Micellar solubilization, interfacial

SEAR) A remediation technology based on reservoir chemical flooding principles (micellar solubilization and/or low interfacial tension flooding) and applied to the treatment of NAPL-contaminated soils. 

The concentration at which this phenomenon occurs is called the critical micelle concentration (CMC). Similar breaks in almost every measurable physical property that depends on size or number of particles in solution, including micellar solubilization of solvent-insoluble material (Chapter 4) and reduction of surface or interfacial tension (Chapter 5), are shown by all types of surfactants—nonionic, anionic, cationic, and zwitterionic in aquecus media. 

Over 50 methods have been employed in the literature to determine CMC values of bile salt solutions (reviewed in [6]). These can be divided into two broad categories (a) methods requiring no physical or chemical additive in the bulk solution and (b) methods involving the use of an additive in the bulk solution. The former methods, also called non-invasive, include surface tension and the measurements of a variety of colligative bulk properties (conductivity, turbidimetry, osmometry, self-diffusion, refractive index, modal volumes, electrometric force) or electromagnetic bulk properties (NMR, sound velocity and adsorption, etc.), all as functions of bile salt concentration. The second set of methods, also called invasive, depends upon a change in some physical or chemical property of an additive which occurs with the formation of micelles. These include the spectral change of a water-soluble dye, micellar solubilization of a water-insoluble dye, interfacial tension at liquid-liquid interfaces, and partition coefficients between aqueous and immiscible non-polar phases. Whereas a detailed discussion of the merits and demerits of both approaches can be found elsewhere [6], non-invasive methods which are correctly utilized provide the most reliable CMC values. 

Surfactants Surfactants are amphiphilic compounds with both hydrophilic and hydrophobic moieties. Because of their amphiphilic nature, surfactants accumulate at interfaces and thus minimize system-free energies. Surfactants increase PAH solubility by lowering interfacial tension as well as by accumulating the hydrophobic materials in micelles (the micellar solubilization) (Rosen, 1978 West and Harwell, 1992). 

A number of batch and column studies have sought to enhance the solubilization and desorption of PAHs from soil. Surfactants are commonly used to remediate PAH-contaminated soil. Two methods are employed micellar solubilization and PAH mobilization by reduction of interfacial tension (West and Harwell, 1992). As surfactant toxicity became a significant issue, biodegradable and biocompatible surfactants have been more widely examined. For example, food-grade surfactants such as Tergitol 15-S-X (X = 7,9, and 12) (Li and Chen, 2002) and other surfactants with indirect food additive status, such as alkyl diphenyl disulfonate (DOWFAX) (Deshpande et /., 2000), have been investigated for use in solubilization/desorption of single PAHs or PAHs mixtures from contaminated soils. 

The enhanced absorption of medicinals on administration with deoxycholic acid may be due to reduction in interfacial tension or micelle formation. The inefficient absorption of reserpine promoted an investigation into its absorption in combination with deoxycholic acid [98]. Deoxycholic acid was found to increase the rapidity of absorption of reserpine and to increase its potency. The solubility of reserpine is increased in hydro-alcoholic deoxycholic acid solutions [99], it being suggested that both micellar solubilization and inclusion formation is responsible. A combination of these effects may facilitate the absorption of the reserpine. 

The solubilization of amino acids in AOT-reversed micelles has been widely investigated showing the importance of the hydrophobic effect as a driving force in interfacial solubihzation [153-157]. Hydrophilic amino acids are solubilized in the aqueous micellar core through electrostatic interactions. The amino acids with strongly hydrophobic groups are incorporated mainly in the interfacial layer. The partition coefficient for tryptophan and micellar shape are affected by the loading ratio of tryptophan to AOT [158],... 

Optimizing the formulation of micellar surfactant solutions used for enhanced oil recovery consists of obtaining interfacial tensions as low as possible in multiphase systems, which can be achieved by mixing the injected solution with formation fluids. The solubilization of hydrocarbons by the micellar phases of such systems is linked directly to the interfacial efficiency of surfactants. Numerous research projects have shown that the amount of hydrocarbons solubilized by the surfactant is generally as great as the interfacial tension between the micellar phase and the hydrocarbons. The solubilization of crude oils depends strongly on their chemical composition [155]. 

Reverse micellar extraction (RME) is another attractive LLE method for DSP of biological products, as many biochemicals including amino acids, proteins, enzymes, and nucleic acids can be solubilized within and recovered from such solutions without loss of native function/activity. In addition, these systems offer low interfacial tension, ease of scale-up, and continuous operation. RME offers a number of unique, desirable features in comparison with ATPE, which has been extensively studied ... 

For small amounts of solubilized water, as a polar additive, the stability of the micelle is markedly increased, as shown by a decrease in the CMC. On the other hand, large amounts of water as a polar additive decrease the stability of the micelle. It is known that a solution of AOT in iso-octane solubilized up to 50 moles of water per mole of surfactant. As the concentration of water increases, the isotropic reverse micellar solution changes to a water-in-oil microemulsion. A clear understanding of the complex analyte-micelle-water pool interactions, especially analyte concentration and pH at the head group interfacial region, is under intensive study (Cline Love and al., 1984 Little and Singleterry, 1964 Luisi and Straub, 1984 Mclntire, 1990). 

