Microemulsion salt effect

Aughel and coworkers [63] studied the phase behavior of hydrocarbon-water mixtures in the presence of alkyl(aryl)polyoxyethylene carboxylates for enhanced oil recovery and found good salt tolerance with an alkyl ether carboxy-late (C13-C15) with 7 mol EO and a good microemulsion forming effect with the 3 EO type. 

Effect of Ionic Strength. Both yE systems were examined for ionic strength effects. Microemulsion compositions were prepared at 70% water, with a cyanide concentration of 0.032 M with respect to the water content. Potassium bromide was used to vary the ionic strength of the reaction mixtures. Ionic strength in the CTAB yE was varied from 0.04 to 0.34. Since the Brij yE tolerated a much higher salt concentration without phase separation, ionic strength in that system was varied between 0.04 and 1.80. As will be seen, the Brij system exhibits a salt effect, while the CTAB yE does not. Rate constants obtained for reaction (1) in the Brij yE were therefore corrected to take into account the effect of ionic strength in that system (vide infra). 

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). 

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]. 

Ayyub, R, Maitra, A., Shah, D.O. Microstructure of the CTAB-butanol-octane-water microemulsion system effect of dissolved salts. J. Chem. Soc. Faraday Trans. 89, 3585-3589 (1993)... 

Kabalnov A, Olsson U, Wennerstrom H. (1995) Salt effects on non-ionic microemulsions are driven by adsoprtion/depletion at the surfectant monolayer. JPhys Chem 99 6220-6230. 

Since some structural and dynamic features of w/o microemulsions are similar to those of cellular membranes, such as dominance of interfacial effects and coexistence of spatially separated hydrophilic and hydrophobic nanoscopic domains, the formation of nanoparticles of some inorganic salts in microemulsions could be a very simple and realistic way to model or to mimic some aspects of biomineralization processes [216,217]. 

The rates of multiphase reactions are often controlled by mass tran.sfer across the interface. An enlargement of the interfacial surface area can then speed up reactions and also affect selectivity. Formation of micelles (these are aggregates of surfactants, typically 400-800 nm in size, which can solubilize large quantities of hydrophobic substance) can lead to an enormous increase of the interfacial area, even at low concentrations. A qualitatively similar effect can be reached if microemulsions or hydrotropes are created. Microemulsions are colloidal dispersions that consist of monodisperse droplets of water-in-oil or oil-in-water, which are thermodynamically stable. Typically, droplets are 10 to 100 pm in diameter. Hydrotropes are substances like toluene/xylene/cumene sulphonic acids or their Na/K salts, glycol.s, urea, etc. These. substances are highly soluble in water and enormously increase the solubility of sparingly. soluble solutes. 

The rate constants for the reaction of a pyridinium Ion with cyanide have been measured in both a cationic and nonlonic oil in water microemulsion as a function of water content. There is no effect of added salt on the reaction rate in the cationic system, but a substantial effect of ionic strength on the rate as observed in the nonionic system. Estimates of the ionic strength in the "Stern layer" of the cationic microemulsion have been employed to correct the rate constants in the nonlonic system and calculate effective surface potentials. The ion-exchange (IE) model, which assumes that reaction occurs in the Stern layer and that the nucleophile concentration is determined by an ion-exchange equilibrium with the surfactant counterion, has been applied to the data. The results, although not definitive because of the ionic strength dependence, indicate that the IE model may not provide the best description of this reaction system. 

The rate constants for the reaction of N-dodecyl-3-carbamoyl-pyridinlum ion with cyanide in both cationic and nonionic o/w microemulsions have been measured as a function of phase volume. Added salt has no effect in the cationic system, but the rate constants in the nonionic system depend upon ionic strength as would be expected for a reaction between two ions. In order to compare the two microemulsions, the ionic strength in the reaction region has been estimated using thicknesses of 2-4A. The former produces values of the effective surface potential which yield... 

