The Structure of Microemulsions

The M phase in zone (III) raises subtle problems. In particular, the above geometrical relations are no longer respected. Experiments attempting to characterise the structures have not revealed well defined elementary objects. 

Two new approaches have thus been developed by reconsidering the following  

Thermodynamics of highly divided systems with extremely low interfacial tension. In this case, to determine stability of the system, we must consider terms usually neglected when dealing with emulsions  

A particularly interesting case is that of systems in which the film has a zero interfacial tension and zero spontaneous curvature (i.e., it is spontaneously flat), together with a great flexibility, so that local curvature may nevertheless occur. P.G. de Gennes was able to predict the existence of bicontinuous phases which provide a good representation of the mysterious zone (III). Note that these sponge phases possess curvatures in opposite directions, unlike droplets in a dispersion. These points and the properties of these phases are discussed further in Chap. 5. 

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The structure of microemulsions have been studied by a variety of experimental means. Scattering experiments yield the droplet size or persistence length (3-6 nm) for nonspherical phases. Small-angle neutron scattering (SANS) [123] and x-ray scattering [124] experiments are appropriate however, the isotopic substitution of D2O for H2O... 

For diffuse and delocahzed interfaces one can still define a mathematical surface which in some way describes the film, for example by 0(r) = 0. A problem arises if one wants to compare the structure of microemulsion and of ordered phases within one formalism. The problem is caused by the topological fluctuations. As was shown, the Euler characteristic averaged over the surfaces, (x(0(r) = 0)), is different from the Euler characteristics of the average surface, x((0(r)) = 0), in the ordered phases. This difference is large in the lamellar phase, especially close to the transition to the microemulsion. x((0(r)) =0) is a natural quantity for the description of the structure of the ordered phases. For microemulsion, however, (0(r)) = 0 everywhere, and the only meaningful quantity is (x(0(r) = 0))-... 

Thus, in summary, self diffusion measurements by Lindman et a (29-34) have clearly indicated that the structure of microemulsions depends to a large extent on the chain length of the oosurfactant (alcohol), the surfactant and the type of system. With short chain alcohols (hydrophilic domains and the structure is best described by a bicontinuous solution with easily deformable and flexible interfaces. This picture is consistent with the percolative behaviour observed when the conductivity is measured as a function of water volume fraction (see above). With long chain alcohols (> Cg) on the other hand, well defined "cores" may be distinguished with a more pronounced separation into hydrophobic and hydrophilic regions. 

Thus it can be concluded that the structure of microemulsions depends on the structure of surfactant and cosurfactant. Moreover, this structure also determines the amount of solubilisation of oil and or water in microemulsions. 

Microemulsions. The structure of microemulsion systems has been reviewed (22). Both bicontinuous and droplet-type structures, among others, can occur in microemulsions. The droplet-type structure is conceptually more simple and is an extension of the emulsion structure that occurs at relatively high values of IFT. In this case, very small thermodynamically stable droplets occur, typically smaller than 10 nm (7). Each droplet is separated from the continuous phase by a monolayer of surfactant. Bicontinuous microemulsions are those in which oil and water layers in the microemulsion may be only a few molecules thick, separated by a monolayer of surfactant. Each layer may extend over a macroscopic distance, with many layers making up the microemulsion. 

A variety of experimental techniques are available to investigate the structure of microemulsions small angle scattering, specific heat, viscosity and electrical conductivity measurements. In the DDAB systems, conductivity measurements eidiibit a dramatic decrease (typically eight orders of magnitude) as water is added to the mixture. Such changes occur over just a few percent variation in water content, apparently difficult to reconcile with the fact that the oil is (relative to water) non-conducting. It implies that the... 

The simplest representation of the structure of microemulsions is the droplet model in which microemulsion droplets are surrounded by an interfacial film consisting of both surfactant and cosurfactant molecules, as illustrated in Fig. 7.18. The orientation of the amphiphiles at the interface will, of course, differ in o/w and w/o microemulsions. As... 

Structural Aspects of Microemulsions. Several investigators have studied the structure of microemulsions using various techniques such as ultracentrifugation, high resolution NMR, spin-spin relaxation time, ultrasonic absorption, p-jump, T-jump, stopped-flow, electrical resistance and viscosity measurements (56-58). The useful compilation of different studies on this subject is found in the books by Robb (68) and Shah and Schechter (69). Several structural models of microemulsions have been proposed and we will discuss only a few important studies here. 

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. 

Olsson, U., Shinoda, K. and Lindman, B. (1986) Change of the structure of microemulsions with the HLB of nonionic surfactant as revealed by NMR self-diffusion studies. /. Phys. Chem., 90, 4083M088. 

Because of the high solubilization power of both hydrophilic and hydrophobic substances and the ultralow interfacial tension as their inherent property, microemulsions are an excellent medium for textile detergency. The structure of microemulsions and its effect on detergency are presented in Chapter 29 by Ddrfier. A complete discussion of aU such applications would render this volume prohibitively long, so only eclectic applicahons are included. 

A typical ESR spectrum is shown in Fig. 2. As described, Xc can be calculated from the spectra. Shah and co-workers [35] used ESR to study the structure of microemulsion, and Rosen et al. [36] used ESR and found the microviscosity gradient in the middle phase of three-phase micellar systems. Shirhama et al. [37] and Witte et al. [38] have used ESR to study the interaction of SDS with PEO and PVP, yielding information on the structure of complex from the polymers and SDS. But the influence of the spin-label molecule on the microstructure should be considered. 

Photon-based synthesis could provide an avenue to preserve the structure of miCToemulsions (in the chemical reduction method, the introduction of the reducing agent into the water core from outside may damage the structure of microemulsions). A photochemically induced metal nanoparticles synthesis in the water-in-scC02 microemulsion involves microemulsion formation followed by metal ion reduction through absorption of incident light by microemulsion which contains the reactants precursors [43]. 

In spite of the aforementioned complications, several studies have used the exothermic scanning mode to obtain more insight into the structure of microemulsions and to identify percolation processes in model systems. 

Simple Alcohols and Their Role in the Structure of Microemulsions... 

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