W/O-droplet microemulsions

Figure 5. Volume fractions of oil (squares), water (triangles), and alcohol (circles) in microemulsions containing O/W and W/O droplets as a function of the volume ratios of alcohol to surfactant in the microemulsions. Filled symbols refer to O/W droplet microemulsions, and open symbols, to W/O droplet microemulsions. All the system characteristics are identical to those described for Figure 3. In all cases, the volume fraction of the surfactant in the microemulsions is 0.01.

Figure 10. Volume fraction of oil in a bicontinuous microemulsion as a function of the volume ratio of alcohol to surfactant in the microemulsion. The values of Xaw and gAi/gsi are respectively 0.001 72 and 1.036 (filled squares) and 0.001 84 and 1.074 (open squares). In all cases, the volume fraction of surfactant is 0.01 and the volume fraction of water is found by subtraction from unity. The system consists of SDS, 1-pentanol, cyclohexane, water, and 0.3 M NaCl. Also shown are the corresponding results for O/W droplet microemulsions (filled circles) and for W/O droplet microemulsions (open circles).

In this equation, hs is the hard sphere volume fraction which is about 14% larger in o/w-droplet microemulsions of non-ionic surfactant than the dispersed volume fraction. This is caused by the water penetration in the surfactant layer [64]. S(q) approaches unity for q values smaller than the minimum of I(q). This behaviour occurs even for fairly high volume fractions in non-ionic surfactant systems (see for example Fig. 8 in Ref. [64]). Seeing that the value of the radius is fixed by the position of the minimum of I(q), the approximation of S(q) 1 in Eq. (2.12) does not lead to a significant error in the determination of Rq if the low q part of the experimental curve is not taken into... 

Figure 2.1 Comparison of SANS curves obtained for the system D20/n-octane-di8/C1oE4 on the (a) water-continuous (o/w-droplet microemulsion) and (b) the oil-continuous (w/o-droplet microemulsion) side, respectively. The solid lines in both plots are from factor curves according to Eq. (2.11). Usually, the polydispersity is slightly higher for w/o-droplet microemulsions. (Figures redrawn with data from Ref. [67].)...

As already pointed out the first work directly measuring the deformation dynamics in an o/w-droplet microemulsion using NSE was published by Huang et al. [45]. In this work, a microemulsion based on the surfactant AOT was studied and it was shown that the intermediate scattering functions contain information about the centre of mass diffusion and in addition also contributions from the deformation dynamics. The intermediate scattering functions obtained in this work are shown in Fig. 2.3. 

Figure 2.3 Intermediate scattering functions obtained for an AOT-based o/w-droplet microemulsion using NSE. The solid lines are fits with a single exponential function yielding an effective diffusion coefficient. Note that this was the first NSE study of a microemulsion showing the calculation of k on the basis of the intermediate scattering functions. (From Ref. [45], reprinted with permission of the American Physical Society.)...

After these first experiments it took 11 years until this problem was studied again exploiting the unique possibilities of NSE with respect to contrast variation and energy resolution [29]. The studied microemulsion was an o/w-droplet microemulsion in the system H2O/ -octane/C10E5. It turned out that the NSE data can be analysed using a double exponential fit according to Eq. (2.8), when the translational diffusion coefficient is already measured in advance using PCS. The same approach was also successfully applied to study another water-continuous microemulsion in the system H2 0/n-dodecane/Cio E-[49]. Since the approach works as well for oil-continuous systems an extended example for the approach will be discussed in the next subsection. 

The difference between PEO being functionalised at one chain end with a hydrophobic sticker and its counterpart containing the hydrophobic units at both chain ends was examined with oil-in-water (o/w) droplet microemulsions [22-24]. Cetyl pyridiniumchlo-ride/octanol or alkylphenol ethoxylate were used as surfactant. The monofunctional polymers were only capable to decorate the oil droplets whereas the difunctional polymers could also bridge them. Experiments were carried out as a function of the droplet volume... 

Figure 4.6 Phase behaviour of o/w-droplet microemulsions containing 0.2 M NaCI-decane-cetyl pyri-dinium chloride-octanol. The PEO additive is hydrophobically modified at one chain end (a) and at both chain ends (b), respectively, r represents the number of polymer chains per droplet. (From Ref. [23], reprinted with permission of the American Chemical Society.)...

Figure 4.8 Phase diagram of the system wafer decane C12E5. The channel in which the o/w-droplet microemulsion (Li) is formed closes slowly upon adding PEO polymer. (From Ref. [37], reprinted with...

Reference [42] also reported on the hydrophilic polymer PNIPAM which is adsorbed at the outer interface of o/w-droplet microemulsions. Here, the fluctuations are increased... 

