Surfactant phase behavior affected

The results obtained with C19 and as model soils suggested that the crystal form of the hydrocarbon can affect surfactant penetration and hence removal. Hexacosane (C26), with a melting point of 57 °C, allows investigation of the effect of temperature over a wider range than the systems described above. Additional details about the relationship of surfactant phase behavior to solid soil removal can be obtained, and the efficiency of the displacement mechanism can be explored further, using C2g as a model soil. 

Variables identified as important in the achievement of the low IFT in a W/O/S/electrolyte system are the surfactant average MW and MW distribution, surfactant molecular structure, surfactant concentration, electrolyte concentration and type, oil phase average MW and structure, temperature, and the age of the system. Salager et al. (1979b) classified the variables that affect surfactant phase behavior in three groups (1) formulation variables those factors related to the components of the system-surfactant structure, oil carbon number, salinity, and alcohol type and concentration (2) external variables temperature and pressure (3) two-position variables surfactant concentration and water/oil ratio. Some of the factors affecting IFT-related parameters are briefly discussed in this section. Some other factors, such as cosolvent, salinity, and divalent, are discussed in Section 7.4 on phase behavior. Healy et al. (1976) presented experimental results on the effects of a number of parameters. 

In some of these models (see Sec. Ill) the surfactants are still treated as flexible chains [24]. This allows one to study the role of the chain length and chain conformations. For example, the chain degrees of freedom are responsible for the internal phase transitions in monolayers and bilayers, in particular the hquid/gel transition. The chain length and chain architecture determine the efficiency of an amphiphile and thus influence the phase behavior. Moreover, they affect the shapes and size distributions of micelles. Chain models are usually fairly universal, in the sense that they can be used to study many different phenomena. 

The interfacial tension behavior between a crude oil (as opposed to pure hydrocarbon) and an aqueous surfactant phase as a function of temperature has not been extensively studied. Burkowsky and Marx T181 observed interfacial tension minima at temperatures between 50 and 80°C for crude oils with some surfactant formulations, whereas interfacial tensions for other formulations were not affected by temperature changes. Handy et al. [191 observed little or no temperature dependence (25-180°C) for interfacial tensions between California crude and aqueous petroleum sulfonate surfactants at various NaCI concentrations. In contrast, for a pure hydrocarbon or mineral oil and the same surfactant systems, an abrupt decrease in interfacial tension was observed at temperatures in excess of 120°C 1 20]. Non ionic surfactants showed sharp minima of interfacial tension for crude... 

It has also been shown [254] that a commercial petroleum sulfonate surfactant which consists of a diverse admixture of monomers does not exhibit behavior typically associated with micelle formation (i.e., a sharp inflection of solvent properties as the concentration of surfactant reaches CMC). These surfactants exhibit gradual change in solvent behavior with added surfactant. This gradual solubility enhancement indicates that micelle formation is a gradual process instead of a single event (i. e., CMC does not exist as a unique point, rather it is a continuous function of molecular properties). This type of surfactant can represent humic material in water, and may indicate that DHS form molecular aggregates in solution, which comprise an important third phase in the aqueous environment. This phase can affect an increase in the apparent solubility of very hydrophobic chemicals. 

The development of improved methods of surfactant design required progress in several other areas (1) understanding the mechanisms of dispersion flow in porous media, to determine which physical properties should be measured, and how their values would affect sweep control (2) measurements of these properties that are valid at the conditions under which the surfactants will be used and (3) understanding of how the values of these parameters depend on phase behavior, molecular structure, and other thermodynamic variables. 

The phase behavior of surfactant formulations for enhanced oil recovery is also affected by the oil solubilization capacity of the mixed micelles of surfactant and alcohol. For low-surfactant systems, the surfactant concentration in oil phase changes considerably near the phase inversion point. The experimental value of partition coefficient is near unity at the phase inversion point (28). The phase inversion also occurs at the partition coefficient near unity in the high-surfactant concentration systems (31). Similar results were also reported by previous investigators (43) for pure alkyl benzene sulfonate systems. 

Surfactant solution phase behavior is strongly affected by the salinity of the brine. In general, increasing the salinity of the brine decreases the solubility of the anionic surfactant in the brine. The snrfactant is driven out of the brine as the electrolyte concentration increases. Fignre 7.3 shows that as the salinity is increased, the surfactant moves from the aqneons phase to the oleic phase. At a low salinity, the typical snrfactant exhibits good aqueous-phase solubility. The oil phase, then, is essentially free of snrfactant. Some oil is solubilized in the cores of micelles. 

Based on this concept, a crude oil/surfactant/brine system should have phase behavior (e.g., optimum salinity, IFT minima) similar to that of the pure alkane/ surfactant/brine system whose ACN is the same as the crude EACN. However, the concept of EACN is not practically applicable for several reasons. First, all the hydrocarbon compositions of a crade oil are not readily identified. Thus, the EACN of a crude oil cannot be calculated directly using Eq. 7.79. Second, measurement of the EACN of a crude oil requires a series of surfactant solutions to be tested to obtain individual minimum ITT. Then these surfactant solutions are tested against increasing alkane carbon numbers to find minimum IFTs. The ACN at which a surfactant solution also gives the lowest IFT for the crude oil is the EACN of the oil. Finding it is not an easy task. Third, several parameters affect the value of the EACN. Variations in EACN with alcohol cosolvent type, total WOR of the sample, and crude oil composition have been observed (Tham and Lorenz, 1981). In practice, we always select surfactants by scan tests using the actual crude oil for a specific application. 

