Stabilization supercritical microemulsions

Supercritical Microemulsions Used as Templates for the Stabilization of Metal Nanoparticles... 

The interfacial tension is a key property for describing the formation of emulsions and microemulsions (Aveyard et al., 1990), including those in supercritical fluids (da Rocha et al., 1999), as shown in Figure 8.3, where the v-axis represents a variety of formulation variables. A minimum in y is observed at the phase inversion point where the system is balanced with respect to the partitioning of the surfactant between the phases. Here, a middle-phase emulsion is present in equilibrium with excess C02-rich (top) and aqueous-rich (bottom) phases. Upon changing any of the formulation variables away from this point—for example, the hydrophilie/C02-philic balance (HCB) in the surfactant structure—the surfactant will migrate toward one of the phases. This phase usually becomes the external phase, according to the Bancroft rule. For example, a surfactant with a low HCB, such as PFPE COO NH4+ (2500 g/mol), favors the upper C02 phase and forms w/c microemulsions with an excess water phase. Likewise, a shift in formulation variable to the left would drive the surfactant toward water to form a c/w emulsion. Studies of y versus HCB for block copolymers of propylene oxide, and ethylene oxide, and polydimethylsiloxane (PDMS) and ethylene oxide, have been used to understand microemulsion and emulsion formation, curvature, and stability (da Rocha et al., 1999). 

As has been mentioned, the phase stability of these microemulsions is dependent upon the fluid density. The continuous phase solvent must have a sufiSciently high dielectric constant to be able to solvate these nanometer-sized droplets. In near-critical and supercritical solvents having low dielectric constants, we observe strong attractive interactions between the droplets giving rise to a limited size of droplet that can be dispersed. Likewise, the magnitude of the predicted van der Waals type of attractive interactions rises sharply as the dielectric constant of the continuous phase is reduced below a region bounded by supercritical and near-critical... 

Reverse Mieelles. Reverse Micelles in supercritical fluids are currently being studied for several distinct applications (15-18). Normal micelles and microemulsions in aqueous solutions are known to be capable of increasing solution viscosity in several applications including the surfactant flooding of petroleum reservoirs.(19) If reverse micelles or microemulsions can be formed in C02> an increase in solution viscosity could possibly occur. The surfactants chosen as candidates for CO2 flooding application should be characterized by low water solubility and a strong CO2 solubilityi minimal adsorption onto the porous media and stability at reservoir conditions. (20)... 

Harrison et al. reported the first w/c microemulsion in 1994 (20). A hybrid surfactant, namely F7H7, made of respectively one hydrocarbon and one fluorocarbon chain attached onto the same sulfate head group, was able to stabilize a w/c microemulsion at 35 and 262 bar. For a surfactant concentration of 1.9 wt %, water up to a w = 32 value ([water]/surfactant]) could be dispersed. A spherical micellar structure was confirmed by small-angle neutron scattering (SANS) experiments (21). This surfactant was later the subject of dynamic molecular simulations (22, 23). The calculations were consistent with the SANS data and high diffusivity was predicted, highlighting this important feature of low-density and low-viscosity supercritical fluids (SCF). 

Fluorinated surfactants can also serve as miceUar stabilizers for nanoparticles in water-in-supercritical CO2 (SCCO2) microemulsions. Recently, Tsang described Ru nanoparticles as catalysts in the presence of ammonium perfluorotetradecano-... 

Beckman et al. observed an effect of the secondary microemulsion structure on the molecular weight and yield of the polymer. Under conditions where extensive micelle-micelle clustering occurred, at lower fluid density the molecular weight of the polymer was as much as two times higher. Thus, the density of the supercritical phase could be used to control the polymer morphology. Beckman and Smith also completed an extensive study [74] of the effect that acrylamide, surfactant, and water concentrations as well as the pressure and temperature had on the phase stability of the microemulsions. The phase behavior of these systems depends on the choice of operating parameters, and this behavior can be exploited to optimize the properties of the polymer. 

A major new development in a related area is the work of DeSimone et al. [26,31,50,51,75,76], who conducted dispersion polymerizations in supercritical CO2. In the early stages of the dispersion-polymerization reaction, the solutions are homogenous microemulsions containing surface-active polymers with C02-philic moieties. The monomer is soluble in the continuous phase. As the polymer grows, its solubility rapidly diminishes to form precipitated polymer particles that are stabilized by the surface-active polymer. This approach has been expanded to several different polymer systems [50]. 

The use of water-in-C02 microemulsions in particle synthesis has been extended recently to palladium [425]. An aqueous solution of PdCl2 was dispersed in supercritical CO2, and the surfactants NaAOT and (about twice its concentration of) perfluoropolyether phosphate (PFPE-PO4) were used for microemulsion stabilization. As in other similar cases, the formation of microemulsion and particle synthesis took place within a high pressure cell (at 200 atm). Reduction of palladium was caused by hydrogen gas (unlike in other similar investigations) because of its... 

The unique density dependence of fluid properties makes supercritical fluids attractive as solvents for colloids including microemulsions, emulsions, and latexes, as discussed in recent reviews[l-4]. The first generation of research involving colloids in supercritical fluids addressed water-in-alkane microemulsions, for fluids such as ethane and propane[2, 5]. The effect of pressure on the droplet size, interdroplet interactions[2] and partitioning of the surfactant between phases was determined experimentally[5] and with a lattice fluid self-consistent field theory[6]. The theory was also used to understand how grafted chains provide steric stabilization of emulsions and latexes. 

Figure 10 A high-pressure system for evaluating stability of water-in-C02 microemulsions in supercritical CO2 used in the author s laboratory. (1) High-pressure view cell (2) video camera (3) temperature controller (4) stirrer (5) hand-operated syringe pump (6) pressure transducer (7) high pressure valve (8) ISCO syringe pump (9) collection vessel.

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