Solubilization middle-phase microemulsions

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). 

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... 

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... 

The physicochemical aspects of micro- and macroemulsions have been discussed in relation to enhanced oil recovery processes. The interfacial parameters (e.g. interfacial tension, interfacial viscosity, interfacial charge, contact angle, etc.) responsible for enhanced oil recovery by chemical flooding are described. In oil/brine/surfactant/alcohol systems, a middle phase microemulsion in equilibrium with excess oil and brine forms in a narrow salinity range. The salinity at which equal volumes of brine and oil are solubilized in the middel phase microemulsion is termed as the optimal salinity. The optimal salinity of the system can be shifted to a desired value hy varying the concentration and structure of alcohol. 

The effect of hydrated radii, valency and concentration of counterions on oil-external and middle phase microemulsions was investigated by Chou and Shah (40). It was observed that 1 mole of CaCl2 was equivalent to 16-19 moles of NaCl for solubilization in middle phase microemulsion, whereas for solubilization in oil-external microemulsions, 1 mole of CaCl2 was equivalent to only 4 moles of NaCl. For monovalent electrolytes, the values for optimal salinity for solubilization in oil-external and middle phase microemulsions are in the order LiCl > NaCl > KC1 > NH Cl, which corre-... 

Huh developed a theoretical relationship between the solubilization parameter and IFT for a middle-phase microemulsion (type III). His equations are... 

We start with the equations by Huh (1979) who developed a theoretical relationship between the solubilization parameter and IFT for a middle-phase microemulsion (type 111). His equations are shown as Eqs. 7.77 and 7.78. According to Eq. 7.96,... 

In middle-phase microemulsion, owing to the lowest ITT, oil and water can be solubilized in each other, and oil droplets can flow more easily through pore throats. The oil droplets move forward and merge with the oil downstream to form an oil bank. Because of the solubilization effect, water and oil volumes are expanded, leading to higher relative permeabilities and lower residual saturations. However, when kj increases faster than k with decreasing IFT, the oil saturation in the oil bank and the oil recovery rate are deterioated, if no viscosity alteration is made. 

In a water-wet reservoir, initially water film sticks to rock surfaces. Because middle-phase microemulsion can solubilize water, some water films will be replaced by the microemulsion. Thus, the rock surfaces will become less water-wet. Similarly, in an oil-wet reservoir, some oil films on rock surfaces will be replaced by microemulsion, and the rock will become less oil-wet. Therefore, microemulsion always behaves as the most wetting phase. 

There are two bulk interfaces in middle phase microemulsions and one in lower or upper phase microemulsions. Thus, one or three values of interfacial tension (IFT) may be measured depending on system composition (1) ymo between microemulsion and excess oil phase, (2) between microemulsion and excess brine phase, and (3) >Vm between excess oil and brine phases. Phase volumes and consequently the volumes of oil (Vo) and brine () solubilized in the microemulsion depend on the variables that control the phase behavior. The solubilization parameters are defined as Vg/Vs and V JV, where Vs is the volume of the surfactant in the microemulsion phase. These parameters are easily determined from phase volume measurements if all the surfactant is assumed to be in the microemulsion phase. The magnitude of decreases as Vg/Vs increases, i.e., as more oil is solubilized. Similarly, the magnitude of decreases as Vg/Vs increases. The salinity at which the values of ymo and are equal is known as the optimal salinity based on IFT. Similarly, the intersection of Vg/Vs and V. /Vs defines the optimal salinity based on phase behavior. The optimal salinity concept is very important for enhanced oil recovery. 

Unlike the experiments carried out below the cloud point temperature, appreciable solubilization of oil was observed in the time-frame of the study, as indicated by upward movement of the oil-microemulsion interface. Similar phenomena were observed with both tetradecane and hexadecane as the oil phases. When the temperature of the system was raised to just below the phase-inversion temperatures of the hydrocarbons with C12E5 (45°C for tetradecane and 50°C for hexadecane), two intermediate phases formed when the initial dispersion of Li drops in the water contacted the oil. One of these was the lamellar liquid crystalline phase L (probably containing some dispersed water). Above this was a middle-phase microemulsion. In contrast to the studies carried out below the cloud point temperature, there was appreciable solubilization of hydrocarbon into the two intermediate phases. A similar progression of phases was found at 35 C when using / -decane as the hydrocarbon. At this temperature, which is near the phase-inversion temperature of the water-C12E5-decane system, the... 

