Solubilization surfactant systems, importance

Surface activity is not limited to aqueous systems, however. AH of the combiaations of aqueous and nonaqueous phases are known to occur, but because water is present as the solvent phase in the overwhelming proportion of commercially important surfactant systems, its presence is assumed in much of the common terminology of industry. Thus, the water-soluble amphipathic groups are often referred to as solubilizing groups. 

This overview will outline surfactant mixture properties and behavior in selected phenomena. Because of space limitations, not all of the many physical processes involving surfactant mixtures can be considered here, but some which are important and illustrative will be discussed these are micelle formation, monolayer formation, solubilization, surfactant precipitation, surfactant adsorption on solids, and cloud point Mechanisms of surfactant interaction will be as well as mathematical models which have been be useful in describing these systems,... 

In the real world, soluble organic materials may be present in the aqueous surfactant system. As with micelles, it is important to know how these affect mixed admicelle formation and how well these organics are solubilized in the admicelle (adsolubi1ized). 

It has been well documented that surfactants self-associate in aqueous solution to minimize the are of contact between their hydrophobictails and the aqueous solution (Mukerjee, 1979 Tanford, 1980). This phenomenon occurs at a critical concentration of surfactant, the critical micelle concentration or CMC (see Figure 12.4) above where the surfactant molecules exist predominantly as monomeric units and above which micelles exist. The CMC can be measured by a variety of techniques, for example, surface tension, light scattering, osmometry, each of which shows a characteristic break point in the plot of the operative property as a function of concentration. Knowing the CMC of the particular surfactant system and understanding the conditions that may raise or lower that critical concentration is important to the design of a formulation based on micellar solubilization. 

The solubilizing capacity for a given surfactant system is a complex function of the physicochemical properties of the two components which, in turn, influence the location or sites where the drug is bound to the micelle. The molar volume of the solubilizate together with its lipophilicity are important factors, the former reducing and the latter increasing solubilization. ... 

Solubilization of hydrophobic soils is perhaps equally important to roU-up for shampoo cleaning. Shampoos are generally used at 1 to 4% surfactant concentration, well above the critical micelle concentration (cmc). In addition, shampoos are actually mixed surfactant systems consisting of mixed micelles, reducing the cmc of the system even further. Thus, hydrophobic soils of sebum and other oily soils can be solubilized by being incorporated into the structure of the micelles of shampoos. Solubilization is a very important mechanism for cleaning oily soils from hair during the shampoo process. 

A hydrophobically modified water-soluble polymer (HM-polymer) can be viewed as a modified surfactant. It forms micelles, or hydrophobic microdomains, on its own at very low concentrations (intramolecularly, at infinite dilution) and these micelles can solubilize hydrophobic molecules. Furthermore, an HM-polymer and a surfactant in general have a strong tendency to form mixed micelles in a similar way as two surfactants. Two stoichiometries are important for HM-polymer-surfactant systems, i.e. the alkyl chain stoichiometry and the charge stoichiometry. 

Other factors that can affect the ability of a particular surfactant system to solubilize materials include pH and pressure. The effects of such factors, however, have not been as extensively reported in the literature as the factors discussed above, and they are often very specific to each surfactant system. Obviously, surfactants that show extreme sensitivity to pH such as the carbox-ylate salts can also be expected to exhibit significant changes in solubilization with changes in that factor. In addition, changes in pH can affect the nature of the additive itself, producing dramatic changes in its interactions with the micelle, including the locus of solubihzation. Such effects can be especially important in many apphcations of solubilization, such as in the pharmaceutical field. 

FIGURE 16.3. The absorption of fatty nutrients in the intestines is an obviously important example of the solubilizing action of surfactant systems. 

