Surfactants and micro-emulsions

Most highly polar and ionic species are not amenable to processing with desirable solvents such as carbon dioxide or any other solvent such as water that has a higher critical temperature well above the decomposition temperature of many solutes. In such instances, the combination of the unique properties of supercritical fluids with those of micro-emulsions can be used to increase the range of applications of supercritical fluids. The resulting thermodynamically stable systems generally contain water, a surfactant and a supercritical fluid (as opposed to a non-polar liquid in liquid micro-emulsions). The possible supercritical fluids that could be used in these systems include carbon dioxide, ethylene, ethane, propane, propylene, n-butane, and n-pentane while many ionic and non-ionic surfactants can be used. The major difference between the liquid based emulsions and the supercritical ones is the effect of pressure. The pressure affects the miscibility gaps as well as the microstracture of the micro-emulsion phase. 

The incorporation of the micro-emulsion phase creates interesting potential advantages for reactions as well as separations. Isolation of components from fermentation broths and garment cleaning appear to be two of the more competitive applications of these systems. 

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Parvinzadeh, M. and Hajiraissi, R. (2008) Effect of nano and micro emulsion silicone softeners on properties of polyester fibers. Tenside Surfactants Detergents, 45 (5), 254-257. 

C02-philic molecules have been utilized for the design of metal-mobilizing ligands to be used in SCCO2 [67-69,135-137], e.g., as shown in Fig. 7a [55] and for the synthesis of surfactants that form micelles, emulsions, and micro emulsions in CO2, e.g., as shown in Fig. 7b. [70] Polymer solubility in SCCO2 has been studied [71] and utilized for polymer synthesis [72-74]. Recently, DeSimone and co-workers synthesized high-molar-mass fluoropolymers in SCCO2, and studied the polymerization kinetics [75]. 

This paper reviews various aspects of macro- and micro-emulsions. The role of interfacial film of surfactants in the formation of these systems has been high-lighted. 

Recently a new field, mesoscopic physics, has emerged. It is interesting to understand the physical properties of systems that are not as small as a single atom, but small enough that the properties can be dramatically different from those in a larger assembly. All these new mesoscopic phenomena can easily be observed in the dielectric properties of colloid systems. Their properties strictly depend on the dimensional scale and the time scale of observation. Self-assembling systems such as micellar surfactant solutions, micro emulsions, emulsions, aqueous solutions of biopolymers, and cell and lidposome suspensions all to-... 

The emulsion copolymerization of VAc/BuA monomer system can be performed by other emulsion pol3mrierization methods, mini-emulsion and micro-emulsion polymerizations. When the mini-emulsion copol5nnerization carries out with the 50/50 molar ratio of VAc/BuA in the presence of sodium hexadecyl sulfate emulsifier, hexadecane co-surfactant and ammonium persulfate initiator, the results of this polymerization conducted in a batch process are different from that of conventional batch polymerization of this monomer couple [94]. For the mini-emulsion polymerization the polymerization rate is slowed done. 

A side-by-side comparison of coarse and micro- emulsions for use as liquid membranes indicates that each possesses unique advantages. The microemulsion displays faster rates of separation yet more difficult demulsification than the coarse or macro- emulsion system. Both systems suffer from swell. A new method of contacting emulsion liquid membranes with the feed solution minimizes swell while maintaining high separation flux. The key advantage of the HFC contactor lies in its ability to stabilize the liquid membrane from leal ge. Thus, surfactant concentration can be minimized and swell essentially eliminated. The system is much like a supported liquid membrane but will not produce short circuits due to solvent loss since the solvent is continuously supplied on the emulsion side of the membrane. Our lab is currently characterizing such systems. 

Uses O/w emulsifier, gellant for surfactants, prep, of clear min. oil gels and micro-emulsion gels, hair telaxers, hair styling prods., creams/lotlons, shampoos, male grooming prods. 

