Three-phase microemulsion systems

In this paper, a molecular thermodynamic approach is developed to predict the structural and compositional characteristics of microemulsions. The theory can be applied not only to oil-in-water and water-in-cil droplet-type microemulsions but also to bicontinuous microemulsions. This treatment constitutes an extension of our earlier approaches to micelles, mixed micelles, and solubilization but also takes into account the self-association of alcohol in the oil phase and the excluded-volume interactions among the droplets. Illustrative results are presented for an anionic surfactant (SDS) pentanol cyclohexane water NaCl system. Microstructur al features including the droplet radius, the thickness of the surfactant layer at the interface, the number of molecules of various species in a droplet, the size and composition dispersions of the droplets, and the distribution of the surfactant, oil, alcohol, and water molecules in the various microdomains are calculated. Further, the model allows the identification of the transition from a two-phase droplet-type microemulsion system to a three-phase microemulsion system involving a bicontinuous microemulsion. The persistence length of the bicontinuous microemulsion is also predicted by the model. Finally, the model permits the calculation of the interfacial tension between a microemulsion and the coexisting phase. 

The partitioning of alcohol into the oil, water, and interfacial layer domains of a microemulsion controls whether a two-phase or a three-phase microemulsion system is formed, as well as the microscopic characteristics of the microemulsion phases. F or the typical alcohols used, the amount of alcohol present in the oil domain can be large and comparable to the amount present in the interfacial layer. This is in contrast to the behavior of the surfactant, most of which remains at the interfacial layer and only a negligibly small amount of which are partitioned into the oil and the water domains. Therefore, the accurate accounting of the partitioning of alcohol into the oil domain is a necessary part of any quantitative theory of microemulsions. Such a theory must account for the facts that the alcohol is present in the oil phase as both monomers and aggregates and that the self-association of alcohol in the oil is responsible for its appreciable presence in the oil domain. 

The problem of large amounts of amphiphiles (i.e., surfactant and cosurfactant) required to form a single-phase microemulsion can be overcome with multiphase microemulsions. Indeed, two- and three-phase microemulsion systems can be obtained with only about 3-5% surfactant (Scheme 22.5). 

DSO via molybdate-catalyzed disproportionation of HjOj provides a readily scalable alternative to photooxidation. It can be carried out in commonly available stirred-tank reactors. However, the reaction does not work at low temperatures and organic media are limitedto alcoholic polar solvents (methanol or the safer ethylene glycol) or to microstructured media such as one-, two-, or three-phase microemulsion systems. The latter based on balanced catalytic surfactants advantageously combine low surfactant concentration with easy product isolation and catalyst recycling via simple phase separation. Safe processing may be further enhanced by microreactors, which minimize peroxide hold-up. 

Microemulsion research has since its inception been stimulated by the great potential for practical applications. In particular, considerable research interest has been invested in the possibility of using microemulsions for enhanced oil recovery (EOR). It was observed that surfactant formulations forming three-phase microemulsion systems, often termed Winsor III systems [29], in the oil well could increase the oil yield considerably. Important contributions to the understanding of the mechanisms involved were made by Shah and Hamlin [30] and the Austin group led by Schechter and Wade (see Bourrel et al. [31]). 

Nevertheless, possibilities for confusion abound. Until recently (50), it was thought that all nonmultiple emulsions were either oil-in-water (O/W) or water-in-oil (W/O). However, from the definitions of microemulsions and macroemulsions and from Fig. 16.2, it follows that in many macroemulsions one of the two or three phases is a microemulsion. The phase diagram of Fig. 16.2 makes clear that there are six possible nonmultiple, two-phase morphologies, of which four contain a microemulsion phase. These six two-phase morphologies are oleic-in-aqueous (017AQ, or O/W) and aqueous-in-oleic (AQ/OL, or W/O), but also, oleic-in-microemulsion (OL/Ml), microemulsion-in-oleic (MI/OL), aqueous-in-microemulsion (AQ/MI), and microemulsion-in-aqueous (MI/AQ) (50). [Although they have not yet aU been reported, theoretically there are 12 three-phase emulsion morphologies formed by the top, microemulsion (i.e., middle), and bottom phases (51,52) of three-phase microemulsion systems.]... 