Micellar-polymer flooding and alkali-surfactant-polymer (ASP) flooding are discussed in terms of emulsion behavior and interfacial properties. Oil entrapment mechanisms are reviewed, followed by the role of capillary number in oil mobilization. Principles of micellar-polymer flooding such as phase behavior, solubilization parameter, salinity requirement diagrams, and process design are used to introduce the ASP process. The improvements in ""classicaV alkaline flooding that have resulted in the ASP process are discussed. The ASP process is then further examined by discussion of surfactant mixing rules, phase behavior, and dynamic interfacial tension. 

Ideally, the injected micellar solutions will be miscible with the fluids that they are in contact with in the reservoir and can thus miscibly displace those fluids. In turn, the micellar solutions may be miscibly displaced by water. Highest oil recovery will result if the injected micellar solution is miscible with the reservoir oil. If there are no interfaces, interfacial forces that trap oil will be absent. Injection of compositions lying above the multiphase boundary initially solubilizes both water and oil and displaces them in a misciblelike manner. However as injection of the micellar solution progresses, mixing occurs with the oil and brine at the flood front, and surfactant losses occur because of adsorption on the reservoir rock. These compositional changes move the system into the multiphase region. The ability of... 

Double Layer Interactions and Interfacial Charge. Schulman et al (42) have proposed that the phase continuity can be controlled readily by interfacial charge. If the concentration of the counterions for the ionic surfactant is higher and the diffuse electrical double layer at the interface is compressed, water-in-oil microemulsions are formed. If the concentration of the counterions is sufficiently decreased to produce a charge at the oil-water interface, the system presumably inverts to an oil-in-water type microemulsion. It was also proposed that for the droplets of spherical shape, the resulting microemulsions are isotropic and exhibit Newtonian flow behavior with one diffused band in X-ray diffraction pattern. Moreover, for droplets of cylindrical shape, the resulting microemulsions are optically anisotropic and non-Newtonian flow behavior with two di-fused bands in X-ray diffraction (9). The concept of molecular interactions at the oil-water interface for the formation of microemulsions was further extended by Prince (49). Prince (50) also discussed the differences in solubilization in micellar and microemulsion systems. 

The effect of alcohol concentration on the solubilization of brine has been studied in this laboratory (41). It was observed that there is an optimal alcohol concentration which can solubilize the maximum amount of brine and can also produce ultralow interfacial tension. The optimal alcohol concentration depends on the brine concentration of the system. The effect of different alcohols on the equilibrium properties and dynamics of micellar solutions has been studied by Zana (42). 

Figure 2 presents a schematic of one scenario in which nonionic surfactant may assist biomineralization. In this situation micellar nonionic surfactant has solubilized HOC from soil. As microorganisms deplete aqueous-phase HOC via mineralization, the micelle releases HOC to solution. HOC exit rates from micelles may be significantly faster than HOC desorption rates from soil, and this condition thereby potentially enhances the availability of HOC to the microorganism. Other researchers (25, 28) suggested that surfactants may make HOCs more available for microbial attack in soil by decreasing the interfacial tension between the compound and water. 

As stated above, emulsions are essentially swollen micellar systems. The differences between a micelle, containing solubilized oil, and an emulsion, comprising an oil droplet surrounded by an interfacial layer composed largely of surfactant, are difficult to assess. The droplet sizes of the dispersed phase of an emulsion can be estimated approximately by its appearance as seen in Table 39.9. 

A neutral molecule solubilized in the micelle can be located in several positions or microenvironments. As early as the 1930s it was suggested by Lawrence that the site of a solubilized molecule would be dependent on the hydrophobic/hydrophilic composition of the solubilizate. Two extremes are easily identified the core of the micelle providing a hydrocarbon-like microenvironment, and the palisade layer providing an aqueous or water-rich interfacial environment. It seems logical to assume, then, that nonpolar solutes like alkanes would prefer the micellar core and that polar molecules would be anchored at the surface. However, this is an oversimplification available data tend to contradict it. First, the solubility of alkanes in micelles is significantly lower than expected if compared to solubility in hydrocarbon solvents. Second, the size of a micelle is normally such that part of the solute would be close to the surface at any time. Sepulveda et al. state that for SDS micelles at least half of the solute will be within 4 to 5 A of the surface. We should also consider the timescale of the experiments, as the timescale for intramicellar migration is short. The rate constants of entry and exit of molecules to and from micelles is of the order 1(F and... 

The effect of the curvation of the micelle on solubilization capacity has been pointed out by Mukerjee (1979, 1980). The convex surface produces a considerable Laplace pressure (equation 7.1) inside the micelle. This may explain the lower solubilizing power of aqueous micellar solutions of hydrocarbon-chain surfactants for hydrocarbons, compared to that of bulk phase hydrocarbons, and the decrease in solubilization capacity with increase in molar volume of the solubilizate. On the other hand, reduction of the tension or the curvature at the micellar-aqueous solution interface should increase solubilization capacity through reduction in Laplace pressure. This may in part account for the increased solubilization of hydrocarbons by aqueous solutions of ionic surfactants upon the addition of polar solubilizates or upon the addition of electrolyte. The increase in the solubilization of hydrocarbons with decrease in interfacial tension has been pointed out by Bourrel (1983). 

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