One-phase microemulsions

The kinetics of an enzyme catalysed reactions in a w/o-microemulsions is dependent on several parameters. For example, the substrates and enzymes distribute within the different parts of a one-phase microemulsion with different concentrations. The enzymes are located in the water and hydrophobic substrates are mainly dissolved in the oil. Additionally, the choice of oil and surfactant, the water concentration, and the structure of the interfacial layer can influence the activity and stability of biocatalysts. The influences of the main parameters on the kinetics will be discussed in this chapter. 

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). 

Figure 8.4. Phase behavior of water/C02/surfactant systems studied to date. r=35°C, P = 414 bar and O T= 35°C, P=138 bar. PFPE COO NH4+ (Johnston et al., 1996) PFPE COO NH4+ (Zielinsky et al., 1997) di-HCF4 (Holmes et al., 1998). The one-phase microemulsion region is to the right of each curve.

For this system the temperature of phase inversion (PIT) is between 45°C and 55°C. Variation of both the temperature and the surfactant concentration in a system with a fixed ratio of water and oil leads to a phase diagram that is called informally the Kahlweit fish due to the shape of the phase boundaries that resemble a fish. In Figure 3.24 (left), this diagram is given for the system water/tetradecane/CnEs. For small surfactant concentrations (<15%), the phases already discussed occur but, at higher emulsifier concentrations, the surfactant is able to solubilise all the water and the hydrocarbon which results in a one-phase microemulsion D or a lamellar phase La. 

Organic reactions in micro emulsions need not be performed in one-phase systems. It has been found that most reactions work well also in two-phase Winsor I or Winsor II systems, i.e. an oil-in-water microemulsion coexisting with excess oil or a water-in-oil microemulsion coexisting with excess water, respectively [7, 8]. A Winsor III system, i.e. a three-phase system in which a middle phase microemulsion coexists with both oil and water, has also been successfully used as reaction medium [9]. The transport of reactants from the excess oil or water phase to the microemulsion phase, where the reaction takes place, is evidently fast compared to the rate of the reaction. This is a practically important aspect on the use of micro emulsions as media for chemical reactions because it simplifies the formulation work. Formulating a Winsor I or Winsor II system is usually much easier than formulating a one-phase microemulsion of the whole reaction mixture. Winsor systems can also be of value to simplify the work-up process, in particular to separate the product from the surfactant, as will be discussed below in Sect. 2.4 (see also [6]). 

Figure 2. Phase boundaries of the system aqueous X% SLS solution, MMA and pentanol or hexanol at 23 °C expressed in weight percent. At each surfactant concentration, the region above the phase boundary (higher MMA content) consists of two phases while the region just below is a one phase microemulsion.

Considering now the variation of the phase behaviour with increasing mass fraction y of surfactant one can see that the volume of the respective microemulsion phase increases (see test tubes in Fig. 1.3(b)) until the excess phases vanish and a one-phase microemulsion is found. The optimal state of the system is the so-called X-point where the three-phase body meets the one-phase region. It defines both the minimum mass fraction y of surfactant needed to solubilise water and oil, i.e. the efficiency of the surfactant, as well as the corresponding temperature f, which is a measure of the PIT. 

Figure 1.4 T(7)-sections through the phase prism of the systems H20-n-octane-C6E2, C8E3, Q0E4 and C12E5 at an oil/(water + oil) volume fraction of = 0.5. In order to determine the respective X-point the phase boundaries are measured only for surfactant mass fractions 7 > 7. An increase of both the hydrophobic chain length / and the size of the hydrophilic head group j shifts the X-point to lower values of 7, i.e. the efficiency increases. Simultaneously the stability range of the bicontinuous one phase microemulsion shrinks dramatically due to the increased extension of the lamellar mesophase (La). (From Ref. [26], reprinted with permission of Elsevier.)...

