Lower-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). 

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

Phase Behavior on Equilibration with Oil. Microemulsions are formed when the aqueous surfactant-cosurfactant solutions are mixed with oil, and allowed to equilibrate. Figure 5(a) shows the phase behavior when 5 ml aqueous surfactant solutions (without any polymer) were equilibrated with equal volumes of n-dodecane. The salinity was varied from 0.8 to 2.2 gm/dl NaCl in 0.2 gm/dl increments. At low salinities a lower phase microemulsion exists in equilibrium with excess oil. The middle phase microemulsion appears at about... 

Lower-phase microemulsion Type ll(-) microemulsion Winsor Type I microemulsion y-type microemulsion Water-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). 

When oil is added to the surfactant and polymer solution, the system follows the typical pattern of a system without polymer—that is, from type I to type 111 to type II as salinity increases. The three-phase region simply shifts a small distance to the left on the salinity scale, compared with the three-phase region without polymer. When polymer is present, it remains almost exclusively in the most aqueous phase whether the phase is lower-phase microemulsion or excess brine. Consequently, polymers affect relative mobility of the phases generated during a chemical flood, but they do not appear to affect phase equilibria significantly (Nelson, 1982). Some of the aqueous phases in the critical region of the shift (which is also just above oil-free CEC salinity) were found to be gel-like in nature. 

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. 

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 Tupper-phase microemulsion in equilibrium 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]. 

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