Phase behavior study, microemulsions

In the Will case, provided that there is enough surfactant but not too much, e.g., 1 wt. %, the system splits into three phases, i.e., a microemulsion in equilibrium with excess water and excess oil. At a higher surfactant concentration than the top vertex of the 3

single phase microemulsion often called WW behavior is attained. However, this occurrence generally requires a large amount of surfactant, e.g., 20 wt. %, which is in most practical cases too much for cost reasons. At a very low surfactant concentration, around the CMC, only two phases are in equilibrium, and the tension is not necessarily very low. Hence, the convenient surfactant concentration to carry out a phase behavior study is in the range 0.5-3 wt. % for which three-phase behavior and a very low inter facial tension is exhibited in most Will cases. 

In order to emphasize the role of the inter facial films and to highlight the most recent viewpoints on the stability of microemulsions, sponge phases, and dilute lamellar phases, some of the experimental facts about phase behavior of microemulsion systems containing alcohol are reviewed in this chapter. The systems investigated consist of water, oil, alcohol, and sodium dodecylsulfate (SDS). In the next section, the theoretical aspects of the stability of surfactant phases are briefly discussed. Then in Secs. Ill and IV the effects of varying alcohol and oil chain lengths and the addition of a water-soluble polymer are examined. The examination of multiphase regions provides the location of lines of critical points or critical endpoints. This chapter also deals with the study of several physical properties in the vicinity of critical points. 

Phase behavior studies are typically conducted in small (up to 100 mL) vials in order to determine what type, if any, of microemulsion is formed with a given micellar-cmde oil system. The salinity of the micellar solution is usually varied around the salt concentration of the field brine where the process will be applied. Besides the microemulsion type, other factors examined could be oil uptake into the microemulsion, ease with which the oil and aqueous phases mix, viscosity of the microemulsion, and phase stability of the microemulsion. 

Another interesting and important parameter that can and should be studied is the temperature level (5). This is not simply to simulate reservoir temperature but also because it is known to affect the physical properties and phase behavior of microemulsions. 

The phase behavior studies reported in Figure 4 were performed in a brine of similar salinity as in Figure 3, but containing additives to ensure long-term chemical stability of the polymer among these additives was 0.4% isopropanol. These additives have a marginal effect on phase behavior and the same increase in microemulsion/ aqueous phase interfacial tension has been observed. However, we did not find the drastic increases in viscosity reported in Figure 3. 

Microemulsions are ternary systems containing oil, water, and surfactant. The terms oil and water in a microemulsion system normally refer to oil phase (oil and oil soluble components such as cyclosporine) and aqueous phase (water and water soluble components such as sodium chloride), respectively. The phase behavior of water-oil-surfactant mixtures was extensively studied by Winsor (1948). Based on his experimental observations, Winsor classified equilibrium mixtures of water-oil-surfactant into four systems (1) type I (Winsor I) system where water continuous or oil-in-water (0/W) type microemulsion coexists with the oil phase. In these systems, the aqueous phase is surfactant-rich (2) type II (Winsor II) system where oil continuous or water-in-oil (W/0) type microemulsion coexists with the aqueous phase. In these systems, the oil phase is surfactant-rich (3) type III (Winsor III) system where bicontinuous type microemulsion (also referred to as surfactant-rich middle-phase) coexists with excess oil at the top and excess water at the bottom and (4) type IV (Winsor IV) system where only a single-phase (microemulsion) exists. The surfactant concentration in type IV microemulsion is generally greater than 30 wt%. Type IV microemulsion could be water continuous, bicontinuous, or oil continuous depending on the chemical composition. The phase behavior of microemulsions is often described as a fish diagram shown in Figure lO.I (Komesvarakul et al. 2006). 