In practice, the excess oleic or aqueous phase usually emulsifies into the conjugate microemulsion phase upon agitation. The reason is that the surfactant-rich continuous microemulsion phase resists coalescence of the surfactant-poor excess phase, presumably because surfactant depletion in the thinning microemulsion phase is counteracted by surfactant diffusion to restore uniform chemical potential (Gibbs Marangoni stability). In addition, macroemulsions formed from three-phase microemulsion systems tend to... 

In an earlier study calorimetry achieved this objective for the compositional boundaries between two and three phases (2). Such boundaries are encountered both in "middle-phase microemulsion systems" of low tension flooding, and as the "gas, oil, and water" of multi-contact miscible EOR systems (LZ). The three-phase problem presents by far the most severe experimental and interpretational difficulties. Hence, the earlier results have encouraged us to continue the development of calorimetry for the measurement of phase compositions and excess enthalpies of conjugate phases in amphiphilic EOR systems. 

R. Abu-Reziq, J. Blum and D. Avnir, Three-Phase Microemulsion/ Sol-Gel System for Aqueous Catalysis with Hydrophobic Chemicals, Chem. Eur. J., 2004, 10, 958. 

Practical Surfactant Mixing Rules Based on the Attainment of Microemulsion-Oil-Water Three-Phase Behavior Systems... 

A different example of triphasic catalysis for the Heck, Stille and Suzuki reactions relied on a three-phase microemulsion/sol-gel transport system. Gelation of an z-octyl(triethoxy)silane, tetramethoxysilane and Pd(OAc)2 mixture in a H2O/CH2CI2 system led to a hydrophobicitized sol-gel matrix that entrapped a phosphine-free Pd(ii) precatalyst. The immobilized precatalyst was added to a preformed microemulsion obtained by mixing the hydrophobic components of a cross coupling reaction with water, sodium dodecyl sulfate and a co-surfactant, typically zz-propanol or butanol. This immobilized palladium catalyst was leach proof and easily recyclable. 

II system). The WI system is particularlyrelevant with regard to the ease of workup since the product is extracted into the excess oil phase and the catalyst remains in the microemulsion phase. However, it is still sensitive to dilution by water arising from H2O2 disproportionation. This can be avoided by using pervaporation membrane [71]. A further improvement has been brought by three-hquid phase microemulsion systems based on balanced catalytic surfactants [72]. This new kind of catalysts is carefully designed in order to provide spontaneously a Winsor... 

Rataj VN, Caron L, Borde C, and Aubry JM. (2008). Oxidation in three lipid phase microemulsion systems using balanced catalytic surfactants. Journal of American Chemical Society, 130, 14914-14915. 

A remarkable achievement was the discovery in 1984 of tricritical-point phenomena (Figure 3) in surfactant-containing systems [19]. He was the first researcher to report such behavior [19, 20] and to show that three-phase microemulsions are related to mulhcritical-soluhon phenomena (when three phases become identical and ultralow interfacial tension is attained) [21], He reported tricritical phenomena in several surfactant systems [19-23]. 

During the oil removal from fabrics or hard surfaces, ternary systems occur where three phases co-exist in equilibrium. These systems are also referred to as three-phase microemulsions. These effects have been studied in detail for alkyl polyglycol ethers (22). Depending on the temperature, different phases exist, having a three-phase region between the temperature T and 71, (Figure 3.31). When these three phases are formed. 

A brief summary of the information available till around 1990 [211] shows the use of three reverse microemulsion systems for the synthesis of (amorphous) silica particles AOT /isooctane /water, AOT /benzyl alcohol/ decane /water and NP-5 /cyclohexane/ water. Note that the dispersed water phase had a dissolved base (NH4OH) or acid (HCl) as catalyst in it. Tetraethyl orthosilicate (TEOS) was added to the reverse microemulsions, leading to hydrolysis-condensation reaction and formation of silica particles. The size of the particles depended on the experimental conditions, but could go down to about 15 nm. 

Arcoleo et a/.[340] used a calorimetric cell to synthesize calcium carbonate from three different microemulsion systems involving one of AOT, DDAB (didodecyldimethylammonium bromide) and C,2E4 as surfactant, n-heptane or isooctane as the oil phase and CaCl2 and Na2C03 or NaHC03 as the precursors in aqueous solutions. The particles were a few nm in size. 