Different surfactants are usually characterised by the solubility behaviour of their hydrophilic and hydrophobic molecule fraction in polar solvents, expressed by the HLB-value (hydrophilic-lipophilic-balance) of the surfactant. The HLB-value of a specific surfactant is often listed by the producer or can be easily calculated from listed increments [67]. If the water in a microemulsion contains electrolytes, the solubility of the surfactant in the water changes. It can be increased or decreased, depending on the kind of electrolyte [68,69]. The effect of electrolytes is explained by the HSAB principle (hard-soft-acid-base). For example, salts of hard acids and hard bases reduce the solubility of the surfactant in water. The solubility is increased by salts of soft acids and hard bases or by salts of hard acids and soft bases. Correspondingly, the solubility of the surfactant in water is increased by sodium alkyl sulfonates and decreased by sodium chloride or sodium sulfate. In the meantime, the physical interactions of the surfactant molecules and other components in microemulsions is well understood and the HSAB-principle was verified. The salts in water mainly influence the curvature of the surfactant film in a microemulsion. The curvature of the surfactant film can be expressed, analogous to the HLB-value, by the packing parameter Sp. The packing parameter is the ratio between the hydrophilic and lipophilic surfactant molecule part [70] ... 

An attempt was also made to accelerate the same reaction performed in a microemulsion based on water, nonionic surfactant and hydrocarbon oil [9]. The reaction was performed in a Winsor III system and the same Q salt, tetra-butyl ammonium hydrogen sulfate, was added to the formulation. In this case the addition of the phase transfer catalyst gave only a marginal increase in reaction rate. Similar results have been reported for an alkylation reaction performed in different types of micellar media [52]. The addition of a Q salt gave no effect for a system based on cationic surfactant, a marginal increase in rate for a system based on nonionic surfactant and a substantial effect when an anionic... 

Since most precursors for solution-phase nanostructural growth are ionic metal salts, a typical micelle would not be effective since the precursor would not be confined to the interior of the microemulsion. Hence, reverse micelles (or inverse micelles, Figure 6.34) are used to confine the precursor ions to the aqueous interior, which effectively serves as a nanoreactor for subsequent reduction, oxidation, etc. en route to the final nanostructure. Not surprisingly, either PAMAMOS dendrimers (Chapter 5) or dodecyl-terminated (hydrophobic) PAMAM dendrimers (Figure 6.35) have been recently employed for this application. 

The presence of high concentrations of salt changed the microemulsion region considerably, but had only limited effect on the transport properties. The main effect was that the duration of the liquid crystal presence was shortened and that an isotropic liquid middle phase was formed. 

These results clearly show that the nost effective polymerizable microemulsions for industrial applications (minimum amount of surfactant) are bicontinuous. It is interesting to note that addition of salting-in-type electrolytes (inducing no W I - W III transition) to AM microemulsions gives unstable and turbid latices. 

The goals of this work have been to determine the effect of polymers on the phase behavior of aqueous surfactant solutions, prior to and after equilibration with oil, to understand the mechanism of the so-called "surfactant-polymer interactions (SPI) in EOR, to develop a simple model which will predict the salient features of the phase behavior in polymer-microeraulsion systems, and to test the concept of using sulfonate-carboxylate mixed microemulsions for increased salt tolerance. 

The effect of 750 ppm Xanthan gum on the microemulsion phase behavior is shown in Figure 5(b). The observed phase behavior is similar except that the extent of the three-phase region is widened. Thus at both 0.8 and 1.0 gm/dl salt concentrations there exists a polymer-containing brine phase in equilibrium with the microemulsion phase. When no polymer is present, the microemulsion phase is in equilibirum with only excess oil. The volumes of the polymer phases are small and the interface between the polymer phase and microemulsion is diffipult to detect in Figure 5(b). However, phase separation is clearly visible in Figure 5(c), which illustrates the oil-equilibrated phase behavior at a higher polymer concentration of 1500 ppm. 

In the methylsulfolane-containing system, it was found that neither incorporation of salts (as seawater) nor alteration of the cosurfactant/surfactant ratio seriously altered the solubilization of chlorethyl ethyl sulfide. Thus, it seems plausible that low concentrations of inorganic buffers and/or oxidants should not seriously degrade the solubilization effectiveness of microemulsions containing sulfones as cosurfactants. 

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. 

A significant number of patents on LDLDs have been issued in the U.S., Europe, and Japan in recent years. Listed in Table 7.10 are examples of LDLD patents formulating for effective soil removal/cleaning. The technology utilized in these patents ranges from special surfactants, surfactant mixtures, salts, and microemulsion to the use of special additives such as lemon juice and abrasives. 

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