Microemulsion middle-phase

Fig. 1. Phase diagram of an amphiphile—oil—water system that forms a middle-phase microemulsion, definition of microemulsion, and illustration of the...

However, often the identities (aqueous, oleic, or microemulsion) of the layers can be deduced rehably by systematic changes of composition or temperature. Thus, without knowing the actual compositions for some amphiphile and oil of poiats T, Af, and B ia Figure 1, an experimentaUst might prepare a series of samples of constant amphiphile concentration and different oil—water ratios, then find that these samples formed the series (a) 1 phase, (b) 2 phases, (c) 3 phases, (d) 2 phases, (e) 1 phase as the oil—water ratio iacreased. As illustrated by Figure 1, it is likely that this sequence of samples constituted (a) a "water-continuous" microemulsion (of normal micelles with solubilized oil), (b) an upper-phase microemulsion ia equiUbrium with an excess aqueous phase, ( ) a middle-phase microemulsion with conjugate top and bottom phases, (d) a lower-phase microemulsion ia equiUbrium with excess oleic phase, and (e) an oA-continuous microemulsion (perhaps containing iaverted micelles with water cores). 

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. 

The temperature (or salinity) at which optimal temperature (or optimal salinity), because at that temperature (or salinity) the oil—water interfacial tension is a minimum, which is optimum for oil recovery. For historical reasons, the optimal temperature is also known as the HLB (hydrophilic—lipophilic balance) temperature (42,43) or phase inversion temperature (PIT) (44). For most systems, all three tensions are very low for Tlc < T < Tuc, and the tensions of the middle-phase microemulsion with the other two phases can be in the range 10 5—10 7 N/m. These values are about three orders of magnitude smaller than the interfacial tensions produced by nonmicroemulsion surfactant solutions near the critical micelle concentration. Indeed, it is this huge reduction of interfacial tension which makes micellar-polymer EOR and its SEAR counterpart physically possible. 

Microemulsions are transparent or translucent, thermodynamically stable emulsion systems (Griffin 1949). Forming a middle phase microemulsion (MPM) requires matching the surfactant system s hydrophobicity with that of the oil. The HLB (hydrophilic-lipophilic balance) number reflects the surfactant s partitioning between water and oil phases higher HLB values indicate water soluble surfactants while lower values indicate oil soluble surfactants (Kunieda et. al. 1980, Abe et. al. 1986). While a balanced surfactant system produces middle phase microemulsions, an underoptimum surfactant system is too water soluble (high HLB) while an over-optimunTSystem is too oil soluble (low HLB). 

Figure 1 shows changes in the system phase behavior as its HLB value is systematically adjusted. The left side of the diagram represents a two-phase system with micellar-solubilized oil in equilibrium with an excess oil phase (Winsor Type I) (Winsor 1954). The right side of the diagram represents a different two-phase system with reversed micellar-solubilized water. In-between these two systems a third phase coemerges which contains enriched surfactant with solubilized water and oil. This new thermodynamically stable phase is known as a Winsor Type HI middle phase microemulsion. 

It is widely recognized that the system IFT reaches a minimum in the middle phase microemulsion region. At the same time, the solubilization parameter (a), defined as mass of oil solubilized per unit mass of surfactant, is maximized in middle phase microemulsion systems (see Figure 1). This inverse relationship between the solubilization parameter (c)and IFT (y)has been defined by the Chun-Huh equation (Huh 1979, Sunwoo et. al. 1992, Abe et. al. 1987) ... 

The goal of surfactant enhanced subsurface remediation is to maximize the contaminant extraction efficiency while optimizing system economics. Since middle phase microemulsions maximize the solubilization while minimizing the oil-water interfacial tension, these systems are highly desirable, especially for NAPLs lighter than water, where downward... 

All middle phase microemulsion and solubilization studies were carried out in 20 ml centrifuge tubes with Teflon screw caps. Experiments were conducted at controlled room temperature of 22°C. Equal volumes of aqueous surfactant solution and oil (10.00 ml each) were placed in the centrifuge tube. The tube was shaken gently on a wrist action shaker for 20 minutes and allowed to stand for at least 12 hours to ensure equilibrium. The volume of each phase was carefully recorded to within 0.01 mis. The surfactant and oil concentrations were quantified in each phase for a few... 

Since the oil EACN is so critical to formulating surfactant middle phase microemulsions (MPMs), we will begin by discussing a method for characterizing the EACN of unknown NAPLs. The relation between the oil-water partitioning of alcohols (k,j) and NAPL EACN is dependent upon the hydrophobicity of the alcohol, as given by ... 

O % Recovery of hexadecane as solubilized inside micelles % Recovery of hexadecane as free phase effluent % Recovery of hexadecane as middle phase microemulsion — — % Hexadecane recovery 40000... 

Wu, B., Harwell, J. H., Sabatini, D. A., and Bailey, J. D. (1999). Alcohol-Free Diphenyloxide Disulfonate (DPDS) Middle Phase Microemulsion Systems, Journal of Surfactants and Detergents. 