In this chapter we examine some issues in mass transfer. The reader has already been introduced to some of the key aspects. In Chapter 3 (Section 7), flocculation kinetics of colloidal particles is considered. It shows the importance of diffusivity in the rate process, and in Equation 3.72, the Stokes-Einstein equation, the effect of particle size on diffusivity is observed, leading to the need to study sizes, shapes, and charges on colloidal particles, which is taken up in Chapter 3 (Section 4). Similarly some of the key studies in mass transfe in surfactant systems— dynamic surface tension, smface elasticity, contacting and solubilization kinetics—are considered in Chapter 6 (Sections 6, 7, 10, and 12 with some related issues considered in Sections 11 and 13). These emphasize the roles played by different phases, which are characterized by molecular aggregation of different kinds. In anticipation of this, the microstructures are discussed in detail in Chapter 4 (Sections 2,4, and 7). Section 2 also includes some discussion on micellization-demicellization kinetics. 

Blending of sucrose distearate with C EOs was shown by Aramaki et al. to have a synergistic effect on lamellar phase production in the pseudobinary system with water [102]. This translated into a substantial increase in ability to solubilize water and -decane in the bicontinuous microemulsion formed by these materials. Addition of only 10% sucrose distearate increased the capacity by approximately three times that of C12EO6 alone. Interestingly, the same effect was not found with sucrose monostearate as the HLB of the added surfactant was important. 

The common point of the different cleaning mechanisms (roll-up, solubilization, emulsification) is the wetting of the surface by an aqueous solution of one or several surfac-tant(s). To clearly understand this essential step of the cleaning process, it is important to get good information on how the surfactant system interacts at the water-solid grease (soil) and water-substrate (clean surface) interfaces. 

Solubilization of dyes and luminescence probes (see Chapters 7 and 9) by surfactants has an important role in the determination of micellar structure. Solubilization of a dye in mixed fluorinated and nonfluorinated surfactant systems is discussed in Section 7.2. 

Microemulsions or solubilized or transparent systems are very important ia the marketing of cosmetic products to enhance consumer appeal (32,41). As a rule, large quantities of hydrophilic surfactants are required to effect solubilization. Alternatively, a combination of a solvent and a surfactant can provide a practical solution. In modem clear mouthwash preparations, for example, the flavoring oils are solubilized in part by the solvent (alcohol) and in part by the surfactants. The nature of solubilized systems is not clear. Under normal circumstances, microemulsions are stable and form spontaneously. Formation of a microemulsion requires Httle or no agitation. Microemulsions may become cloudy on beating or cooling, but clarity at intermediate temperatures is restored automatically. 

For many solubilized enzymes the greatest catalytic activity and/or changes in conformation are found at R < 12, namely, when the competition for the water in the system between surfactant head groups and biopolymers is strong. This emphasizes the importance of the hydration water surrounding the biopolymer on its reactivity and conformation [13], It has been reported that enzymes incorporated in the aqueous polar core of the reversed micelles are protected against denaturation and that the distribution of some proteins, such as chymotrypsine, ribonuclease, and cytochrome c, is well described by a Poisson distribution. The protein state and reactivity were found markedly different from those observed in bulk aqueous solution [178,179],... 

The amount of water solubilized in a reverse micelle solution is commonly referred to as W, the molar ratio of water to surfactant, and this is also a good qualitative indicator of micelle size. This is an extremely important parameter since it will determine the number of surfactant molecules per micelle and is the main factor affecting micelle size. For an (AOT)/iso-octane/H20 system, the maximum Wq is around 60 [16], and above this value the transparent reverse micelle solution becomes a turbid emulsion, and phase separation may occur. The effect of salt type and concentration on water solubilization is important. Cations with a smaller hydration size, but the same ionic charge, result in less solubilization than cations with a large hydration size [17,18]. Micelle size depends on the salt type and concentration, solvent, surfactant type and concentration, and also temperature. 

The most important property of micelles in aqueous or nonaqueous solvents is their ability to dissolve substances that are insoluble in the pure solvent. In aqueous systems, nonpolar substances are solubilized in the interior of the micelles, whereas polar substances are solubilized in the micellar core in nonaqueous systems. This process is called solubilization. It can be defined as the formation of a thermodynamically stable isotropic solution with reduced activity of the solubilized material (8). It is useful to further differentiate between primary and secondary solubilization. The solubilization of water in tetrachloroethylene containing a surfactant is an example of primary solubilization. Secondary solubilization can be considered as an extension of primary solubilization because it refers to the solution of a substance in the primary solubilizate. 

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