Upper-phase microemulsion

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). 

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. 

Upper-Phase Microemulsion A microemulsion, with a high oil content, that is stable while in contact with a bulk-water phase, and in laboratory tube or bottle tests tends to be situated at the top of the tube, above the water phase. See also Microemulsion. 

The dynamic behavior of the PDM system in the salinity range between optimum salinity and the upper-phase microemulsion region was similar to that below optimum in that intermediate brine and microemulsion phases formed. Figure 13 shows these phases for the 1.5 gm/dl-salinity PDM system. The middle-phase type of microemulsion, being high in oil content at these salinities, grew more rapidly in the direction of the oil phase than it did at low... 

The term "microemulsion" will be used as defined by Healy and Reed (2) "a stable translucent micellar solution of oil, water that may contain electrolytes, and one or more amphiphilic compounds (surfactants, alcohols, etc.)". Microemulsions have been classified as lower phase (A), upper phase (u), or middle phase (m) in equilibrium with excess oil, excess water, or both excess oil and water respectively. On increasing salinity, phase transitions take place in the direction of lower middle - upper phase microemulsions. 

When the NaOH-free aqueous solutions were equilibrated with equal amounts of dodecane, the resulting systems contained two phases, an upper phase microemulsion and an excess brine phase, at salinities exceeding 0.8 gm/dl NaCl. Three phases were observed at 0.8 gm/dl NaCl, an excess-oil phase, a middle-phase microemulsion and an excess-brine phase. Small aliquots of concentrated NaOH were added to the other systems to observe the uppers middle lower phase transitions where possible. 

The effect of pH on phase behavior of microemulsions has been discussed in a different paper (4). In general, an increase in pH by addition of NaOH at constant salinity makes surfactant more hydrophilic by ionizing the carboxylic acid. Therefore, under appropriate conditions, the effect of salinity which is to make the surfactant hydrophobic, can be counterbalanced by an appropriate change in pH. The amount of NaOH, or equivalently, the pH needed for an upper phase microemulsion to shift to a middle phase increases with increasing salinity. Thus, the concentrations are 0.03M and 0.1M NaOH for 2 and 7 gm/dl NaCl, respectively. The upper + middle + lower phase transitions were observed with pH adjustment for salinities less than 5 gm/dl NaCl. For higher salinities, the microemulsion remained as a middle phase even with an excess of NaOH. All the surfactant molecules are ionized in such a situation, and the salinity is too high to be counterbalanced by pH adjustment only. 

Upper-phase microemulsion Type ll(-i-) microemulsion Winsor Type II microemulsion a-type microemulsion Oil-external microemulsion... 

The system has two phases an excess oil phase and a water-external microemulsion phase. Because microemulsion is the aqueous phase and is denser than the oil phase, it resides below the oil phase and is called a lower-phase microemulsion. At a high salinity, the system separates into an oil-external microemulsion and an excess water phase. In this case, the microemulsion is called an upper-phase microemulsion. At some intermediate range of salinities, the system could have three phases excess oil, microemulsion, and excess water. In this case, the microemulsion phase resides in the middle and is called a middle-phase microemulsion (Healy et al., 1976). Such terminology is consistent with their relative positions in a test tube (pipette) with the water being the dense liquid. In the environmental sciences and engineering, however, a dense nonaqueous phase liquid (DNAPL) could be denser than water (UTCHEM-9.0, 2000). Fleming et al. (1978) used y, P, and a to name the lower-phase, middle-phase, and upper-phase microemulsions, respectively. 

In the cases in which surfactant concentrations are low, the actual salinity range for type III most likely would be wider than we measure in the salinity scans. Thus, IFT measurements for salinity scans for low surfactant concentration are usually between the upper and lower phases observed in the sample tubes. The phases may be (1) lower-phase microemulsion and excess oil [type II(-)], (2) excess brine and upper-phase microemulsion [type II (-F)], or (3) excess brine and excess oil (type III). 