The importance of a surfactant - rich phase, particularly a lamellar one, to detergency performance was noted for liquid soils such as C16 and mineral oil (3.6). Videomicroscopy experiments indicated that middle phase microemulsion formation for C12E04 and Cjg was enhanced at 30 °C, while at 18 °C, oil - in - water, and at 40 °C, water - in - oil microemulsions were found to form at the oil - bath interface (3.6). A strong temperature dependence of liquid soil removal by lamellar liquid crystals, attributed to viscosity effects, has been noted for surfactant - soil systems where a middle - phase microemulsion was not formed (10). 

Type III microemulsions, in which the aqueous and oleic phases are in equilibrium with a third, surfactant-rich, phase called the middle-phase microemulsion, which can contain bi-continuous emulsion. This is illustrated in Figure 3.28 [226],... 

Figure 3.28 Illustrative section from the phase prism of a mixture of oil, water, and surfactant. This section is for constant surfactant concentration (T is temperature). The section shows a middle-phase microemulsion phase existing together with oil (upper) and water (lower) phases. The surfactant is partitioned among all of the phases. The cross-hatching shows how the microemulsion can be O/W (to the left), or W/O (to the right), or bicontinuous (centre). From Schwuger et al. [226]. Copyright 1995, American Chemical Society.

A two-phase system in which both phases are continuous phases. For example, a possible structure for middle-phase microemulsions is one in which both oil and water phases are continuous throughout the microemulsion phase. An analogy can be drawn from the structure of porous and permeable rock in which both the mineral phase and the pore or throat channels can be continuous at the same time. See also Middle-Phase Microemulsion. See Bi-molecular Film. 

A microemulsion that has high oil and water content and is stable while in contact with either bulk oil or bulk water phases. This stability can be caused by a bi-continu-ous structure in which both oil and water phases are simultaneously continuous. In laboratory tube or bottle tests involving samples containing unemulsified oil and water, a middle-phase microemulsion tends to situate between the two phases. See also Winsor Type Emulsions. 

In microemulsions, the salinity for which the mixing of oil with a surfactant solution produces a middle-phase microemulsion containing an oil-to-water ratio of 1. In micellar enhanced oil recovery processes, extremely low interfacial tensions result, and oil recovery tends to be maximized when this condition is satisfied. 

Several categories of microemulsions that refer to equilibrium phase behaviours and that distinguish, for example, the number of phases that can be in equilibrium and the nature of the continuous phase. They are denoted as Winsor Type I (oil-in-water), Type II (water-in-oil), Type III (most of the surfactant is in a middle phase with oil and water), and Type IV (water, oil, and surfactant are all present in a single phase). The Winsor Type III system is sometimes referred to as a middle-phase microemulsion , and the Type IV system is often referred to simply as a microemulsion . An advantage of the Winsor category system is that it is independent of the density of the oil phase and can lead to less ambiguity than do the lower-phase or upper-phase microemulsion type terminology. Nelson type emulsions are similarly identified, but with different type numbers. 

The interfacial tension y at the planar interface has a minimum near the temperature Indeed, at the latter temperature r is small, A/jt0 = 0 and because d ij w/d J and dfi /dT have opposite signs and s is also small (because T is small), dy/d I 0. The temperature T0 is provided by Eq. (25) and is independent of the concentration of surfactant. In other words, the two minima of Fig. 4 which correspond to the phase inversion temperatures of a macroemulsion (the curve with a positive minimum) and microemulsion (the curve with a negative minimum) are the same. When emulsions are generated from a microemulsion and its excess phase, the emulsion is of the same kind as the microemulsion, the phase inversion temperature is obviously located in the middle of the middle phase microemulsion range and the above conclusion remains valid. The above results explain the observation of Shinoda and Saito [6,7] that the phase inversion temperature (PIT) of emulsions can be provided by the ternary equilibrium phase diagram. 

The preceding condition of thermodynamic equilibrium implies that the curvature of the dispersed phase becomes zero at the transition from two to three phases. In the middle-phase microemulsion, the pressures p and p2 fluctuate in time and space because of the instability of the interface between the two media (see below), and intuition suggests that Eqs. (37) and (40) be replaced with their average, hence that the condition of zero curvature be replaced with the condition of mean (with respect to time) average curvature. 

Microemulsions are thermodynamically stable dispersions of oil and water stabilized by a surfactant and, in many cases, also a cosurfactant.1-4 The microemulsions can be of the droplet type, either with spherical oil droplets dispersed in a continuous medium of water (oil-in-water microemulsions, O/W) or with spherical water droplets dispersed in a continuous medium of oil (water-in-oil microemulsions, W/O). The droplet-type microemulsions can be either a single-phase system or part of a two-phase system wherein the microemulsion phase coexists with an excess dispersed phase (an upper phase of excess oil in the case of O/W and a lower phase of excess water in the case of W/O microemulsions). There are also nondroplet-type microemulsions, referred to as middle-phase microemulsions. In this case, the microemulsion phase is part of a three-phase system with the microemulsion phase in the middle coexisting with an upper phase of excess oil and a lower phase of excess water. One possible structure of this middle-phase microemulsion, characterized by randomly distributed oil and water microdomains and bicontinuity in both oil and water domains, is known as thebiccntinuous microemulsion. Numerous experimental studies have shown1 2 4 that one can achieve a transition... 

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