Chain length microemulsions

These microdroplets can act as a reaction medium, as do micelles or vesicles. They affect indicator equilibria and can change overall rates of chemical reactions, and the cosurfactant may react nucleophilically with substrate in a microemulsion droplet. Mixtures of surfactants and cosurfactants, e.g. medium chain length alcohols or amines, are similar to o/w microemulsions in that they have ionic head groups and cosurfactant at their surface in contact with water. They are probably best described as swollen micelles, but it is convenient to consider their effects upon reaction rates as being similar to those of microemulsions (Athanassakis et al., 1982). 

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

Before describing how microemulsion nature and structure are determined by the structure and chain length of surfactant and cosurfactant, it is necessary first to briefly review the theories of microemulsion formation and stability. These theories will highlight the important factors required for microemulsion formation. This constitutes the first part of this review. The second part describes the factors that determine whether a w/o or o/w microemulsion is formed. This is then... 

TADROS Structure, Chain Length Influence on Microemulsions... 

With ionic surfactants for which V/1 <0.7, microemulsion formation needs the presence of a cosurfactant. The latter has the effect of increasing V without affecting 1 (if the chain length of the cosurfactant does not exceed that of the surfactant). These cosurfactant molecules act as "padding" separating the head groups. 

Both the structure and chain length of surfactants and cosurfactants have a striking influence on the structure of the micremulsion formed. The most systematic studies have been on the influence of the cosurfactant chain length and structure on the nature of the microemulsion region. Two main studies have been carried out to elucidate the difference obtained, namely electrical conductivity and NMR investigations. As we will see... 

These systems were referred to by Clausse t a (21) as Type U systems. On the other hand, with cofurfactants with chain length Cg to Cy (Figur 3 e-g), the Winsor IV domain is split into two disjointed areas that are separated by a composition zone over which viscous turbid and birifringent media are encountered. This second class of systems was referred as Type S systems (24). It can also be seen that the Winsor IV domain reaches its maximum extension at reducing in size below and above C. Moreover, at C, one observes a small monophasic region near the W apex (probably o/w microemulsion of the Schulman s type) which vanishes as the alcohol chain length is increased to Cg. 

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. 

We have examined the photosensitized reduction of a series of + 4,4 -bipyridinium salts, CnV2+, (2), (where n=l-16) with Ru(bpy)l as sensitizer and (NHi jEDTA as electron donor in a water-in-toluene microemulsion media. Under steady state illumination the quantum yield of CjjVt" formation strongly depends on the alkyl chain length of the electron acceptor (Figure 4). It improves as the hydro-... 

The above-mentioned artificial microbubble surfactant mixtures, and other successful mixtures found for stable microbubble production (ref. 544-546), all contain saturated glycerides (with acyl chain lengths greater than 10 carbons) combined with cholesterol and cholesterol derivatives (cf. Chapters 9 and 10, and ref. 544). As described earlier, long chain lengths in nonionic (or even unionized) surfactants are known to favor the formation of both large, rodlike micelles (as opposed to small spherical micelles) and macroemulsions (as opposed to microemulsions) (see... 

Constantinides, P. P., and Scalart, J. P. (1997), Formulation and physical characterisation of water-in-oil microemulsions containing long versus medium chain length glycerides, Int. I. Pharm., 58, 57-68. 

Most single-chain surfactants do not lower the oil-water interfacial tension sufficiently to form microemulsions nor are they of the correct molecular structure, and short- to medium-chain length alcohols are necessary as cosurfactants. The cosurfactant also ensures that the interfacial film is flexible enough to deform readily around each droplet as their intercalation between the primary surfactant molecules decreases both the polar head group interactions and the hydrocarbon chain interactions. Medium-chain alcohols such as pentanol and hexanol have been used by many investigators as they are particularly effective... 

Effect of Chain Length Compatibility on Monolayers, Foams, and Macro- and Microemulsions... 

The gas/liquid and liquid/liquid systems are relevant to biomedical and engineering applications. The large interfacial area in foams, macro- and microemulsions is suitable for rapid mass transfer from gas to liquid or liquid to gas in foams and from one liquid to another or vice versa in macro- and microemulsions. The formation and stability of these systems may be influenced by the chain length compatibility which may also influence the flow through porous media behavior of these systems. Therefore, the present communication deals with the effect of chain length compatibility on the properties of monolayers, foams, macro- and microemulsions. An attempt is made to correlate the chain length compatibility effects with surface properties of mixed surfactants and their flow behavior in porous media in relation to enhanced oil recovery. 

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