A typical system composed of EAN/l-hexadecyl-3-methyllmidazolium chloride [Cj mim][Cl]/dodecane at ambient temperature with high thermal stability (stable between 30 and 150°C) was reported by Zech et al. [100]. Inherent properties were characterized by SANS, DLS, generalized indirect Fourier transformation, and Teubner and Strey model. A phase behavioral study with respect to temperature and IL mass fraction of a system comprising water/TX-100/[bmim][PFJ was reported by Anjum et al. [101]. SANS and polarized microscopic techniques revealed the existence of discontinnons-type microemulsion droplets in the system. 

Phase Behavior Study For characterization of different microemulsions, we studied their phase behavior. We constructed the partial phase diagram of the ternary systems RTILs/[C mim][AOT]/benzene at 298 K by visually observing the transition from the clear transparent solution to the turbid solution. 

Phase behavior study showed that with an increase in the alkyl chain of the IL anion, the single-phase region increased. Variation in chain length of IL anions was used for tuning the range of microemulsion area [42]. 

In real petroleum applications, surfactant-based systans are applied as corrosion inhibitors in the beginning of pipeline flows, together with the petroleum emulsion (crude oil + brine). The surfactant may be used either as a micellar solution or in the miCToemulsified form. Its maximal solubility must be known in advance therefore, phase behavior studies must be previously made. If the right conditions are promoted, dilution of the inhibitor occurs spontaneously along the emulsion flow. When microemulsions are used, this phenomenon is more effective, since more surfactant molecules can be solubilized in the medium. 

In this study, the phase behavior of microemulsions consisting of alkyl polyglycosides and ethoxylates as hydrophilic emulsifiers, a lipophilic coemulsifier, an oily component, and water is evaluated in terms of micro-emulsion formation and stability. Parameters such as temperature, oil polarity, and composition of the surfactant mixture are discussed. It was shown that both the concentration range and the temperature stability could be extended by using suitable mixtures of emulsifiers and coemulsifiers. 

Aughel and coworkers [63] studied the phase behavior of hydrocarbon-water mixtures in the presence of alkyl(aryl)polyoxyethylene carboxylates for enhanced oil recovery and found good salt tolerance with an alkyl ether carboxy-late (C13-C15) with 7 mol EO and a good microemulsion forming effect with the 3 EO type. 

Physical-chemical studies require traces of additives (reactants, catalysts, electrolytes) with respect to the concentration of the basic components of the microemulsion, and this causes only a minor change in the phase behavior of the system. However, when the amounts of additives are on the scale used in organic synthesis, the phase behavior, which is very sensitive to the concentration of the reactants, is sometimes difficult to control and the reaction is carried out in a one-, two- or three-phase state. 

Recently, the phase equilibria of a microemulsion were reported. The phase behavior of a microemulsion formed with food-grade surfactant sodium bis-(2-ethylhexyl) sulfosuccinate (AOT) was studied. Critical microemulsion concentration (cpc) was deduced from the dependence of the pressure of cloud points on the concentration of... 

Figure 8.4. Phase behavior of water/C02/surfactant systems studied to date. r=35°C, P = 414 bar and O T= 35°C, P=138 bar. PFPE COO NH4+ (Johnston et al., 1996) PFPE COO NH4+ (Zielinsky et al., 1997) di-HCF4 (Holmes et al., 1998). The one-phase microemulsion region is to the right of each curve.

Stoffer and Bone (15.16) studied the phase behavior and morphology of polymers obtained by polymerization of w/o microemulsions. They observed that the cosurfactant pentanol acts as a chain transfer agent. They also found that the polymer formed in a microemulsion is as large as the droplet size in the macroemulsion, thus explaining problems encountered with phase separation. Gan, Chew and Friberg (17) studied the stability behavior of w/o microemulsions containing styrene and polystyrene with particular reference to the effects of pentanol and butylcellosolve. 