In summary, various phenomena occurring at an optimal salinity in relation to enhanced oil recovery by macroemulsion and microemulsion flooding are schematically shown in Figure 6. It has been demonstrated that a maximum in oil recovery correlates well with several equilibrium and transient properties of surfactant flooding in the form of macroemulsion and microemulsion systems. Results have shown that a maximum in oil recovery, a minimum in surfactant adsorption, a minimum in apparent viscosity of the emulsion, a minimum in phase separation time, an equal solubilization of oil/brine phases in middle phase microemulsion, and a minimum in interfacial tension occur at an optimal salinity of the system. 

In high surfactant concentration systems, a middle phase microemulsion forms in equilibrium with excess oil and brine in a given salinity range. The middle phase microemulsion contains equal volumes of oil and brine and practically all of the surfactant at a specific salinity defined as the optimal salinity of the given system. The interfacial tension of the two interfaces, middle phase/brine and middle phase/oil, depends on the extent of solubilization of oil and brine in the middle phase microemuIsion. The higher the solubilization of oil and brine in the middle phase, the lower is the interfacial tension at both these interfaces. We have... 

Figure 13 schematically illustrates our proposed mechanism for the formation of middle phase microemulsions (13). At low salinities, micelles are formed in the aqueous phase in equilibrium with oil. As the salinity increases, the solubilization of oil within the micelles increases and the thickness of electrical double layer around the micelles decreases. The reduction in repulsive forces allows micelles to approach each other closely and subsequently a micelle-rich phase separates out due to the density difference from the aqueous phase forming the middle phase microemulsion. Hence, the middle phase microemulsion is similar to coacervation process in micellar solution where a micelle-rich phase separates out upon addition of salts. The presence of oil only contributes towards the solubilization of oil within the micelles. Ultimately, at higher salinities, surfactant preferen-... 

In summary, the formation of middle phase microemulsion at the optimal salinity is an important phenomenon with respect to ultralow interfacial tension, solubilization, rate of coalescence and oil displacement efficiency in porous media (10,11,25). Also, the optimal salinity can be shifted to a desired value by adjusting several variables. 

Scaling theory (47, 48) also shows that there is a simple, universal, thermodynamic relationship between the width of the three-phase region, the composition (i.e., amount of solubilized oil or brine) of the middle-phase microemulsion at optimum, and the optimal tensions. The results show that, except for small exceptions the goals of simultaneously lowering the tensions and increasing the width of the three-phase region are mutually... 

A typical variation of the interfacial tension and solubility parameters is shown in Fig. 20 [63] 7mo and 7mw refer to the interfacial tensions between the microemulsion (water phase) and the excess oil, and between the microemulsion (oil phase) and the excess water, respectively. The solubilization parameters are also defined for two- and three-phase systems. In the last case, the two solubilization parameters can be measured at the same time, Vo and Vw being the amount of oil and the amount of water, respectively, that are solubilized into the middle-phase microemulsion. 

It is believed that the formation of a microemulsion can enhance detergent action since the oil-water interfacial tension can be lowered considerably, which facilitates solubilization of oily dirt particles by surfactant. The microemulsion is of Winsor type III (Section 3.13.2), with small amounts of surfactant forming a middle phase microemulsion in equilibrium with excess oil and water. The oil-water interfacial tension is a minimum at the phase inversion temperature (PIT) of an oil-water-surfactant system, so it is desirable to optimize the properties of the detergent mixture so that the system is close to the PIT at the washing temperature. Microemulsions made from mixtures of nonionic surfactants are used in hard surface cleaning products. Usually they are sold in concentrated form and diluted prior to use. 

Figure 8.32 Mechanism of phase inversion of microemulsions. Adapted from Shah et aL [168] per Zajic and Panchal. [169]. The lamellar phase ("middle phase microemulsions) coexist with bulk aqueous and non-aqueous phases. These systems and their solubilizing ability has been treated theoretically by Huh [170].

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