Propane microemulsion

The SANS method has also been applied to microemulsions formed by using cationic and nonionic surfactants in near-critical propane [47]. A propane microemulsion formed with the cationic surfactant didodecyldimethylammonium bromide (DDAB) exhibits a percolation of the electrical conductivity as the pressure is reduced toward the phase boundary [48]. 

DDAB-water (IT =24)-propane microemulsion. The reduced rate constant (i propane viscosity is also shown. (From Ref. 43.)... 

Figure 11 A scanning electron micrograph of Al(OH)3 particles formed in a supercritical propane microemulsion at llO C and 200 bar using the surfactant AOTat IV=5. (From Ref 67.)...

Another promising application of supercritical microemulsions is in the broad area of polymerization reactions. An example of one such system is in the work by Beckman et al. [41,74]. In this study, acrylamide monomer dissolved in the surfactant interfacial region of a supercritical ethane-propane microemulsion was catalyzed by azobis(isobutylnitrile) (AIBN) dissolved in the reverse micelle core to produce the micelle-soluble polyacrylamide. 

Tingey et al. [128] reported an "antipercolation" feature of a water/dodecyldi-methylammonium bromide/supercritical propane microemulsion in the pressure range of 80-400 bar the conductance decreased by three orders of magnitude. The interconnected channels of the studied bicontinuous system were anticipated to break down into dispersed droplets. 

The locations of the tietriangle and biaodal curves ia the phase diagram depead oa the molecular stmctures of the amphiphile and oil, on the concentration of cosurfactant and/or electrolyte if either of these components is added, and on the temperature (and, especially for compressible oils such as propane or carbon dioxide, on the pressure (29,30)). Unfortunately for the laboratory worker, only by measuriag (or correcdy estimatiag) the compositions of T, Af, and B can one be certain whether a certain pair of Hquid layers are a microemulsion and conjugate aqueous phase, a microemulsion and oleic phase, or simply a pair of aqueous and oleic phases. 

The first generation of research involving surfactants in SCFs addressed water/oil (w/o) microemulsions (Fulton and Smith, 1988 Johnston et al., 1989) and polymer latexes (Everett and Stageman, 1978) in ethane and propane (Bartscherer et al., 1995 Fulton, 1999 McFann and Johnston, 1999). This work provided a foundation for studies in C02, which has modestly weaker van der Waals forces (polarizability per volume) than ethane. Consequently, polymers with low cohesive energy densities and thus low surface tensions are the most soluble in C02 for example, fluor-oacrylates (DeSimone et al., 1992), fluorocarbons, fluoroethers (Singley et al., 1997), siloxanes, and to a lesser extent propylene oxide. Since C02 is... 

Reverse micelle and microemulsion solutions are mixtures of a surfactant, a nonpolar fluid and a polar solvent (typically water) which contain organized surfactant assemblies. The properties of a micelle phase in supercritical propane and ethane have been characterized by conductivity, density, and solubility measurements. The phase behavior of surfactant-supercritical fluid solutions is shown to be dependent on pressure, in contrast to liquid systems where pressure has little or no effect. Potential applications of this new class of solvents are discussed. 

Most of the early work involving microemulsions in supercritical fluids utilized the supercritical alkanes, ethane and propane, with the surfactant AOT. Table 1 gives a summary of the surfactant systems that have been studied in supercritical hydrocarbon solvents. More recently, there has been some success with the formation of... 

Figure 2. Phase diagram of an. 4077water/supercritical propane system in the oil-rich comer of the temaiy phase diagram. The regions to the right of the solid lines are the one-phase, clear microemulsions. The various IV value lines ([H2O]/ [AOT]) are also indicated. The temperature of the system is 103°C. (Compositions are given in weight percent.)...

The rate at which the microemulsion droplets exchange the contents of their aqueous cores is another important transport variable. Exchange kinetics have been measured using fluorescence techniques. Dynamic fluorescence quenching has also been used to study micelle size. f The technique is, however, more useful as a tool to explore the exchange kinetics between microemulsion droplets. The systems studied were AOT or didodecyldimethyl ammonium bromide and water in propane up to 100°C and... 

