The Surfactant Phase

It may not be difficult to understand that while pure water and an immiscible liquid can form emulsions, no purpose is served by such emulsions if particle synthesis is the goal. Obviously, some precursor of the expected particulate end product must be present in one of the phases. Of the two phases, water is logically a significantly better choice as host, as the polar molecules of water can easily harbor cations and anions from dissolved salts, as also other charged and neutral species by forming hydration shells. Thus, this water containing dissolved species must be used as the dispersed phase in the emulsions we are talking about, i.e. the W/O type, commonly known as the reverse or invert type. This will be further discussed in Section 1.6 and Chapter 3. 

The water phase, then, cannot be just pure water in the present case. The simplest substitute is a salt solution in water [18]. Another option, used extensively [30, 31], is an aqueous sol. This necessitates further discussion on sols and their preparative routes. 

Inorganic polymers can also be produced in aqueous solutions by dissolving a salt in water and heating the solution near the boiling point with dropwise addition of ammonium hydroxide [34]. This leads to cation hydrolysis and formation of mono- to polynuclear species. In another route, an aqueous solution is treated with a solution of an organic amine in a water-immiscible organic liquid. In the process, anions of the salt are extracted and hydroxyls are generated in the aqueous part [35]. This part can be separated with ease and used in emulsions. 

Particulate sols are often prepared via the following steps  

Surface active agents are divided into four major types according to the charge carried by the surface-active group [37]  

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In this study we examined the influence of concentration conditions, acidity of solutions, and electrolytes inclusions on the liophilic properties of the surfactant-rich phases of polyethoxylated alkylphenols OP-7 and OP-10 at the cloud point temperature. The liophilic properties of micellar phases formed under different conditions were determined by the estimation of effective hydration values and solvatation free energy of methylene and carboxyl groups at cloud-point extraction of aliphatic acids. It was demonstrated that micellar phases formed from the low concentrated aqueous solutions of the surfactant have more hydrophobic properties than the phases resulting from highly concentrated solutions. The influence of media acidity on the liophilic properties of the surfactant phases was also exposed. 

In the PIT range Shinoda observed an isotropic liquid phase called the surfactant phase (31) the general features of the micellar solution regions are illustrated in Figure 3. The influence of several factors on the size of the various regions has been amply described (32, 33). 

Figure 4. Formation of normal micelles (left), the surfactant phase (middle), and inverse micelles (right) may be referred to the relative size of the interfacial tensions against the oil (yo/n) and the water (jw/p) a plane interface (32)...

In this context it is instructive to ruminate on the structure of the surfactant phase. A representative composition of the phase would be 10% emulsifier and equal amounts of water and hydrocarbon. The conclusions giving a layer structure (31, 32, 33) appear to be a reasonable basis for discussing the energy conditions implied in the structure. If an area per molecule of 10-18 m2 is considered reasonable (39), the water and oil layers are approximately 1.2 X 10"8 m thick. Low angle x-ray determinations have shown that the structure does not consist of regular layers with constant spacings a structure which would accommodate the factors which determine stability would be difficult to envision. Further, since the phase is an isotropic liquid, a regularly layered structure is excluded. 

Figure 7. Close to the PIT value two phases with a lamellar structure exist. One of these, the surfactant phase (S), is an isotropic liquid, the other one, (N), is an optically anisotropic liquid crystal with a lamellar structure.

Figure 8. The possibility of a non-regular structure of the surfactant phase containing both oil ana water dispersed and continuous is attractive but does not appear probable...

The presence of a liquid crystalline phase at high surfactant concentrations has been shown by Shinoda (31), but the method of presentation renders the evaluation of the temperature dependence of necessary emulsifier concentrations to obtain the liquid crystalline phase difficult. Although several phase diagrams of the system (water, emulsifier, and nonionic surfactant) have been published (4, 45, 46, 47, 48), no results have been given on the relation between the surfactant phase and the lamellar liquid crystalline phase in these systems. 

The variation of the phase regions with temperature is illustrated in Figure 9 (38). At the PIT value (A) the surfactant phase forms an... 

The surfactant phase diagrams for several surfactants have been developed in order to understand the phase structure of surfactants in solution at high concentration. With these... 

