Microemulsions microstructures

The last, and less extensively studied field variable driving percolation effects is chemical potential. Salinity was examined in the seminal NMR self-diffusion paper of Clarkson et al. [12] as a component in brine, toluene, and SDS (sodium dodecylsulfate) microemulsions. Decreasing levels of salinity were found to be sufficient to drive the microemulsion microstructure from water-in-oil to irregular bicontinuous to oil-in-water. This paper was... 

R. Strey Microemulsion, Microstructure and Interfacial Curvature. Colloid Polym. Sci. 272, 1005 (1994). 

Two main microemulsion microstructures have been identified droplet and biconti-nuous microemulsions (54-58). In the droplet type, the microemulsion phase consists of solubilized micelles reverse micelles for w/o systems and normal micelles for the o/w counterparts. In w/o microemulsions, spherical water drops are coated by a monomolecular film of surfactant, while in w/o microemulsions, the dispersed phase is oil. In contrast, bicontinuous microemulsions occur as a continuous network of aqueous domains enmeshed in a continuous network of oil, with the surfactant molecules occupying the oil/water boundaries. Microemulsion-based materials synthesis relies on the availability of surfactant/oil/aqueous phase formulations that give stable microemulsions (54-58). As can be seen from Table 2.2.1, a variety of surfactants have been used, as further detailed in Table 2.2.2 (16). Also, various oils have been utilized, including straight-chain alkanes (e.g., n-decane, /(-hexane),... 

Strey, R. (1994) Microemulsion microstructure and interfacial curvafflofl. Polym. Sci., 272 1005-1019. 

Th. Zemb. The doc model of microemulsions Microstructure, scattering, conductivity and phase limits imposed by sterical constraints. Colloid Surface A, 129 435, 1997. 

Kaler, E. W., Bennett, K. E., Davis, H.T., and Scriven, L. V. (1983),Toward understanding microemulsion microstructure I A small-angle x-ray scattering study,/. Phys. Chem.,19, 5673-5684. 

Strey R. Microemulsion microstructure and interfacial curvature. Colloid Po-lym Sci 1994 272 1005-1019. 

The field of microemulsion microstructure started to develop forcefully in the 1970s. Realizing the thermodynamic stability and the relation to other surfactant self-assemblies, it became natural to assume that they were akin to micellar solutions, the most studied aspect of surfactant-water systems, and they were discussed in terms such as swollen micelles. ... 

Here we mainly address one problem of microemulsion microstructure, namely that of connectivity, in particular the distinction between uni- and bicontinuous structures. We also, to some extent, consider the problem of size and shape of aggregates for discrete particle structures and that of different bicontinuous structures. NMR can also shed light on other aspects of microstructure, such as the distribution of surfactant molecules between surfactant films and the oil and water domains and the local packing and ordering of surfactant molecules. Here we merely note that there is overwhelming evidence (mainly from NMR) that, locally, surfactant aggregates in different phases (micellar solutions, microemulsions, different liquid crystalline phases) show only quite minor differences. 

In this review, we discuss the development and application of cryogenic TEM techniques in the study of microemulsion microstructures. Our goal is to illustrate the power of the technique to directly image a variety of microemulsion microstructures that were once only figments of an artist s imagination. The chapter opens with a brief description of the sample preparation techniques, presents a series of micrographs... 

Microemulsion microstructures are best imaged by cryo-TEM. A description of sample preparation methods with a view to obtaining direct and artifact-free images is described in this section as it is the key to microscopy. 

C. Imaging of Microemulsion Microstructures in the H20-n-Dodecane-DDAB and Related Systems... 

Figure 12 Micrographs from a series of control experiments to establish the globular-to-bicontinuous transition in microemulsion microstructures. Bar = 250 nm. (From Ref 9.)...

Several of my collaborations with Friberg, Shinoda, and Ekwall concerned microemulsion microstructure, where they provided enlightening systems for structural investigation and deep insight into the subject. 

Ethoxylated alcohols are amphiphiles containing hydrocarbon tails which prefer oil environments and ethoxylated alcohol groups which prefer water environments. However, short-chain amphiphiles such as C4EJ are inefficient at mixing oil and water and only form weakly structured solutions (22). Longer-chained amphiphiles such as C12E5 are true surfactants which efficiently mix oil and water, and also form distinct droplet and bicontinuous microemulsion microstructures (23). 

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. 

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