Middle-phase microemulsions macroemulsions

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 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. 

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. 

The hydrophilic-lipophilic deviation (HLD) is a dimensionless representation of SAD, given by HLD = SAD/RT. Either SAD or HLD values can be used to determine composition regions for which macroemulsions or microemulsions are likely to be stable, break or invert. Negative SAD or HLD values refer to Winsor type 1 systems (O/W), positive SAD or HLD values refer to Winsor type II systems (W/O) and SAD = HLD = 0 refers to Winsor type III systems (most of the surfactant is in a middle phase with oil and water). Much of the use of SAD and HLD has been in developing surfactant formulations. 

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.]... 

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