Several years ago we reported initial observations of reverse micelles and microemulsions in supercritical fluid solvents (JL) These studies suggested the possibility of creating a previously unsuspected broad range of organized molecular assemblies in dense gas solvents. Such systems are of interest due to potential applications which exploit the readily variable properties of supercritical fluids as well as the unique solvent environments of reverse micelles and microemulsions. These initial studies showed that even gram quantities of proteins, such as Cytochrome-c (Mwt. 12,842 dalton) could be solvated in a liter of supercritical ethane or propane due to the microemulsion solvent environment, something which is not achievable with "conventional"... 

In this paper we use dynamic light scattering (DLS) methods to examine micelle size and clustering in (1) supercritical xenon, (2) near-critical and supercritical ethane, (3) near-critical propane as well as (4) the larger liquid alkanes. Reverse micelle or microemulsion phases formed in a continuous phase of nonatomic molecules (xenon) are particularly significant from a fundamental viewpoint since both theoretical and certain spectroscopic studies of such systems should be more readily tractable. Diffusion coefficients obtained by DLS for AOT microemulsions for alkanes from ethane up to decane are presented and discussed. It is shown that micelle phases exist in equilibrium with an aqueous-rich liquid phase, and that the apparent hydrodynamic size, in such systems is highly pressure dependent. 

Although water allows for greater uptake of acrylamide by the microemulsion, water alone ([acrylamide] 0) will not produce a one-phase system with the Brij 52/30 blend in an ethane/propane continuous phase. As postulated earlier, acrylamide is a cosurfactant with the B52/B30 blend, as evidenced by the results in Figure 5. The existence of the maximiun in the allowable water as a function of [AM] has been observed in other micelle systems where acrylamide behaves as a co-surfactant ). When more than the maximum allowable water level is added at a particular acrylamide content, the system becomes turbid, followed by the appearance of what appears to be a solid second phase. That acrylamide behaves as a co-surfactant is possibly due to its... 

Effect of Pressure on Apparent Micelle Size. The pressure-induced color changes that are observed in the one-phase region of the Brij-based microemulsion systems suggest that the size of the organized assemblies in the ethane/propane mixtures is changing as the pressure is changed. Initial considerations of dynamic light... 

Investigation of the phase behavior of the Brij-based microemulsion system in ethane/propane mixtures defines the operating conditions for the polymerization process and provides evidence of formation of stable microemulsions in supercritical... 

First, it is apparent that the density of the ethane/propane continuous phase, rather than the molecular coxtqposition, determines the stability of the microemulsion. Stable microemulsions can be prepared in mixtures of ethane and propane over the entire concentration range. This allows examination of the effect of continuous-phase density on reaction rate, etc., while temperature and pressure remain constant. 

The reverse micelles refer to the aggregates of surfactants formed in nonpolar solvents, in which the polar head groups of the surfactants point inward while the hydrocarbon chains project outward into the nonpolar solvent (Fig. 7) [101-126], Their cmc depends on the nonpolar solvent used. The cmc of aerosol-OT (sodium dioctyl sulfosuccinate, AOT) in a hydrocarbon solvent is about 0.1 mM [102]. The AOT reverse micelle is fairly monodisperse with aggregation number around 20 and is spherical with a hydrodynamic radius of 1.5 nm. No salt effect is observed for NaCl concentration up to 0.4 M. Apart from liquid hydrocarbons, recently several microemulsions are reported in supercritical fluids such as ethane, propane, and carbon dioxide [111-113]. 

Despite all difficulties mentioned above, examples for C02 containing microemulsion stabilised by a technical grade non-ionic surfactant have been found (see Fig. 11.3 in Chapter 11). The studied system consists ofwater/NaCl- -propane/C02-Lutensol XL70 with varying amounts of C02 in n-propane/C02 mixtures. All measurements were carried out at p = 220 bar and at equal volume fractions of the two solvents [56], The respective phase diagrams have been studied as a function ofthe temperature T and the total surfactant... 

Sommerville, M.L., Johnson, C.S., Cain, J.B., Rypacek, F. and Hickey, A.J. (2002) Lecithin microemulsions in dimethyl ether and propane for the generation of pharmaceutical aerosols containing polar solutes. Pharm. Dev. Tech., 7, 273-288. 

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