Figure 4.9 Phase diagram of the system water-decane-CioE4 at equal volume fractions of water and decane as a function of the temperature T and the surfactant concentration cf>7. At low (f>7 there is a three-phase coexistence, while at moderate cf>7 the one-phase bicontinuous microemulsion appears. At even higher cf>7 the lamellar phase appears. At high and low temperatures a microemulsion phase coexists with either excess water or oil. The polymer fraction cf>p is raised symmetrically for the water- and oil-soluble polymers, and the one-phase microemulsion window closes continuously. The 2 K temperature shift is due to the use of heavy water. (From Ref. [40], reprinted with permission of the American Chemical Society.)...

The above-mentioned reaction between sodium phenoxide and 1-bromooctane to synthesise 1 -phenoxyoctane has been carried out in different types of microemulsion systems, all based on the same non-ionic surfactant, Triton X-100 (an octylphenol ethoxylate), the same surfactant concentration (20 wt.%), the same oil to water ratio (2 3) but different hydrocarbons as oil component [28]. This results in different phase volume ratios for the different hydrocarbons. A one-phase microemulsion is only obtained with toluene as oil component. The more hydrophobic oils, i.e. cumene, isooctane, hexadecane and paraffin oil, all give a microemulsion in equilibrium with an excess oil phase, i.e. a Winsor I system. With the more hydrophilic chlorobenzene as oil a microemulsion coexisting with an excess water phase, i.e. a Winsor II system, is obtained. As is also shown in Fig. 5.4, the reactivity is highest in the chlorobenzene- and the paraffin oil-based microemulsions, i.e. in the systems... 

The observation that the reaction runs approximately as fast in a Winsor system as in a one-phase microemulsion has later been seen also for another bimolecular substitution reaction [16]. The fact that a Winsor system can be used instead of a one-phase microemulsion is practically important. Formulation of a one-phase microemulsion is often a problem, particularly when one wants a high loading of reactants into the oil and water domains, and one may end up with various types of two-phase or three-phase systems. Evidently, such systems may be just as useful as reaction media, as long as one of the phases is a microemulsion. The excess phase (or phases) can be regarded as reservoirs for the reactant (or reactants) while the reaction occurs at the oil-water interface of the microemulsion phase. 

Microemulsions based on non-ionic surfactants of alcohol ethoxylate type are sensitive to temperature changes and those based on ionic surfactants are sensitive to variations in the electrolyte concentration. Such variations may cause a one-phase microemulsion to form a two- or a three-phase system in which a microemulsion phase coexists with one or two excess phases. As a work-up approach the concept is particularly useful for microemulsions based on non-ionic surfactants because the transitions obtained by temperature variations are reversible. 

Figure 8.6 Pseudoternary phase diagram of a system containing 20 wt.% emulsifier (Cs/io-APG, Q2/14-APG and GMO), 20 wt.% perfume oil, 0.6 wt.% oil (dicapryl ether, octyldodecanol) and 59.4 wt.% water at 25°C. The formation of microemulsions was studied as a function of the emulsifier s composition. The dotted lines separate the o/w- from the w/o-region. ME indicates a one-phase microemulsion. (From Ref. [39], reprinted with permission of Elsevier.)...

The size of w/c microemulsion droplets has been measured by neutron scattering for a di-chain hybrid surfactant (C7Hi5)-(C7Fi5)CHS04 Na [32], 667 g/mol PFPE-C00"NH4 [33], and for a partially fluorinated di-chain sodium sulfo-succinate surfactant [34]. For the PFPE-COO NH4 surfactant, the droplet radius increases from 20 A to 36 A for W o values of 14 and 35, respectively. For the di-chain sodium sulfosuccinate surfactant, droplet radius varied linearly from 12 to 36 A as Wo increased from 5 to 30. This linear relationship has also been shown for AOT reverse micelles in organic solvents [7]. In each of these studies for a one-phase microemulsion, droplet size and Wq were found to be only a weak function of pressure, unless the pressure is reduced to the phase boundary where droplets aggregate. This trend was calculated theoretically [6,23] and has been measured in AOT w/o microemulsions in supercritical propane [35,36]. 