Davis summarized the concepts about HLB, PIT, and Windsor s ternary phase diagrams for the case of microemulsions and reported topologically ordered models connected with the Helfrich membrane bending energy. Because the curvature of surfactant lamellas plays a major role in determining the patterns of phase behavior in microemulsions, it is important to reveal how the optimal microemulsion state is affected by the surface forces determining the curvature... 

To summarize, the lecithin studies provide a qualitative picture of how the solvent affects the phase behavior for a zwitterionic surfactant with a large hydro-phobic moiety. Lecithin forms lamellar lyotropic liquid crystals with a wide variety of solvents a sufficiently hydrophilic solvent — and indeed even amphiphilic compounds with a hydrophilic moiety — stabilizes lamellar phases. 

For these transient networks formed by the interaction of an ABA triblock copolymer and a microemulsion it has been shown that their principal viscoelastic properties are not affected significantly by the chemical nature of the microemulsion, i.e., they are similar for systems with both nonionic and ionic surfactants. Also it should be noted that the phase behavior of the corresponding microemulsion is qualitatively preserved, i.e., the reversible aggregation of the nanodroplets and the phase transitions to lyotropic liquid crystalline phases remain essentially unchanged (although the concentrations at which they occur might... 

Most studies have focused on ternary and quaternary systems (with an electrolyte as the fourth component or an alcohol as cosurfactant in the latter) of both ionic [6,37-41] and nonionic [28,42] surfactants. The shape of the transient birefringence signal and the number, amplitude, and rate of the relaxations typically depend on composition, temperature, and field strength. Since the thermodynamic conditions affect the aggregation number ( , size, and stability of the particles as well as the phase behavior of the system, the distance (Tc — T) from a critical temperature and the distance from a critical composition Cc also have a major influence. 

Table I summarizes the qualitative changes in the phase behavior of microemulsions containing ionic surfactants. Some details of the effects of different variables are available in Ref. 13 and various chapters in this book. The phase transitions are generally understood in terms of relative strengths of hydrophilic and hydrophobic properties of the surfactant film in the microemulsion. The phase behavior depends strongly on the type and structure of the surfactant. For example, microemulsions containing nonionic surfactants are less sensitive to salinity but are more sensitive to temperature than those with ionic surfactants. The partitioning of cosolvents such as alcohols between the surfactant film, the organic phase, and the aqueous phase also affects the phase behavior. Microemulsions can be tailored for specific applications by adjusting an appropriate variable. For example, as indicated in Table 1, the effect of salinity on the phase behavior can be counterbalanced by an increase in the pH of an appropriate microemulsion [18,19].

The phase behavior of surfactant formulations for enhanced oil recovery is also affected by the oil solubilization capacity of the mixed micelles of surfactant and alcohol. For low concentration surfactant systems, the surfactant concentration in the oil phase changes considerably near the phase inversion point. 

During formulation of a suitable microemulsion liquid membrane, the researcher must be able to incorporate a certain amount of additives such as the liquid ion exchanger for metal ion separations. This additive will affect the microemulsion phase behavior and require a screening of surfactant types and concentrations to obtain the desired microemulsion properties discussed in Sec. III. A. In some cases (e.g., quaternary amines), the additive itself is so interfacially active that a microemulsion cannot form. 

The phase behavior of ternary and quaternary systems of the type water-oil-surfactant-cosurfactant is affected strongly by the addition of other components. Therefore, it is questioned how the solubilization of soil during the use of microemulsions as cleaning media in the washing process influences their existence region in the phase diagram and their solubilization power. To test this effect, the temperature dependence of the phase behavior of samples 10-28 (see Table 2) after their use in model... 

Jandera and Fischer [7] pointed out to the severe inconsistency of the equations of retention stated above, to describe the behavior of antibinding solutes. They postulated that as the repulsion forces hinder the molecules of a solute to come into close contact with the molecules of surfactant, a part of the stationary phase with adsorbed surfactant becomes inaccessible. This means that only a fraction of the real volume of stationary phase, Vso, affects the phase ratio in the column. The relative reduction in accessible volume, AVs, is proportional to the amount of adsorbed surfectant, Qcmo which is assumed to be constant above the cmc ... 

The presence of both polymers and surfactants affects phase behavior as well. For instance, the presence of SDS increases the cloud point temperature of PEO. The negatively charged micelles produce an electrical repulsion between polymer chains that impedes their aggregation to form a polymer-rich phase. (SDS... 

For certain classes of anionic surfactants, other effects are also important. For example, changes in pH affect the degree of ionization and hence the phase behavior when the surfactants include organic adds, amines, or other pH-sensitive compounds. An increase in the degree of ionization produces a more hydrophilic surfactant film and hence, other things bdng equal, increases optimal salinity (Qumbuddin et al., 1984). 

Multivalent cations affect phase behavior, hence, optimal salinity, more than the effect of an equal molar quantity of monovalent cations and the multivalent to monovalent cation effectiveness ratio increases with decreasing surfactant concentration. Consequently, ion exchange during a chemical flood can influence... 

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