Structure of Microemulsions

2 Structure of Microemulsions. - A microemulsion can exhibit different phases depending on the temperature as well as the nature of the surfactant, co-surfactant, oil phase, and their relative concentrations (i) Two phases, the microemulsion in equilibrium with the oil phase, (ii) Two phases, the microemulsion in equilibrium with the water phase, (iii) Three phases, the microemulsion in equilibrium with both oil and water phases, (iv) Single phase, oil, water and surfactant being homogeneously mixed. 

Once a microemulsion is formed, depending on the concentration of the different components, the mixture will consist of (i) spherical micelles either of an oil in water microemulsion, with aggregates called micelles, or a water in oil microemulsion with reversed micelle aggregates, (ii) rod-like micelles or bi-continuous phases . Recently, Hellweg has reviewed the state-of-the-art of this topic s. For the use of ionic type surfactants, particularly AOT, a recent review by Nave et al is recommended.  

Schematic representation of transition from W/0 to 0/Why way of continuous structure for ternary system based on non-ionic surfactant. Influence of temperature adapted from reference 11 

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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))-... 

Influence of Structure and Chain Length of Surfactant on the Nature and Structure of Microemulsions... 

Further information on the dependence of structure of microemulsions formed on the alcohol chain length was obtained from measurement of self diffusion coefficient of all the constitutents using NMR techniques (29-34). For microemulsions consisting of water, hydrocarbon, an anionic surfactant and a short chain alcohol and C ) the self diffusion... 

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. 

B. Lindman and U. Olsson. Structure of microemulsions studied by NMR. Berichte Der Bunsen-Gesellschaft-Phys. Chem. Chem. Phys., 100(3) 344—363, 1996. 

J.H. Schulman, W. Stoeckenius and L.M. Prince, Mechanism of formation and structure of microemulsions by electron microscopy, J. Phys. Chem. 63 (1959) 1677-1680. 

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. 

Friberg et al. (73) have proposed a random structure of microemulsions with varying curvatures. Taupin and co-workers (74) have considered the presence of hard oil and water droplets with a relatively sharp transition between these, while Shinoda (75.761 bas proposed a lamellar structure with alternating water, amphiphilic... 

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