Microemulsion stability pressures

Table 1. Aqueous Phase Critical Micelle Concentrations (erne s), Limiting Surface Tensions yeme s and Microemulsion Stability Pressures for Fluorinated 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...

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... 

It has to be underlined that, in comparison to LS containing nucleic acid molecules inside the particle, the production of CLS may be performed obviously without considering the stability problems of nucleic acid molecules. In this view, some preparation procedures are not considered, such as the microemulsion technique [55] that represents a favorable method when working with substances unstable because of the high mechanical stress produced by high-pressure homogenization. 

Typical approaches to this biphasic system have involved the immobilization of catalysts in the aqueous phase as colloids [53] or using water-soluble catalysts based on ligands such as the trisulfonated TPPTS [55, 56]. Particularly high reaction rates have been obtained with surfactant-stabilized microemulsions and emulsions that allow for intimate contact of all reagents with the catalyst during the reaction [57]. The emulsions separate readily into two phases by small pressure changes and the C02-phase is then vented to isolate the products. The catalyst RhCl(tppds)3 (tppds =... 

The above concept of duplex film can be used to explain both the stability of microemulsions and the bending of the interface. Considering that initially the flat duplex film has different tensions (i.e., different values) on either side of it, then the deriving force for film curvature is the stress of the tension gradient which tends to make the pressure or tension in both sides of the curved film the same. This is schematically shown in Figure 1. For example if ir > ir on the flat... 

The stability of miniemulsion droplets against diffusional degradation results from an osmotic pressure in the droplets, which controls the solvent or monomer evaporation. The osmotic pressure is created by the addition of a substance, which has extremely low water solubility, the so-called hydrophobe. This crucial prerequisite is usually not present in microemulsions, but... 

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

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. 

Recently, Steinbach and Sucker (23) reported about the formation of l+-H20-molecule structures that may develop on the hydrophilic groups of surface active compounds upon dilatation of a l-H20-molecu-le structure, by adsorbing 3-water molecules from the subphase at a water-air interface. In the case of the water-oil interphase of the microemulsion, the dispersed droplet consits of an interphasal choro-na that surrounds an inner water core the free water fraction of the latter (bulk-H20)is the subphase that, acting as a reservoir, supplies H2O molecules to the interphase region. Since the formation of hydrated structures takes place at ons ant sur ace tension (23), the above mechanism allows the water-oil interface to expand without affecting the surface pressure necessary to maintain the system s equilibrium. In this way while the area of every polar head of the amphi-phile remains constant, the interphase area stabilized by a single polar head increases up to the amount corresponding to the definite area requirement of the it-H20-molecule structure (23) (3-6). 

The goal of the present work is to obtain a consistent structural model for a microemulsion system. In particular, we are interested in carrying this model down to the molecular level so that the intermolecular effects which are responsible for the stability of these systems can be elucidated. We have studied the system consisting of water, SLS and MMA with and without n-hexanol or n-pentanol. We have determined the phase boundaries of the isotropic microemulsion and Lj phases and determined how these are affected by surfactant concentration and alcohol chain length. Measurements were also made of the vapor pressure of MMA over these systems to determine the concentration of MMA in the water surrounding the microemulsion droplets. From these data, the energetics of transfer of the MMA from aqueous to micellar solution were determined. Finally, a 1,C NMR chemical shielding study was performed to find how the MMA and the alcohol were distributed within the microemulsion. 

Another approach developed to increase the solubility of proteins in a bulk aqueous phase is the use of reverse microemulsions. Zhang et al. (84) reported the GAS-based precipitation of lysozyme solubilized in AOT reverse micelles in iso-octane using pressurized CO2. Comparing the on-line UV-vis (ultraviolet-visible) spectra of processed and unprocessed lysozyme, the authors concluded that the lysozyme was not denatured. The use of reverse micelles to dissolve proteins in a bulk organic phase is a promising variation of the GAS technique. The use of reverse micelles could potentially increase the stability of proteins because they would be in a primarily aqueous local environment until precipitation. 

Based on the success of these fluoro-sulfosuccinates described above di-fluorocarbon phosphates have also been investigated. In terms of synthesis and raw materials costs these surfactants have significant advantages over the sulfosuccinates. Surfactants of this kind have also been studied by DeSimone et al (27 c), and the synthesis and purification are described elsewhere (27b, c). Detailed SANS experiments are described in these papers (27b, c), and it is clear that surfactants of diis kind stabilize aqueous nano-droplets. Hence, anionics other than sulfosuccinates may be employed in water-in-C(>2 microemulsions. Significantly, one of these conq>ounds (di-HCF6-P, see ref 27 b) stabilizes microemulsions in liquid CO2 at vapor pressure a potentially useftil result that may be of importance in facilitating applications. 

A clear correlation has been observed between limiting surface tension ycmc and surfactant performance in water-in-C02 microemulsions, as measured by the phase transition pressure Ptnms- These results have important implications for the rational design of C02-philic surfactants. Studies of aqueous solutions are relatively easy to carry out, and surface tension measurements can be used to screen target compounds expected to exhibit enhanced activity in CO2. Therefore, potential surfactant candidates can be identified before making time-consuming phase stability measurements in high-pressure CO2. 

Water-in-C02 microemulsion was used to dissolve metal salts in the production of nanoparticles via RESOLV. In order to evaluate the solubility of Cu(N03)2, for example, the same microemulsion as that used in the rapid expansion was prepared in a high-pressure optical cell. With Cu(N03>2 in the water phase, which exhibited the distinctive blue color of aqueous Cu (70), the microemulsion appeared homogenous. According to the observed absorbance (the band centered at 740 nm), the Cu(N03)2 salt was completely dissolved in the PFPE-NH4-stabilized water-in-C02 microemulsion. The other metal salts were similarly soluble, resulting in microemulsions that appeared equally homogeneous. 

According to Zielinski et al, the PFPE-NH4-stabilized water-in-COj microemulsion of Wo equal to 11 contains water droplets of - 4 nm in average diameter, and the droplets and droplet structure are little affected by experimental parameters such as the system pressure (28). Thus, the reverse micellar core in the microemulsion used in this study (Wo = 10) should have an average diameter close to 4 nm. Since there is evidence that the average size of the nanoparticles produced via RESOLV is dependent on the size of the pre-expansion reverse micelles in CO2 (9), it may be more than just a coincidence that the metal sulfide nanoparticles are of average sizes comparable to that of the pre-expansion water core (Table 1). The... 

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