Pressure-temperature microemulsion

High-pressure FT-IR spectroscopy has been used to clarify (1) the rotational isomerism of molecules, (2) characteristics of water and the water-head group, and (3) RSO3 Na4- interactions in reverse micellar aggregates in supercritical ethane. This work demonstrates interesting pressure, temperature, and salt effects on an enzyme-catalyzed esterification and/or maintenance of a one-phase microemulsion in supercritical fluids from practical and theoretical points of view (Ikushima, 1997). 

For a pure supercritical fluid, the relationships between pressure, temperature and density are easily estimated (except very near the critical point) with reasonable precision from equations of state and conform quite closely to that given in Figure 1. The phase behavior of binary fluid systems is highly varied and much more complex than in single-component systems and has been well-described for selected binary systems (see, for example, reference 13 and references therein). A detailed discussion of the different types of binary fluid mixtures and the phase behavior of these systems can be found elsewhere (X2). Cubic ecjuations of state have been used successfully to describe the properties and phase behavior of multicomponent systems, particularly fot hydrocarbon mixtures (14.) The use of conventional ecjuations of state to describe properties of surfactant-supercritical fluid mixtures is not appropriate since they do not account for the formation of aggregates (the micellar pseudophase) or their solubilization in a supercritical fluid phase. A complete thermodynamic description of micelle and microemulsion formation in liquids remains a challenging problem, and no attempts have been made to extend these models to supercritical fluid phases. 

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. 

Holmes et al. reported the first enzyme catalyzed reactions in water-in-CO2 microemulsions (67). Two reactions, a lipase-catalyzed hydrolysis and a lipoxygenase-catalyzed peroxidation, were demonstrated in water-in-C02 microemulsions using the surfactant di(l/7,l/7,5/7-octafluoro- -pentyl) sodium sulfosuccinate (di-HCF4). A major concern of enzymatic reactions in CO2 is the pH of the aqueous phase, which is approximately 3 when there is contact with CO2 at elevated pressures. Holmes et al. examined the ability of various buffers to maintain the pH of the aqueous solution in contact with CO2. The biological buffer 2-(A-morpholino)ethanesulfonic acid sodium salt (MES) was the most effective, able to maintain a pH of 5, depending on the pressure, temperature, and buffer concentration. The activity of the enzymes in the water-in-C02 microemulsions was comparable to that in a water-in-heptane microemulsion stabilized by the surfactant AOT, which contains the same head group as di-HCF4. 

Figure 5. Pressure-temperature phase diagram ofW/C02 microemulsion stabilized by di-HCF4 with different additives Rh(CO) cac = 0.01 mmol Rh/TPPTS = 4, syngas = di-HCF4 = J.tfg, 7w/ 0.2Maqueous solution...

Amphiphiles, the representatives of which are soap, surfactant and lipid, have a hydrophilic polar head and lipophilic nonpolar tails. They always remain on the interface between water and oil and form monolayers of surfactants in a water/oil/amphiphile ternary system. This monolayers or interfacial film reduce the surface tension between water and oil domains. In a three-component system the surfactant film exists in various topologically different structures such as micelles, vesicles, bicontinuous microemulsions, hexagonal arrays of cylinders or lamellar structures depending upon the pressure, temperature and the concentration of the components [1,2]. Microemulsions are thermodynamically stable, isotropic and transparent mixtures of ternary amphiphilic systems. When almost equal volume fractions of water and oil are mixed with a dilute concentration of surfactants, they take... 

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. 

Skauge A, Fotland P (1990) Effect of Pressure and Temperature on the Phase Behavior of Microemulsions. SPE Reserv Engin 5 601-608... 

For emulsions, the interfacial tension is usually of most interest. Here, the du Noiiy ring, Wilhelmy plate, drop volume, pendant, or sessile drop methods are the most commonly used. The spinning drop or captive drop techniques are applicable to the very low interfacial tensions encountered in the enhanced oil recovery and microemulsion fields. The maximum droplet pressure technique can be used when there is little or no density contrast between the phases, such as in bitumen-water systems at elevated temperature. 

II, 2. The Equilibrium Radius and a Generalized Laplace Equation The equilibrium state of the system is completely determined by, the temperature T and the external pressure p. Therefore, the values of the radius r and of emerge from the condition that the microemulsion be in Internal equilibrium, i.e., f be a minimum with respect to r and. This leads to the equations ... 

Once the reactor pressure and temperature has stabilized, inject the microemulsion solution using the cosolvent pump. 

The engine used was a one-cylinder direct-injection test engine at Saab-Scania, Sodertalje, Sweden. During the tests, maximum pressure, fuel consumption, exhaust temperature, CO, CO2, NO, NO2, O2, HC, and smoke were registered at varied injection timings, loads, and speeds. The performance of three microemulsion fuels was compared with a reference fuel of pure diesel oil with the cetane number of 43. The data of the microemulsion fuels are given in Table II. 

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