In summary, several phenomena occurring at the optimal salinity in relation to enhanced oil recovery by macro- and microemulsion flooding are schematically shown in Figure 18. It is evident that the maximum in oil recovery efficiency correlates well with various transient and equilibrium properties of macro- and microemulsion systems. We have observed that the surfactant loss in porous media is minimum at the optimal salinity presumably due to the reduction in the entrapment process for the surfactant phase. Therefore, the maximum in oil recovery may be due to a combined effect of all these processes occurring at the optimal salinity. 

The experimental data for this example are those shown earlier in Table 7.2, and the solubilization data (V Afs = C23/C33 and V Afs = Cu/Css) are shown in Figure 7.10. In the phase behavior tests, the surfactant concentration is 1 wt.% that is treated approximately as vol.%. The water/oil ratio is 1. Find the surfactant phase behavior parameters required in simulation C33maxo, C33maxi/ C33max2, Qpl Qpr/ Csei, and Cse . 

The amphiphile-rich phase is also called the surfactant phase or the middle phase. These terms, due to Shinoda, result from the physical appearance of a three-phase system ... 

The maximum volume of the surfactant (middle) phase D at the temperature where all three phases exist is dependent upon the percentage of surfactant in the system. If the percentage is very small, the surfactant phase may not be visible to the naked eye and the system may appear to contain only two phases if the... 

From the above discussion, it should be apparent that for POE nonionics, there is a particular temperature where the hydrophilic and lipophilic characters of the surfactant balance each other and yow is at, or close to, its minimum value. It is usually defined operationally, for example, as the temperature where the surfactant phase solubilizes equal volumes of water and nonpolar material or the temperature at which an emulsion (Chapter 8) of the surfactant, water, and nonpolar material inverts. In the latter case, it is known as the phase-inversion temperature (PIT) (Chapter 8, Section IVB). Similarly, there is an electrolyte content at which the hydrophilic and lipophilic characters of ionic surfactants balance. The point at which equal volumes of water and nonpolar material are solubilized into the surfactant is known as the optimal salinity (Healy, 1974) and has been extensively investigated for enhanced oil recovery (Healy, 1977 Hedges, 1979 Nelson, 1980). The optimal salinity or PIT is at or close to the point where the parameter Vh/lcao (Chapter 3, Section II) equals 1 and lamellar normal and reverse micelles are readily interconvertable. 

The larger the volume of water (VV) (or nonpolar material Vo) solubilized into the surfactant phase relative to its volume Vs, the lower the interfacial tensions yDW, yOD, and yow (Robbins, 1974 Healy, 1976). This is understandable, since for both normal and reverse micelles, the interfacial tension against the second liquid phase decreases as the amount of second phase solubilized increases. The greater the amount solubilized in the presence of excess solubilizate, the more closely the natures of the two phases approach each other. 

As was mentioned earlier in this chapter, it is not necessary to transfer every reaction mixture into a thermodynamically stable one-phase system. Often the presence of one organised surfactant phase in equilibrium with one or two excess phases is sufficient to give an appropriate reaction rate. In such two- or three-phase systems the reaction occurs in the surfactants phase while the coexisting phases act as reservoir for the reactants. This approach has been demonstrated for alkylation of phenol [28] and for rhodium catalysed hydroformylation of dodecene [50]. A major practical advantage with the multi-phase systems is that substantially less surfactant is needed. This reduces costs and simplifies the work-up. 

The prerequisites to be fulfilled for a structural preservation of a preformed LLC assembly are as follows (i) the sohdification has to be irreversible, (ii) the resulting sohd product should not compete with the surfactant head groups for water, as this would result in substantial changes of the composition with the consequence of phase changes, (iii) no macroscopic demixing must occur, which indicates that the sohd substance has to be compatible either with the hydrophobic, or (more commonly) with the hydrophilic domains of the phase, (iv) the presence of reactants or the release of by-products should not affect the surfactant phase structure, and (v) the synthesis has to occur at moderate temperature or at least far below the boiling point of the least volatile component (usually water). 

A microemulsion is thus anomalous, because it is relatively fluid and much less viscous than expected. This has been linked with the flexibility of the structure and its transient behavior, as early mentioned byWinsor and Shinoda for the surfactant phase, as they called it (they did not use the word microemulsion). This is particularly true in the case in which extremely low interfacial tension allows easy deformation and low interfacial curvature, a feature that can be attained by adjusting the physicochemical formulation. 

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