Systems containing equal amounts of oil and water often can be classified according to the four Winsor types [34]. These classical configurations are known as type I (normal micelles in equilibrium with excess oil, often designated 2, which means two phases with the surfactant in the lower phase), type II (reverse micelles in equilibrium with excess water, 2), type III (middle phase microemulsion with excess water and excess oil phases, 3), and type IV (one-phase microemulsion, 1). 

It was observed in certain experiments that even though particle synthesis was initiated in a clear one-phase microemulsion, the fluid phase became unstable during the reaction and phase separation occurred [81]. The continuing nucleation and growth of silica in the resulting biphase system resulted in a bimodal size distribution. With the aid of phase diagrams of temperature versus weight percent aqueous phase and temperature versus the ethanol/H20 mole ratio, the phase separation was traced to microemulsion destabilization via H2O depletion and ethanol release [81]. 

The role of the microemulsion environment in this zeolite synthesis process is undoubtedly complex. Presumably, addition of the sodium aluminate solution to the silicate-containing microemulsion first results in the formation of an amorphous aluminosilicate precipitate. Apparently this material does not remain as dispersed particles in the microemulsion fluid phase. One possible reason is that the stability field of the one-phase microemulsion regions shrinks with an increase in temperature. The ability of NaOH to promote the hydrolytic decomposition of AOT [150,151] will also contribute to the destabilization of the surfactant aggregates. It is likely that the adsorbed AOT molecules associated with the amorphous precipitate play some role in the subsequent transformation to crystalline zeolite. However, the details are yet to be determined. 

Yet another related approach is to use the strong temperature dependence of microemulsions based on nonionic surfactants. After the reaction is completed in a one-phase microemulsion, the temperature is raised (or lowered) so that a two-phase system forms, consisting of microemulsion in equilibrium with excess water (or oil) phase. If the reaction product is hydrophilic, a temperature increase is chosen and the product is recovered from the aqueous phase. If the product is lipophilic, the temperature is instead decreased, and the product is recovered from the excess hydrocarbon phase. The principle has been applied successfully to an HLADH-catalyzed reaction in microemulsion based on C,2E5 [132],... 

The first step in selecting an optimal micellar system for any field application is to define a complete one-phase microemulsion without any oil or water phase separation, whose viscosity is in the range of the equivalent viscosity of the oil-bank, and which can be prepared from a minimum amount of surfactant, corresponding to the smallest multiphase area in phase diagrams or to the highest solubilization parameters. Such a system involves a surfactant... 

The scheme outlined here has turned out to make up a convenient approach, first, to treating Winsor I and Winsor II microemulsion systems where an excess phase is present [38], in which case it is sufficient to consider merely the first interfacial term of Eq. (191), and, second, to the corresponding one-phase microemulsions elose to the two-phase region (i.e., on the border to what occasionally is referred to as emulsification failure) [47]. 

On the other hand, the coordinates of the fishtail are shghtly affected by a change of [bmim][PF j fraction. The variation of f and T with a allows the determination of the trajectory of the middle phase. Its schematic projection onto the (7, 7) plane is voluntarily amplified and displayed in Figure 11.4. We notice, from this trajectory (dotted line), that the surfactant amount needed to produce one-phase microemulsion is highest when a = 0.58, which corresponds to equal water and [bmim][PFJ volumes. An increase or a decrease in a from 0.58 systematically, although only to a little extent, decreases the optimal amount of surfactant f. The small variation in 7 and r is a significant difference between this system and water-u-dodecane systems [40-42]. A careful examination of absolute 7 values brings the present water-[bmim] [PFJ-TX-100 closer to water-u-dodecane-C Ej. Indeed, we observe that both are inefficient nearly 50% of surfactant is needed to solubilize water with the immiscible solvent. 

If CO is added to such a two-phase equilibrium at fairly high surfactant concentration all oil is taken up and a one-phase microemulsion appears. 

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