Another important interaction that needs to be considered is the hydrophobic interaction. This can be most easily thought of in terms of two immiscible liquids such as oil and water being induced to mix by adding surfactants, to form (micro) emulsions. The exact structure of the phase formed depends heavily on the relative compositions of the various phases and the structure of the surfactant (see Figure 6.4). 

Silica particles synthesized in nonionic w/o microemulsions (e.g., poly-oxythylene alkyl phenyl ether/alkane/water) typically have a narrow size distribution with the average value between 25 and 75 nm [54,55]. Both water and surfactant are necessary components for the formation of stable silica suspensions in microemulsions. The amounts of each phase present in the micro emulsion system has an influence on the resulting size of the silica nanoparticle. The role of residual water (that is the water that is present in the interface between the silica particle and the surfactant) is considered important in providing stability to the silica nanoparticle in the oil... 

Many reports are available where the cationic surfactant CTAB has been used to prepare gold nanoparticles [127-129]. Giustini et al. [130] have characterized the quaternary w/o micro emulsion of CTAB/n-pentanol/ n-hexane/water. Some salient features of CTAB/co-surfactant/alkane/water system are (1) formation of nearly spherical droplets in the L2 region (a liquid isotropic phase formed by disconnected aqueous domains dispersed in a continuous organic bulk) stabilized by a surfactant/co-surfactant interfacial film. (2) With an increase in water content, L2 is followed up to the water solubilization failure, without any transition to bicontinuous structure, and (3) at low Wo, the droplet radius is smaller than R° (spontaneous radius of curvature of the interfacial film) but when the droplet radius tends to become larger than R° (i.e., increasing Wo), the microemulsion phase separates into a Winsor II system. 

An alternative to the injection method for importing enzymes into a microemulsion is the phase transfer method. In this method, a layer of an aqueous enzyme solution is located under a mixture of surfactant and oil. Upon gentle shaking, the enzyme is transferred into the reverse micelles of the hydrocarbon phase. Finally, the excess of water is removed and the hydrophobic substrates can be added. The main advantage of this method is that it ensures thermodynamically stable micro emulsions with maximum water concentrations. However, the method is very time consuming. The method is often applied in order to purify, concentrate or renaturate enzymes in the reverse micellar extraction process [54-58]. 

The determination of the enzyme activity as a function of the composition of the reaction medium is very important in order to find the optimal reaction conditions of an enzyme catalysed synthesis. In case of lipases, the hydrolysis of p-nitrophenyl esters in w/o-microemulsions is often used as a model reaction [19, 20]. The auto-hydrolysis of these esters in w/o-microemulsions is negligible. Because of the microstructure of the reaction media itself and the changing solvent properties of the water within the reverse micelles, the absorbance maximum of the p-nitrophenol varies in the microemulsion from that in bulk water, a fact that has to be considered [82]. Because of this, the water- and surfactant concentrations of the applied micro emulsions have to be well adjusted. 

There is a considerable patent art concerning preparation of transparent mixtures of water with low molecular weight silicone oils using polymeric silicone surfactants. Some representative early references are Keil [47], Gee [48, 49], Gum [50] and Terae [51]. These compositions are called micro emulsions in the patents in the sense of being transparent mixtures of water, surfactant and oil - but note that they are transparent because of small particle size or because of index of refraction matching. 

While microemulsions are thermodynamically stable, and the stability of emulsions has a kinetic origin, in both cases the adsorption of the dispersant upon the interface of the globules is responsible for stability. For this reason it appears natural to attempt to explain the above equality between the two inversion temperatures on the basis of surfactant adsorption. In addition, both the micro and macro-emulsions obey in many cases the Bancroft rule [8,9], which indicates that the phase in which a larger amount of dispersant is present becomes the continuous phase there are, however, some violations of this rule which will be discussed later in the paper. 

Let us now represent a micro emulsion as a dispersion of globules of water in oil or of oil in water, with the surfactant and cosurfactant distributed at equilibrium among the dispersed and continuous media of the microemulsion and their interface. Assuming the globules to be spherical and of uniform radius, one can write the following expression... 

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