Finally, in the discussion of reverse microemulsion systems, mention should be made of one of the most widely studied systems. The surfactant, sodium bis(2-ethylhexyl) sulfosuccinate or Aerosol-OT (AOT), is one of the most thoroughly studied reverse micelleforming surfactants since it readily forms reverse micelle and microemulsion phases in a multitude of different solvents without the addition of cosurfactants or other solvent modifiers. The phase behavior of AOT in liquid alkane/water systems is already well documented. Indeed, the first report of the existence of the formation of microemulsions in a supercritical fluid involved an AOT/alkane/ water system. A The spherical structure of an AOT/nonpolar-fluid/ water microemulsion droplet is shown in Fig. 1. In the now well-known structure, it can be seen that the two hydrocarbon tails of each AOT molecule point outward into the nonpolar phase (e g., supercritical fluid). These tails are lipophilic and are solvated by the nonpolar continuous phase solvent whereas the hydrophilic head groups are always positioned in the aqueous core. 

In this article we describe the phase behavior of a microemulsion system chosen for the free radical polymerization of acrylamide within near-critical and supercritical alkane continuous phases. The effects of pressure, temperature, and composition on the phase behavior all influence the choice of operating parameters for the polymerization. These results not only provide a basis for subsequent polymerization studies, but also provide data on the properties of reverse micelles formed in supercritical fluids from nonionic surfactants. 

Theoretical diffusion path studies were made with a model system for comparison to the experimentally observed phenomena. A pseudoternary representation was chosen for modeling the phase behavior, and brine and oil were chosen as the independent diffusing species. For simplicity and because their exact positions and shapes were not known, phase boundaries in the liquid crystal region were represented as straight lines. Actually, studies indicate a rather complex transition from liquid crystal to microemulsions as system oil content is increased, especially near optimum salinity (15-16). A modified Hand scheme was used to model the equilibria of binodal lobes (14,17). Other assumptions are described in detail elsewhere (13). 

The effect of polymers on microemulsions phase behavior has been reported by Hesselink and Faber (8). They have described the surfactant-polymer phase separation in terras of the incompatibility of two different polymers in a single solvent, considering the microemulsion as a pseudo-polymer system. The effect of polymers on the phase behavior of micellar fluids has been recently studied by Pope et al. (9) and others (10,11). 

This chapter covers the fundamentals of surfactant flooding, which include microemulsion properties, phase behavior, interfacial tension, capillary desaturation, surfactant adsorption and retention, and relative permeabilities in surfactant flooding. It provides the basic theories for surfactant flooding and presents new concepts and views about capillary number (trapping number), relative permeabilities, two-phase approximation of the microemulsion phase behavior, and interfacial tension. This chapter also presents an experimental study of surfactant flooding in a low-permeability reservoir. 

Dogra, A. and Rakshit A.K. (2004) Phase behavior and percolation studies on microemulsion system water/SDS + Myrj 45/cyclohexane in the presence of various alcohols as cosurfactants. /. Phys. Chem. B, 108, 10053-10061. 

Engelskirchen, S., Eisner, N., Sottmann, T. and Strey, R. (2007) Triacylglycerol microemulsions stabilized by alkyl ethoxylate surfactants - A basic study Phase behavior, interfacial tension and microstructure. /. Colloid Interface Sci., 312(1), 114—121. 

The next section describes measurements of interfacial tension and surfactant adsorption. The sections on w/c and o/c microemulsions discuss phase behavior, spectroscopic and scattering studies of polarity, pH, aggregation, droplet size, and protein solubilization. The formation of w/c microemulsions, which has been achieved only recently [19, 20], offers new opportunities in protein and polymer chemistry, separation science, reaction engineering, environmental science for waste minimization and treatment, and materials science. Recently, kinetically stable w/c emulsions have been formed for water volume percentages from 10 to 75, as described below. Stabilization and flocculation of w/c and o/c emulsions are characterized as a function of the surfactant adsorption and the solvation of the C02-philic group of the surfactant. The last two sections describe phase transfer reactions between lipophiles and hydrophiles in w/c microemulsions and emulsions and in situ mechanistic studies of dispersion polymerization. 

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