Microemulsion-forming systems

At that time, some large-scale field tests on surfactant and microemulsion technologies had alreadybeen performed [50-55]. In most cases, the applied surfactants or microemul-sion components were selected in laboratory experiments by determining phase behaviour, interfacial tension, solubilisation capacity, viscosity and extracting power in soil columns. 

The expertise with EOR was used for finding suitable microemulsion-forming systems for LNAPL. However, the high polarity of chlorinated hydrocarbons with very low or even negative equivalent alkane carbon numbers (EACN) required novel types of surfactants [56]. The enhanced solubility of surfactants in the oil phase makes most surfactants less effective for solubilisation. DNAPL extraction by mobilisation, however, is problematic owing to the high density of the pollutants, since they may be displaced into deeper soil compartments [57]. This probably happened in at least one field test [58]. 

Mobilisation of NAPL generally leads to the formation of an oil bank (see Chapter 10.3) in front of the surfactant solution. If the solubilisation capacity of the surfactant solution is too low, large amounts of emulsions will be formed, which can clog the pore space. As the flow in columns is forced, these experiments may not correctly reflect the behaviour of the multiphase system under free flowing conditions in a three-dimensional pore space. 

All types of micro emulsions were obtained in salinity scans with mixtures of Aerosol MA (sodium dihexyl sulphosuccinate) and twin-tailed (Guerbet and Exxon type) alcohol ethoxy and propoxy sulphates for perchloroethylene (PCE), carbon tetrachloride, 1,2-dichlorobenzene and trichloroethylene [59] at 25°C. At lower temperatures, however, stable macro emulsions are formed. Chloroform, 1,2-dichloroethane and other chlorinated hydrocarbons were found to be too polar for those anionic surfactants. Extremely hydrophilic and temperature-insensitive surfactants are necessary for effective solubilisation of chlorinated hydrocarbons yielding Winsor III systems. N-methyl-N-D-glucalkaneamide surfactants showed good performance for DNAPL solubilisation even at 16°C [56]. 

All the systems described thus far require relatively high salinity, which is considered to be critical in soil remediation processes [60]. Another approach for effective microemulsification of organic liquids is the use of co-surfactants. Sodium mono- and dimethyl naphthalene sulphonate were found to be effective co-surfactants in formulations with Aerosol OT (sodium bis(2-ethylhexyl) sulphosuccinate) for diverse chlorinated hydrocarbons and their mixtures between 15 and 25°C [60, 61]. All types of microemulsions could be obtained with this approach. 

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It is interesting to note that by replacing octane for pentane both and n° produced minimum at 308 K when aUcanols were butanol, pentanol, and hexanol (data not shown). The value on the other hand exhibited minimum at 308 K for both butanol and hexanol in the case of pentanol, there was a regular increase in K. Digout et al. [42] also studied microemulsion forming systems with nonane and decane (data not shown) as oil, and observed almost similar types of behavior as found for systems with pentane and octane. This simply indicated that chain length of alkanols and hydrocarbon oils have a large say on the overall microemulsion formation and their structures. 

If the system of interest contains additives that are soluble in water, such as salt or polymer, a gradual step-by-step addition of these ingredients also allows one to follow the direction in which the phase diagram moves. Key to the step-by-step approach is starting from a known microemulsion-forming system, and looking at which direction the phase diagram moves upon substitution of oil or aqueous components. 

Cationic surfactants may be used [94] and the effect of salinity and valence of electrolyte on charged systems has been investigated [95-98]. The phospholipid lecithin can also produce microemulsions when combined with an alcohol cosolvent [99]. Microemulsions formed with a double-tailed surfactant such as Aerosol OT (AOT) do not require a cosurfactant for stability (see, for instance. Refs. 100, 101). Morphological hysteresis has been observed in the inversion process and the formation of stable mixtures of microemulsion indicated [102]. 

Microemulsion Polymerization. Polyacrylamide microemulsions are low viscosity, non settling, clear, thermodynamically stable water-in-od emulsions with particle sizes less than about 100 nm (98—100). They were developed to try to overcome the inherent settling problems of the larger particle size, conventional inverse emulsion polyacrylamides. To achieve the smaller microemulsion particle size, increased surfactant levels are required, making this system more expensive than inverse emulsions. Acrylamide microemulsions form spontaneously when the correct combinations and types of oils, surfactants, and aqueous monomer solutions are combined. Consequendy, no homogenization is required. Polymerization of acrylamide microemulsions is conducted similarly to conventional acrylamide inverse emulsions. To date, polyacrylamide microemulsions have not been commercialized, although work has continued in an effort to exploit the unique features of this technology (100). 

This transition may j-.e. reducing the specific surface energy, f. The reduction of f to sufficiently small values was accounted for by Ruckenstein (15) in terms of the so called dilution effect". Accumulation of surfactant and cosurfactant at the interface not only causes significant reduction in the interfacial tension, but also results in reduction of the chemical potential of surfactant and cosurfactant in bulk solution. The latter reduction may exceed the positive free energy caused by the total interfacial tension and hence the overall Ag of the system may become negative. Further analysis by Ruckenstein and Krishnan (16) have showed that micelle formation encountered with water soluble surfactants reduces the dilution effect as a result of the association of the the surfactants molecules. However, if a cosurfactant is added, it can reduce the interfacial tension by further adsorption and introduces a dilution effect. The treatment of Ruckenstein and Krishnan (16) also highlighted the role of interfacial tension in the formation of microemulsions. When the contribution of surfactant and cosurfactant adsorption is taken into account, the entropy of the drops becomes negligible and the interfacial tension does not need to attain ultralow values before stable microemulsions form. 

