Droplet size surfactant role

It is generally accepted that one of the attractive features of the microemulsion environment for materials synthesis is the stabilization of the produced particles by the microemulsion surfactants. However, in the specific case of alkoxide/ microemulsion systems, there have been no investigations into the manner in which this stabilization is effected. For example, when the particle size exceeds the microemulsion droplet size, are the particles expelled from the water pools, or do the particles rather induce the enlargement of the microemulsion water droplets There have been no investigations into the role of surfactant adsorption in the colloidal... 

In the same work, it is also supposed that colloidal stability, rather than monomer ripening, plays an effective role in determining the final droplet size. Such a conclusion was supported by two different experimental results. First, it was noticed that droplet size increases right after the emulsification process stops, and a stable situation is typically achieved after just a few hours. However, if surfactant is added immediately after, this growth in size does not occur. Second, it is shown that there is a clear correlation between final droplet size and amount of oil phase used in the recipe. In particular, when the oil fraction in the system increases, droplet size also increases. 

There have been many attempts to explain the bell-shaped curve of enzyme activity versus Wo. It is likely that several factors contribute and that the relative importance of different parameters varies with the type of enzyme studied [40,41]. However, it seems probable that diffusion effects play a major role, and a diffusion model applicable to a hydrophilic enzyme located in the core of the water droplet and hydrophilic substrates also situated in the droplets was worked out by Walde and coworkers [42,43]. Before the enzyme-catalyzed reaction can take place, two different diffusion processes must occur. In the first of these, an interdroplet diffusion step, drops containing the substrate and drops containing the enzyme must collide. In the second process, an intradroplet diffusion step, the substrate reaches the enzyme s active site. Whereas the rate of the first process increases with droplet radius, the reverse is true for the second process. These two counteracting dependencies of reaction rate on droplet size (and thus on Wo at constant surfactant concentration) may lead to a bell-shaped activity versus Wo curve. 

In a spraying process, a liquid is forced through an orifice (the spray nozzle) to form droplets by the application of hydrostatic pressure. The effect of surfactants and/or polymers on the droplet size spectrum of a spray can be described in terms of their effects on the surface tension. Since surfactants lower the surface tension of the liquid, one would expect that their presence in the spray solution would result in the formation of smaller droplets. However, when considering the role of surfactants in droplet formation, one should consider the dynamics of surfactant adsorption at the air/liquid interface. In a spraying process, a fresh liquid surface is continuously being formed. The surface tension of this... 

ILs can be incorporated in the microemulsion formulation as substituents of the polar and nonpolar phases or as surfactants. In recent studies, Koetz and coworkers [103] showed that the role of cosurfactant can also be assumed by an IL in the stabilization of water-in-oil microemulsions. SDS- and CTAB-based water-in-toluene/ pentanol microemulsions have been formulated with the aid of ethyl-methylimidazolium hexylsulfate, [C2mim][CgSOJ. Their experimental results showed that replacing water by the IL increases the isotropic phase region of the system. The authors assume the formation of a palisade layer (Scheme 13.4), where the IL plays a similar role like a cosmfactant, changes the spontaneous curvature of the interfacial film, and decreases the droplet size. 

The role of surfactant in emulsion formation is crucial and is described in detail in Chapter 6. It reduces the oil-water interfacial tension, yow adsorption at the interface. The droplet size R is directly proportional to yow It enhances deformation and break-up of the droplets by reducing the Laplace pressure p,... 

To break up a drop into smaller ones, it must be strongly deformed and this deformation increases p [87]. Surfactants play major roles in the formation of emulsions By lowering the interfacial tension, p is reduced and hence the stress needed to break up a drop is reduced (2,3) and surfactants prevent coalescence of newly formed drops. To describe emulsion formation one has to consider two main factors hydrodynamics and interfacial science. To assess emulsion formation, one usually measures the droplet size distribution using for example laser diffraction techniques. A useful average diameter d is. 

In ordinary batch macroemulsion polymerization reactions, monomer macroemulsion droplets (tf = 1 -10 p.m) are in equilibrium with excess surfactant in the form of micelles d 0.01 xm). The emulsion polymerization reaction can be divided into three intervals (Figure 9.14). In Interval I, nucleation of particles takes place by invasion of radicals from the aqueous phase into micelles or by precipitation of oligomer particles in the aqueous phase outside the particles. Macroemulsion droplets play little role in Interval I owing to the fact that they are of a large size... 

The basic role of the interfacial film between the two phases has already been pointed out. It is clear, then, that the surfactant(s) employed should produce as strong an interfacial film as possible, one with high viscosity and tenacity, consistent with their ability to produce the required droplet size under the conditions of emulsification. It is useful, therefore, to choose a surfactant system with maximum lateral interaction among the surfactant molecules concurrent with efficient and effective lowering of the interfacial tension. 

The reverse microemulsion method can be used to manipulate the size of silica nanoparticles [25]. It was found that the concentration of alkoxide (TEOS) slightly affects the size of silica nanoparticles. The majority of excess TEOS remained unhydrolyzed, and did not participate in the polycondensation. The amount of basic catalyst, ammonia, is an important factor for controlling the size of nanoparticles. When the concentration of ammonium hydroxide increased from 0.5 (wt%) to 2.0%, the size of silica nanoparticles decreased from 82 to 50 nm. Most importantly, in a reverse microemulsion, the formation of silica nanoparticles is limited by the size of micelles. The sizes of micelles are related to the water to surfactant molar ratio. Therefore, this ratio plays an important role for manipulation of the size of nanoparticles. In a Triton X-100/n-hexanol/cyclohexane/water microemulsion, the sizes of obtained silica nanoparticles increased from 69 to 178 nm, as the water to Triton X-100 molar ratio decreased from 15 to 5. The cosurfactant, n-hexanol, slightly influences the curvature of the radius of the water droplets in the micelles, and the molar ratio of the cosurfactant to surfactant faintly affects the size of nanoparticles as well. 

The size of the monomer droplets plays the key role in determining the locus of particle nucleation in emulsion and miniemulsion polymerizations. The competitive position of monomer droplets for capture of free radicals during miniemulsion polymerization is enhanced by both the increase in total droplet surface area and the decrease in the available surfactant for micelle formation or stabilization of precursors in homogeneous nucleation. 

In practice, the emulsions are formed in the presence of surfactants. At concentrations above the critical micellization concentration (CMC) the swollen micelles can serve as carriers of oil between the emulsion droplets of different size. In other words, surfactant micelles can play the role of mediators of the Ostwald ripening. Micelle-mediated Ostwald ripening has been observed in solutions of nonionic surfactants. In contrast, it was found that the micelles do not mediate the Ostwald ripening in undecane-in-water emulsions at the presence of an ionic surfactant (SDS). It seems that the surface charge due to the adsorption of ionic surfactant (and the resulting double layer repulsion) prevents the contact of micelles with the oil drops, which is a necessary condition for micelle-mediated Ostwald ripening. 

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