Figure 4.12 Phase diagram of the system water-isooctane-AOT-PEO as a function of the temperature T and the surfactant concentration y. The isooctane content was fixed at 40wt.% and the polymer content in the aqueous phase was either Cp = 5 or 20 wt.%. The phase regions are 2 for microemulsion coexisting with excess water, 2 for microemulsion coexisting with excess oil, 3 for three-phase coexistence of a microemulsion with two excess phases. Inside the fish-tail there are one-phase regions L Li for o/w-droplet microemulsions, L2 for w/o-droplet microemulsions, La for the lamellar phase and 2a for a coexistence of a lamellar with a microemulsion phase. Note that the whole fish-tail is shifted to higher surfactant concentrations upon polymer addition. (From Ref. [53], reprinted with permission of the American Chemical Society.)...

It is now clear enough that in a surfactant-oil-water system (with or without additional components), there should be at least two microemulsion phases that are relevant for our work water solubilized in oil (W/O droplet microemulsions, commonly called the L2 phase) and oil solubilized in water (O/W droplet microemulsions, commonly called the Li phase). Indeed, the situation is much more complex than this [3,41,47, 104]. Figure 3.2 is indicative of some common phases that can be identified in a water/oil/surfactant system under selected conditions. This, however, does not concern us much, as most of the present book deals with spherical micelles and microemulsions (generally reverse, but also normal in some specific cases) as they are the most popular templates for synthesis. 

The equations developed in previous sections can be used to calculate the structural features of microemulsions, provided explicit expressions for the standard free energies of transfer of surfactant and alcohol molecules from their infinitely dilute states in water and of oil molecules from the pure oil phase to the interfacial layer of the microemulsion droplets are available. Such expressions are given below for spherical layers of O/W droplets and W/O droplets and also for flat layers. The difference in the standard state free energy consists of a number of contributions ... 

Estimates of the size and composition dispersions of droplets in O/W and W/O droplet-type microemulsions, obtained using the maximum-term method described in Appendix C, are summarized in Table 1. The calculations for the O/W droplet-type microemulsions were carried out... 

Meanwhile, there are a variety of large-scale applications of microemulsion systems. Many products used in daily life contain micro emulsions or formulations which are able to form microemulsions (some prominent examples are discussed in Chapters 8 and 9 of this book). Concentrates, surfactants or surfactant mixtures which can be used for microemulsification are frequently applied. All these materials are produced and handled in large quantities. In particular, oil-in-water (o/w) droplet and water-in-oil (w/o) droplet microemulsions are found in many products or technical processes today. Whereas their usage is not very different from ordinary solvents in most cases, the use of bicontinuous microemulsions poses specific problems which will be discussed later on. 

For the case of droplet microemulsions, tq = ri = r2, and so the mean curvature H = 1/ro. According to convention, H is positive for oil-in-water (o/w) droplets, and negative for water-in-oil (w/o) droplets. For bicontinuous microemulsions, which according to freeze-fracture electron microscopy have saddle-shaped surfaces of negative and positive curvature (13), c —C2, and so the mean curvature // 0. Note that lamellar liquid crystalline phases (Lc,), which are planar layers of oil and water, also have zero mean curvature (c = c 2 = 0), and are often located at higher surfactant concentrations nearby bicontinuous microemulsion phases (19). 

A (macro)emulsion is formed when two immiscible Hquids, usually water and a hydrophobic organic solvent, an oil, are mechanically agitated (5) so that one Hquid forms droplets in the other one. A microemulsion, on the other hand, forms spontaneously because of the self-association of added amphiphilic molecules. During the emulsification agitation both Hquids form droplets, and with no stabilization, two emulsion layers are formed, one with oil droplets in water (o /w) and one of water in oil (w/o). However, if not stabilized the droplets separate into two phases when the agitation ceases. If an emulsifier (a stabilizing compound) is added to the two immiscible Hquids, one of them becomes continuous and the other one remains in droplet form. 

For a typical biomolecule-containing w/o-ME system, only a fraction of the w/o-ME population (e.g., 0.1-1%) will contain proteins. Because of their small concentration and the short (microsecond-scale) lifetime of any given w/o-ME droplet, due to the rapid collision and exchange rate for w/o-ME systems, isolation of the protein-containing, or filled w/o-ME populations is difficult to achieve. Various techniques have demonstrated that encapsulated enzymes can alter the structural properties and behavior of the w/o-MEs, and that filled, w/o-MEs may differ in properties from the empty w/o-MEs in a given microemulsion system [46-51]. However, a clear understanding of the structural and dimensional differences between filled and empty w/o-MEs has yet to be achieved. 

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). 

The term microemulsion is applied in a wide sense to different types of liquid liquid systems. In this chapter, it refers to a liquid-liquid dispersion of droplets in the size range of about 10-200 nm that is both thermodynamically stable and optically isotropic. Thus, despite being two phase systems, microemulsions look like single phases to the naked eye. There are two types of microemulsions oil in water (O/W) and water in oil (W/O). The simplest system consists of oil, water, and an amphiphilic component that aggregates in either phase, or in both, entrapping the other phase to form... 

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),... 

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