Parameters that may be used to tune the phase behavior of microemulsions include salinity, surfactant type and concentration, cosolvent type and concentration, pH, oil composition, temperature, and pressure. As salinity increases, there is a steady progression from lower phase to middle phase to upper phase microemulsions. This reflects a continuous evolution of the preferred curvature of the surfactant film and corresponds to an increase in hydrophobicity with added electrolyte such as NaCl. At low salinity the droplet size in the water-continuous lower phase increases with increasing salinity. This corresponds to an increase in the solubilization of oil and is reflected in increased light scattering. As salinity increases further, the middle phase appears and is initially water-continuous. 

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. 

The importance of phase behavior on oil recovery was mentioned in the section on micellar pol3nner fluids. Healy, Reed and Stenmark (93) showed that most common types of phase behavior for these complicated surfactant-oil-water systems can be classified as lower, middle and upper phase microemulsions. Their lower phase microemulsion corresponds to the alcohol-oil-water phase behavior in Figure 6-A while the upper phase corresponds to the slope of the tie lines and the phase behavior in Figure 6-B. The phase diagram for a surfactant system which has a middle phase microemulsion as described by Healy, Reed and Stenmark is not illustrated here. 

Fig. 12. The formation of lower phase, middle phase and upper phase microemulsions due to increase in salinity of oil/brine/surfactant/alcohol systems.

The salinity of poljmier solution can influence four major parameters of surfactant-pol)mi r flooding process, namely, interfacial tension, mobility control, surfactant loss and phase behavior. When polymer solution of various salinities are equilibrated with surfactant solution in oil, the formation of lower, middle and upper phase microemulsion has been observed (1) similar to the effect of increasing connate water salinity (2,3). In general, there is an optimal salinity (2) which produces minimum interfacial tension and maximal oil recovery (1,4). On the basis of interfacial tension alone, the salinity of polymer solution should then be designed at or near the optimal salinity of the preceding surfactant formulation. 

Upper-Phase Microemulsion. A microeniulsion with a high oil... 

Figure 16.3 also shows the phase diagrams of a nonionic amphiphile-oil-water system at TT c, respectively. Because of the relative densities of the phases, for Tequilibrium with an aqueous phase, whereas for T>Tuc the system may be said to contain a lower-phase microemulsion in equilibrium with an oleic phase. However, if the existence of middle-phase microemulsions at intermediate temperatures is unknown, the respective phase pairs are likely to be called simply oil and water. 

Considerable studies have been carried out on microemulsions during the past quarter-century, during which time it has been recognized that there are three types of microemulsions lower-phase, middle-phase, and upper-phase microemulsions. The lower-phase microemulsion can remain in equilibrium with excess oil in the system, the upper-phase microemulsion can remain in equilibrium with excess water, and the middle-phase microemulsion can remain in equilibrium with both excess oil and water. As a result, the lower-phase microemulsion has been considered to be an oil-in-water microemulsion, the upper-phase microemulsion has been considered to be a water-in-oil microemulsion, whereas the middle-phase microemulsion has been the subject of much research and has been proposed to be composed of bicontinuous or phase-separated swollen micelles from the aqueous phase [33-44]. Figure 14 shows the lower-, middle-, and upper-phase microemulsions, as represented by the darker liquid in each tube [45-48]. 

The formation of lower-, middle-, and upper-phase microemulsions is related to the migration of surfactant from lower phase to middle phase to upper phase. Figiue 15 illustrates that migration of the surfactant from the... 

FIG. 14 Samples of (a) lower-, (b) middle-, and (c) upper-phase microemulsions in equilibrium with excess oil, excess water and oil, or excess water, respectively. 

One parameter that has been discovered to be crucially important in the successful implementation of the surfactant-polymer flooding process is the salinity of the aqueous phase. As discussed previously, addition of salt to the microemulsion system induces the change from lower- to middle- to upper-phase microemulsion (Fig. 15) [33]. It was found that at a particular salt concentration, deemed the optimal salinity, a number of important parameters were optimized for the oil recovery process. The optimal salinity was found to occur when equal amounts of oil and brine were solubilized by the middle-phase microemulsion [50]. 

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