Reverse micelle-fluid phases

Fig. 12.5 Temperature-composition phase diagram of the monoolein/water system (up to 50 wt% water). A cartoon representation of the various phase states is included in which colored zones represent water. The mesophases are as follows Lc-crystalline lamellar, La-lamellar, la3d-gyroid inverted bicontinuous cubic, Pn3m-primitive inverted bicontinuous cubic, Hn-inverted hexagonal, and Fl-reverse micelles fluid phases (Taken from [8])...

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. 

The surfactant AOT forms reverse micelles in non-polar fluids without addition of a cosurfactant, and thus it is possible to study simple, water/AOT/oil, three component systems. To determine micelle structure and behavior in water/AOT/oil systems, investigators have studied a wide range of properties including conductivity (15), light (JL ), and neutron (12) scattering, as well as solution phase behavior (1 ). From information of this type one can begin to build both microscopic models and thermodynamic... 

In the studies described here, we examine in more detail the properties of these surfactant aggregates solubilized in supercritical ethane and propane. We present the results of solubility measurements of AOT in pure ethane and propane and of conductance and density measurements of supercritical fluid reverse micelle solutions. The effect of temperature and pressure on phase behavior of ternary mixtures consisting of AOT/water/supercritical ethane or propane are also examined. We report that the phase behavior of these systems is dependent on fluid pressure in contrast to liquid systems where similar changes in pressure have little or no effect. We have focused our attention on the reverse micelle region where mixtures containing 80 to 100% by weight alkane were examined. The new evidence supports and extends our initial findings related to reverse micelle structures in supercritical fluids. We report properties of these systems which may be important in the field of enhanced oil recovery. 

The phase boundary lines for supercritical ethane at 250 and 350 bar are shown in Figure 2. The surfactant was found to be only slightly soluble in ethane below 200 bar at 37 C, so that the ternary phase behavior was studied at higher pressures where the AOT/ethane binary system is a single phase. As pressure is increased, more water is solubilized in the micelle core and larger micelles can exist in the supercritical fluid continuous phase. The maximum amount of water solubilized in the supercritical ethane-reverse micelle phase is relatively low, reaching a W value of 4 at 350 bar. 

The existence of a reverse micelle phase in supercritical fluids has been confirmed from solubility, conductivity and density measurements. The picture of the aggregate structure in fluids is one of a typical reverse micelle structure surrounded by a shell of liquid-like ethane, with this larger aggregate structure dispersed in a supercritical fluid continuous phase. 

The reverse micelle phase behavior in supercritical fluids is markedly different than in liquids. By increasing fluid pressure, the maximum amount of solubilized water increases, indicating that these higher molecular weight structures are better solvated by the denser fluid phase. The phase behavior of these systems is in part due to packing constraints of the surfactant molecules and the solubility of large micellar aggregates in the supercritical fluid phase. 

In this chapter, the recent progress in the understanding of the nature and dynamics of excess (solvated) electrons in molecular fluids composed of polar molecules with no electron affinity (EA), such as liquid water (hydrated electron, and aliphatic alcohols, is examined. Our group has recently reviewed the literature on solvated electron in liquefied ammonia and saturated hydrocarbons and we refer the reader to these publications for an introduction to the excess electron states in such liquids. We narrowed this review to bulk neat liquids and (to a much lesser degree) large water anion clusters in the gas phase that serve as useful reference systems for solvated electrons in the bulk. The excess electrons trapped by supramolecular structures (including single macrocycle molecules ), such as clusters of polar molecules and water pools of reverse micelles in nonpolar liquids and complexes of the electrons with cations in concentrated salt solutions, are examined elsewhere. 

Finally, in the discussion of reverse microemulsion systems, mention should be made of one of the most widely studied systems. The surfactant, sodium bis(2-ethylhexyl) sulfosuccinate or Aerosol-OT (AOT), is one of the most thoroughly studied reverse micelleforming surfactants since it readily forms reverse micelle and microemulsion phases in a multitude of different solvents without the addition of cosurfactants or other solvent modifiers. The phase behavior of AOT in liquid alkane/water systems is already well documented. Indeed, the first report of the existence of the formation of microemulsions in a supercritical fluid involved an AOT/alkane/ water system. A The spherical structure of an AOT/nonpolar-fluid/ water microemulsion droplet is shown in Fig. 1. In the now well-known structure, it can be seen that the two hydrocarbon tails of each AOT molecule point outward into the nonpolar phase (e g., supercritical fluid). These tails are lipophilic and are solvated by the nonpolar continuous phase solvent whereas the hydrophilic head groups are always positioned in the aqueous core. 

Shield et al. (22) have demonstrated that reverse micelles can be used in organic solvents to recover proteins selectively from aqueous solutions. Protein denaturation can occur, however, during recovery from the organic phase, which requires changes in pH or ionic strength. Supercritical fluid solvents offer the potential advantage that proteins could be recovered simply by changing the pressure. Additional potential applications of surfactants in supercritical fluids... 

Structure of Reverse Micelle and Microemulsion Phases in Near-Critical and Supercritical Fluid as Determined from Dynamic Light-Scattering Studies... 

In this article we describe the phase behavior of a microemulsion system chosen for the free radical polymerization of acrylamide within near-critical and supercritical alkane continuous phases. The effects of pressure, temperature, and composition on the phase behavior all influence the choice of operating parameters for the polymerization. These results not only provide a basis for subsequent polymerization studies, but also provide data on the properties of reverse micelles formed in supercritical fluids from nonionic surfactants. 

As shown in Figure 3.9, the L2 phase is able to solubilize a very large amount of a hydrocarbon such as decane or hexadecane. In fact, a composition containing up to 75% decane and water/surfactant/cosurfactant proportions corresponding to the L2 phase is still clear, fluid and isotropic, forms spontaneously, and is thermodynamically stable. The structure of this microemulsion can be (to some extent) regarded as a dispersion of tiny water droplets (reverse micelles) in a continuous phase of the hydrocarbon. The surfactant and cosurfactant are mainly located at the water/oil interface. This type of system is often referred to as a w/o microemulsion. 

Reverse micelles also form in supercritical fluids, as evidenced by changes in the fluorescence and absorption spectra of probes, in the two phase region -... 

It should be noted that LLC phases are different from the ubiquitous individual, phase-separated aggregate structures commonly formed by amphiphilic molecules or surfactants, such as micelles, reverse micelles, vesicles, and lipid microtubules. These discrete aggregate structures formed from amphiphiles lack periodic order, and are not condensed-phase materials—two defining characteristics of LLC phases. For the purposes of this review, LLC phases will he defined as fluid, condensed-phase materials composed of amphiphilic molecules that have periodic order and are formed via phase separation of the amphiphiles around an added solvent as a secondary component (i.e., mixtures). Consequently, functional normal micelle, reverse micelle,... 

Reverse micelles and microemulsions formed in supercritical CO2 allow highly polar conq>ounds and electrolytes to be dispersed in the non-polar fluid phase. Searching for C02-soluble s actants that would form stable water-in-CO2 microemulsions started a decade ago. A review of the design and performance of surfactants for making stable water-in-C02 microemulsions with all relevant references is presented by Eastoe in Chiq>ter 19. 

There has been much interest in recent years to exploit the properties of microemulsion phases in supercritical fluids (23-33). A reverse micelle or microemulsion system of particular interest is one based on CO2 because of its minimum environmental impact in chemical applications. Since water and CO2... 

The reverse micelles get transformed into a liquid crystalline phase or vesicle dispersion, when it comes in contact with the aqueous body fluids. This reduces the rate of release of the solubilized drugs. ... 

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