A typical ESR spectrum is shown in Fig. 2. As described, Xc can be calculated from the spectra. Shah and co-workers [35] used ESR to study the structure of microemulsion, and Rosen et al. [36] used ESR and found the microviscosity gradient in the middle phase of three-phase micellar systems. Shirhama et al. [37] and Witte et al. [38] have used ESR to study the interaction of SDS with PEO and PVP, yielding information on the structure of complex from the polymers and SDS. But the influence of the spin-label molecule on the microstructure should be considered. 

In order to clarify the relation between the phase behavior, interactions between droplets, and the Ginzburg number, we have undertaken further SANS studies of critical phenomenon in a different three-component microemulsion system called WBB, consisting of water, benzene, and BHDC (benzyldimethyl-n-hexadecyl ammonium chloride). This system also has a water-in-oil-type droplet structure at room temperature and decomposes with decreasing temperature. Above the (UCST) phase separation point, critical phenomena have been investigated by Beysens and coworkers [9,10], who obtained the critical indexes, 7 = 1.18 and v = 0.60, and concluded that their data could be interpreted within the 3D-Ising universality. However, Fisher s renormalized critical exponents were not obtained. 

Similarly to anionic surfactants, alkyl polyglycosides react to the addition of cosolvents that increase the solubility of the surfactant in the oil phase. In the decane-water-alkyl polyglycoside system, the addition of the cosolvent i-butanol results in a drastic reduction in the interfacial tension between oil and aqueous phase and, hence, in the formation of a third phase, the microemulsion [70]. As expected, the range in which this three-phase microemulsion exists is only slightly dependent on temperature and, in contrast to anionic surfactants, is also hardly affected by electrolytes [70]. Systematic investigations of the phase behavior confirm these initial results for a number of simple hydrocarbons from hexane to hexadecane and aromatics [71,72]. 

These three-phase behavior systems have been found to be associated with ultealow interfacial tension and, thus, were the target of the enhanced oil-recovery research in the seventies. Note that the ultralow interfacial tension is perfectly consistent with the possibility of entropy stabilization of microemulsions, because it decreases the interfacial free energy term yAA [50]. 

Abu-Reziq, R., Blum, J., and Avnir, D. (2004) Three-phase microemulsion/sol-gel system for aqueous catalysis with hydrophobic chemicals. Chem. Eur. /.,... 

Figure 12.14 Phase behavior and interfacial tension of water/decane/Ci2E4 systems 3-PM = three-phase microemulsion (Reproduced by permission of Steinkopff from ref. 81)...

Figure 8.10 Proposed mechanism of the catalytic oxidative desulfurization with SEPs/HjOj/iomimJPFj in three-liquid-phase microemulsion system. (Reprinted with permission from Ref. [20]. Copyright 2013 john wiley. Sons.)...

In three-phase systems the top phase, T, is an oleic phase, the middle phase, Af, is a microemulsion, and the bottom phase, B, is an aqueous phase. Microemulsions that occur ia equiUbrium with oae or two other phases are sometimes called "limiting microemulsions," because they occur at the limits of the siagle-phase regioa. 

The function /[0(r)] has three minima by construction and guarantees three-phase coexistence of the oil-rich phase, water-rich phase, and microemulsion. The minima for oil-rich and water-rich phases are of equal depth, which makes the system symmetric, therefore fi is zero. Varying the parameter /o makes the microemulsion more or less stable with respect to the other two bulk uniform phases. Thus /o is related to the chemical potential of the surfactant. The constant g2 depends on go /o and is chosen in such a way that the correlation function G r) = (0(r)0(O)) decays monotonically in the oil-rich and water-rich phases [12,13]. This is the case when gi > 4y/l +/o - go- Here we take, arbitrarily, gj = 4y l +/o - go + 0.01. 

Physical-chemical studies require traces of additives (reactants, catalysts, electrolytes) with respect to the concentration of the basic components of the microemulsion, and this causes only a minor change in the phase behavior of the system. However, when the amounts of additives are on the scale used in organic synthesis, the phase behavior, which is very sensitive to the concentration of the reactants, is sometimes difficult to control and the reaction is carried out in a one-, two- or three-phase state. 

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