence of the formation of microemulsions in a supercritical fluid involved an AOT/alkane/ water system.l The herical structure of an AOT/nonpolar-fluid/ water microemulsion droplet is shown in Fig. 1. In the now wdl-known structure, it can be seen that the two hydrocarbon tails of each AOT molecule point outward into the nonpolar phase (e.g., supercritical fluid). These tails are lipophilic and are solvated by the nonpolar continuous phase solvent whereas the hydrophilic head groups are always positioned in the aqueous core. 

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Friberg S E and Liang Y-C 1987 Nonaqueous microemulsions Microemulsions Structure and Dynamics ed S E Friberg and Bothorel (Booa Raton, FL Chemioal Rubber Company) pp 79-91... 

Friberg S E and Bothorei P (eds) 1987 Microemulsions Structure and Dynamics (Boca Raton, FL Chemicai Rubber Company)... 

S. E. Ftiberg and P. Bothorel, Microemulsions Structure and Dynamics, CRC Press, Boca Raton, Fla., 1987. 

Solubilization of biomolecules could induce change in the microemulsion structure. For example, in the presence of the human serum albumin and at low R value, the ternary microemulsion AOT/water/isoctane shows a transition to a bicontinuous microstructure [172],... 

Maidment LJ, Chen V, Warr GG (1997) Effect of added cosurfactant on ternary microemulsion structure and dynamics. Colloids Simf A 130 311-319... 

Figures 7, 8 and 9 are plots at 25 C of specific conductance and density versus volume fraction of methanol in 2/1 triolein/ surfactant systems which are 4/1 molar ratios of 2-octanol to bis(2-ethylhexyl) sodium sulfosuccinate, triethylammonium linoleate and tetradecyldimethylammonium linoleate, respectively. For each surfactant system, a maximum for specific conductance and a minimum for density was observed at the same volume fraction, but this volume fraction of methanol varied between the three surfactant systems. At volume fractions of methanol above these abrupt changes, each system exhibited translucence, and it appears that gel-like structures form. These data are consistent for microemulsion structures that are based largely on geometric considerations (16-18).

Fribeig, S. E. Bothorel, P. Microemulsions Structure and Dynamics CRC Press Boca Raton, FL, 1987. 

Figure 7. The percolation behavior in AOT-water-decane microemulsion (17.5 21.3 61.2 vol%) is manifested by the temperature dependences of the static dielectric permittivity es (A left axis) and conductivity r (Q right axis). Toa is the temperature of the percolation onset Tp is the temperature of the percolation threshold. Insets are schematic presentations of the microemulsion structure far below percolation and at the percolation onset. (Reproduced with permission from Ref. 149. Copyright 1998, Elsevier Science B.V.)...

Ceglie, A., Das, K. P, and Lindman, B. (1987), Microemulsion structure in four component systems for different surfactants, Coll. Surf, 28,29 40. 

Zana R, Lang J (1987) In Frieberg SE, Bothorel (eds) Microemulsions structure and dynamics. CRC, Boca Raton, FL, Ch 6... 

STXM and SPM showed that bicontinuous microemulsion structure with well-defined wave vector has been formed, indicating that the interfacial ten-... 

Fig. 5 shows a hypothetical phase diagram with representation of microemulsion structures. At high water concentrations, microemulsions consist of small oil droplets dispersed in water (o/w microemulsion), while at lower water concentrations the situation is reversed and the system consists of water droplets dispersed in oil (w/o microemulsions). In each phase, the oil and water droplets are separated by a surfactant-rich film. In systems containing comparable amounts of oil and water, equilibrium bicontinuous structures in which the oil and the water domains interpenetrate in a more complicated manner are formed. In this region, infinite curved channels of both the oil and the water domains extend over macroscopic distances and the surfactant forms an interface of rapidly... 

Figure 7.18 Diagrammatic representation of microemulsion structures (a) a water-in-oil microemulsion droplet (b) an oil-in-water microemulsion droplet and (c) an irregular bicontinuous structure.

Figure 4. Idealized AOT reverse micelle or microemulsion structure and a proposed aggregation (or clustering) mechanism which maintains the distinct solvent environments for the reverse micelle conqponents.

The behavior of water in oil microemulsions has been studied using different techniques light scattering, electrical conductivity, viscosity, transient electrical birefringence, ultrasonic absorption. All these experiments lead us to propose a picture of the microemulsions structure which assignes an important role to the fluidity of the interfacial region. 

Microemulsions are transparent fluid mixtures of water, oil, surfactant and cosurfactant (alcohols). At small water fraction, w/o microemulsions are dispersions of water droplets surrounded by a surfactant layer in a continuous oil phase. The microemulsion structure at larger water fraction has been studied with different techniques and some results are presented subsequently. A qualitative microemulsion picture is proposed to explain the data. 

The preceding analysis assigns an important role to the interaction between droplets or equivalently the fluidity of the interfacial region in the microemulsions structure. 

Close to the boundaries Sj and S2 in the three phase domain, the interfacial tensions were found to be very low. In that case, the theoretical model presented above is no longer valid, first of all because the middle phase microemulsion structure is not simply a droplet dispersion. Furthermore the interaction term F becomes evidently dominant and is difficult to evaluate since the nature of the forces is not perfectly known. However, such low interfacial tensions are characteristic of critical consolute points. It was then tempting to check that the behavior of the interfacial tensions was compatible with the universal scaling laws obtained in the theory of critical phenomena. In these theories the relevant parameter is the distance e to the critical point defined by ... 

Effect of Microemulsion Structure on the Transport Properties. It appears from the discussion above that the reduction in the ionic conductivity and water self-diffusion coefficient is primarily attributable to hydration effects, not principally to changes in the structure of the microemulsion with higher phase volume. 

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