In contrast to the conventional emulsions or macroemulsions described earlier are the disperse systems currently termeraiicroemulsions. The term was Lrst introduced by Schulman in 1959 to describe a visually transparent or translucent thermodynamically stable system, with much smaller droplet diameter (6-80 nm) than conventional emulsions. In addition to the aqueous phase, oily phase, and surfactant, they have a high proportion of a cosurfactant, such as an alkanol of 4-8 carbons or a nonionic surfactant. Whereas microemulsions have found applications in oral use (as described in the next chapter), parenteral use of microemulsions has been less common owing to toxicity concerns (e.g., hemolysis) arising from the high surfactant and cosolvent levels. In one example, microemulsions composed of PEG/ethanol/water/medium-chain triglycerides/Solutol HS15/soy phosphatidylcholine have been safely infused into rats at up to 0.5 mL/kg. On dilution into water, the microemulsion forms a o/w emulsion of 60-190 nm droplet size (Man Corswant et al., 1998). 

R.P. Scherersol system Liquid formulations for softgels, which incorporates the drug in a microemulsion preconcentrate or microemulsion form. Oral delivery of proteins and peptides enhanced oral bioavailability with reduced inter- and intraindividual variability in pharmacokinetics of certain drugs. Sandimmune Neoral (cyclosporine). 

Mixtures containing 1 wt% of the pure nonionic surfactant C,2E5 in water were contacted with pure n-hexadecane and n-tetradecane at various temperatures between 25 and 60°C using the vertical cell technique. Similar experiments were performed with C,2E4 and n-hexadecane between about 15 and 40°C. In both cases the temperature ranged from well below to well above the phase inversion temperature (PIT) of the system, i.e., the temperature where hydrophilic and lipophilic properties of the surfactant are balanced and a middle phase microemulsion forms (analogous to the optimal salinity for ionic surfactants mentioned above). The different intermediate phases that were seen at different temperatures and the occurrence of spontaneous emulsification in some but not all of the experiments could be understood in terms of known aspects of the phase behavior, e.g., published phase diagrams for the C12E 5-water-n-tetradecane system, and diffusion path theory. That is, plausible diffusion paths could be found that showed the observed intermediate phases and/or spontaneous emulsification for each temperature. 

The choice of suitable surfactants and additional chemicals for the decontamination of source zones largely depends on the type of pollutant and the structure of the soil (mainly on adsorption behaviour and hydraulic conductivity). Adsorbed and solid pollutants or very viscous liquid phases cannot be mobilised. Preformed microemulsions, co-solvents or co-surfactants can be favourably used for such contaminations in order to enhance the solubilisation capacity of surfactants. NAPL with low viscosity can easily be mobilised and also effectively solubilised by microemulsion-forming surfactant systems. Mobilisation is usually much more efficient. It is achieved by reducing the interfacial tension between NAPL and water. Droplets of organic liquids, which are trapped in the pore bodies, can more easily be transported through the pore necks at lower interfacial tension (see Fig. 10.2). The onset of mobilisation is determined by the trapping number, which is dependent on... 

The propane system shown in Fig. 11.3 is clearly subcritical as the critical temperature of propane is about 96°C. An increase of the C02 fraction ((3) in the mixture of C02 and propane shifts the one-phase region (1), i.e. the bicontinuous microemulsion, to lower temperatures. For pure C02 the bicontinuous microemulsion (1) exists around 35°C, which is higher than the Tc = 31°C of C02. In other words, the C02 solubilised in the microemulsion is supercritical Knowing how to tune the phase behaviour of these systems, one can easily shift phase diagrams on the temperature scale by simply choosing an appropriate surfactant. Other tuning parameters are the oil-to-water fraction and the temperature which maybe adjusted such that, e.g. a C02-in-water droplet microemulsion forms. 

When the surfactant was sodium dodecyl sulfate, some butanol had to be present to achieve the microemulsion. A typical run used 2 mmol aryl iodide, 0.5 g surfactant, 1 mL butanol, 10 mL water, and 2 mmol base. The butanol was unnecessary with nonionic surfactants, such as the one derived from 1-dodecanol with 23 eq of ethylene oxide. The advantages of such a system include the following (a) No organic solvent is needed. The substrate is the oil phase, (b) The microemulsions form without the need for vigorous agitation, (c) No excess base is needed, in contrast with some reactions in which phase-transfer catalysis is used, (d) The surfactants can be recovered and recycled. They are inexpensive and biodegradable. 

The quantity of insoluble substance which can be solubilised in micelles depends to a considerable extent on the chemical structure of the surfactant and is influenced by the presence of other components, which may influence either the micelle formation concentration (CMC) or the micelle geometry (aggregation number, shape). The transition from solubilisation to another important phenomenon, the formation of a micro-emulsions, is continuous. Microemulsions form spontaneously, whereas typical solubilisation systems attain their equilibrium state often only after extreme long periods of intensive mixing of both phases. 

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). 

As a general description, a microemulsion is a homogeneous phase that contains a substantial fraction of both oil and water, their mixing being induced by the dissolved surfactant. In contrast with macroemulsions, microemulsion systems form spontaneously in solution. As with micelles, microemulsions are thermodynamically stable. As a result, in contrast to macroemulsions, microemulsion formation is reversible. For example, if changes in a system parameter (e.g., temperature) alter the microemulsion system, when the system parameter returns to its original state, so does the microemulsion system. In contrast to macroemulsions, a microemulsion forms virtually independent of the volume of the oil phase, but once formed, the microemulsion enhances the aqueous solubility of the oil phase. Most often, an excess oil phase persists in the presence of a microemulsion phase—the oil and water phases do not completely mix. 

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