Structure droplet

The droplet structure in attractive emulsions can be directly observed under a microscope. Besides direct observations, a precise determination of the structure... 

Both O/W and W/O droplet structures have a core region surrounded by an interfacial film, depicted schematically in Figure 2. The interfacial film excludes the headgroups of the surfactant and the alcohol. This region is analogous to the micelle core and, as confirmed by many experiments, is free of water molecules and is thus hydrophobic in character. Each droplet contains gk molecules of kind k... 

Eastoe J, Bayazit Z, Martel S, Ste5dler DC, Heenen RK. Droplet structure in a water-in-C02 microemulsion. Langmuir 1996 12 1423-1424. 

The ideas of the relevance of phase diagrams and thermodynamic stability as well as the bicontinuous structure were certainly not accepted immediately and many publications until well into the 1990s caused confusion as some authors still took droplet structures for granted. A title for a paper [31] in Nature as late as 1986 entitled Occurrence of liquid-crystalline mesophases in microemulsion dispersions illustrates both the slow acceptance and the ignorance of previous work on phase diagrams. 

Scattering techniques provide the most definite proof of micellar aggregation. Zielinski et aL (34) employed SANS to study the droplet structures in these systems. Conductivity measurements (35) and SANS (36) were also used to study droplet interactions at high volume fraction in w/c microemulsions formed with a PFPE-COO NH4 surfactant (MW = 672). Scattering data were successfully fitted by Schultz distribution of polydisperse spheres (see footnote 37). A range of PFPE-COO NH/ surfactants were also shown to form w/c emulsions consisting of equal amount of CO2 and brine (38-40). 

According to Zielinski et al, the PFPE-NH4-stabilized water-in-COj microemulsion of Wo equal to 11 contains water droplets of - 4 nm in average diameter, and the droplets and droplet structure are little affected by experimental parameters such as the system pressure (28). Thus, the reverse micellar core in the microemulsion used in this study (Wo = 10) should have an average diameter close to 4 nm. Since there is evidence that the average size of the nanoparticles produced via RESOLV is dependent on the size of the pre-expansion reverse micelles in CO2 (9), it may be more than just a coincidence that the metal sulfide nanoparticles are of average sizes comparable to that of the pre-expansion water core (Table 1). The... 

Micelles (normal and reverse) are aggregates of surfactants, which are formed spontaneously in a liquid phase when the surfactant concentration is increased than the critical micelle concentration (CMC). Micellar solutions are formed in aqueous continuous systems, whereas, the reverse micelles are formed in oily continuous systems. In micelles (oil-in-water micelle), the polar heads of the surfactant lie outside in the aqueous phase, whereas the lipophilic hydrocarbon chains lie inside (Figure 58.3). When the surfactant concentration is increased, the free energy of the system increases due to the inauspicious interactions between the water molecules and the lipophilic portions of the surfactant. The water molecules around the oil droplets structures themselves, thereby, resulting in the decrease in the entropy. The reverse micelles (water-in-oil micelle) have opposite structure, that is, the polar heads lie at the centre, while the lipophilic tails are present outside in the oil phase (Figure 58.3). The surfactant concentration need not necessarily be higher than the CMC for the formation of reverse micelle. Many scientists reported formation of reverse micelles by lecithin in different oil phases. " ... 

In order to be able to extend the approach described in this contribution beyond microemulsions with a droplet structure, the (in our opinion) most important question to be resolved is What is the structure of the middle-phase microemulsion, and how does it contribute to the mixing entropy of the system An answer to this question may also provide a (qualitative) explanation regarding the limited swellability of a middle-phase microemulsion, that is, the observation that the surfactant-rich phase remains in between excess oil and water in spite of the fact that the middle phase is probably bicontinuous. 

The alternative NMR approach that has provided information on microemuisions is relaxation. However, on the whole, relaxation has been less informative than anticipated from earlier studies of micellar solutions and has provided little unique information on microemulsion structure, although in the case of droplet structures it is probably the most reliable way of deducing any changes in droplet size and shape, particularly for concentrated systems. The reason for this is that NMR relaxation probes the rotational diffusion of droplets, which is relatively insensitive to interdroplet interactions. This is in contrast to, for example, translational collective and self-diffusion and viscosity which depend strongly on interactions. Furthermore, NMR relaxation is a useful technique for characterizing the local properties of the surfactant film. 

Figure 18 Illustration of the double-oil self-diffusion experiment with a cyclohexane-hexadecane mixture. K = D /D2, where D and Dj are the cyclohexane and hexadecane diffusion coefficients, respectively. In the pure oil mixture the ratio of the two diffusion coefficients is K = Kq— 1.69. For a water-in-oil droplet structure the two oil molecules have the same diffusion coefficient, that of the micelle, and the ratio A equals unity. In a bicontinuous structure, on the other hand, a molecular diffusion mechanism is dominating and the ratio K equals that of the pure oil mixture, Kq. By monitoring the diffusion coefficient ratio, the droplet-to-bicontinuous transition could be studied.

It is interesting to note that in microemulsion systems where one can go from one structural type to another it has been observed that typically around the transition from a droplet structure to a bicontinuous structure a maximum in the viscosity occurs. The viscosity change can also be used to determine the percolation threshold in microemulsions, since here at this point the viscosity shows a maximum slope in a logarithmic plot. 

B. Direct Observation of Droplet Structure in a Vitrified Water- Propylene Glycol-lWeen 8O-CCI4 System... 

Figure 7 Droplet structure of oil-continuous microemulsions of the D20-n-decane-A0T system with varying droplet volume fraction. Bar = 2000 A. (From Ref 21.)...

Polymerization in microemulsions with a water/oil droplet structure yields closed-cell porous polymeric solids having a morphology characterized by a disjointed cellular structure in which the water pores are distributed as discrete pockets throughout the solid. 

It is necessary, but not sufficient, to characterize microemulsion systems thermodynamically in terms of phase equilibria. To obtain an understanding on the molecular level we have also to study the molecular behavior. One of the early scientific challenges in the microeraulsion field was to understand the transition from an oil-in-water to a water-in-oil droplet structure. The question was clearly formulated by Friberg, but it turned out to be difficult to find an experimental method that was suitable for settling the... 

Having established the existence of oil-in-water droplets at low temperatures, an unbiased bicontinuous structure at the balance point and water-in-oil droplet structure at high temperatures there still remains to establish how these... 

Nanoemulsions do not form spontaneously and so the droplet structure is predominantly a product of the sequence and magnitude of shear stresses used in their formation. These shear stresses have to work against the interfacial... 

PP/PA6 The dispersed phase featured a droplet structure and a fibrous structure near the center line and wall of the channel Wang et al. 2012... 

---