Solubilization limit

For a given surfactant, the ability to form a single-phase w/o microemulsion is a function of the type of oil, nature of the electrolyte, solution composition, and temperature (54-58). When microemulsions are used as reaction media, the added reactants and the reaction products can also influence the phase stability. Figure 2.2.4 illustrates the effects of temperature and ammonia concentration on the phase behavior of the NP-5/cyclohexane/water system (27). In the absence of ammonia, the central region bounded by the two curves represents the single-phase microemulsion region. Above the upper curve (the solubilization limit), a water-in-oil microemulsion coexists with an aqueous phase, while below the lower curve (the solubility limit), an oil-in-water water microemulsion coexists with an oil phase. It can be seen that introducing ammonia into the system results in a shift of the solubilization... 

The very complex variation of the amount solubilized, as well as physico-chemical properties, with chemical structure of solubilizate and surfactant as well as with surfactant concentration cannot be adequately discussed solely in terms of the energetical conditions of the solubilizate in the micelles. Thus one should also consider the conditions in the phase which separates out at the solubilization limit this is in most cases a liquid crystalline phase. A fundamental basis for a proper understanding of solubilization in surfactant systems is, therefore, a detailed information on phase equilibria in three-component systems surfactant-solubilizate-water. Due in particular... 

The actual solubilization limit depends on the temperature, the nature of surfactant, the concentration of water, and on the nature of the acid. Irrespective of size or the specific properties of the solubilized molecules, very little is known about the thermodynamics or the kinetics of the solubilization process. The association of the solute with the interface can be checked using techniques capable of yielding detailed microscopic information at the molecular level (e.g. NMR, ESR, fluorescence, hydrated electrons). 

Solubilization Limits. The solubility regions were determined by titration with the sodium chloride solution until turbidity and the results checked by long-time storage of suitable compositions. 

We begin with a relatively simple model, which was suggested some years ago by Mukeqee and which provides considerable insight on solubilization of small amounts of solutes in spherical micelles. Suppose that an aqueous micellar solution has reached its solubilization limit and is in equilibrium with an excess liquid phase of a pure hydrocarbon or some other compound of low polarity. Equating the chemical potentials j,g and of the solute in the bulk organic phase and in the micelles, we have... 

The maximum solubilization may be determined by measuring the turbidity of a micellar solution w hile adding successive increments of bulk liquid (or solid) solute. When each increment is first injected, turbidity increases because the drops of solute scatter light. If complete solubilization occurs, turbidity returns to its initial low value. If not, the solubilization limit has been reached. Caution is necessary in using this technique because many hours may be required for each increment to be solubilized as the solubilization limit is approached. When dealing with certain liquid solutes such as aromatics and amphiphilic compounds added to solutions of nonionic surfactants, one must be careful that the solute concentration where the solution becomes turbid is a true solubilization limit with an excess solute phase and not a cloud point with all the solute incorporated into micelles but with an excess water phase. 

If small amounts of hydrocarbons or long-chain polar compounds are added to an aqueous solution of a surfactant above its CMC, these normally water-insoluble materials may be solubilized in the micelles (Chapter 4). This solubilization generally causes an increase in the aggregation number of the micelle, and as the amount of material solubilized by the micelle increases, the aggregation continues to increase until the solubilization limit is reached. 

FIGURE 4-3 The effect of temperature on the solubilization of n-heptane in 1% aqueous solution of I, POE (9.2) nonylphenyl ether and II, POE (9.0) dodecylphenyl ether. , Cloud point Q, solubilization limit. Reprinted with permission from K. Shinoda and S. Friberg. J. Colloid Interface Sci. 24, 4 [1967]. 

The classic biological example of these systems is bile salt (BS)-lecithin (L)-cholesterol (Ch) micelles which have been studied in detail by QLS [239], In TC-L-Ch systems, particle size and polydispersity were studied as functions of Ch mole fraction (= 0-15%), L/TC molar ratio (0-1.6), temperature (5-85°C), and total lipid concentration (3 and 10 g/dl) in 0.15 M NaCl. For values below the established solubilization limits (A )> added Ch has little influence on the size of simple TC micelles, on the coexistence of simple and mixed TC-L micelles, or on the growth of mixed disk TC-L micelles. For supersaturated systems >1), 10 g/dl simple micellar systems (L/TC = 0) exist as metastable micellar solutions even at = 5.3. Metastability is decreased in coexisting systems... 

Figure 12.11 Decane solubilization limit in CisSE/cosurfactant/water systems. W, is the weight fraction of cosurfactant in the CisSE + cosurfactant mixture. Filled symbols ...

In some cases it is found that the action of defoaming agents may depend on the concentration of the surfactant present. If the surfactant concentration is below the cmc, the defoamer will usually be most effective if it spreads as a lens on the surface rather than as a monolayer film. Above the cmc, however, where the defoamer may be solubilized, the micelles may act as a reservoir for extended defoaming action by adsorption as a surface monolayer. If the solubilization limit is exceeded, initial defoaming effect may be due to the lens spreading mechanism with residual action deriving from solubilized material. 

High and low molecular weight homopolymers also have opposite effects on the domain size. Experimentally [110-114], the solubilization of very low molecular weight homopolymer causes either a decrease or a very slight increase in the domain size. The addition of high molecular weight homopolymer causes an increase in domain size, up to the solubilization limit of the homopolymer. 

This behavior offu means that there is a threshold effect there is a distinct solubilization limit for the homopolymer. The maximum homopolymer volume fraction corresponding to this limit, is directly proportional to the total volume fraction of core-forming material of the copolymers, 0CB. that is, a CB- The proportionality constant needs to be calculated numerically, and is discussed in Ref. 143. However, it behaves approximately as... 

This expression captures the dominant behavior of the solubilization limit. First, it is a rapidly decreasing function of the ratio of the degrees of polymerization of the two components of the core, the homopolymer and copolymer. Second, it is a decreasing function of the product /Acb-... 

Examples of the solubilization limits calculated numerically for the case of PEO swelling PEO-PS micelles in a PS matrix are shown in Figure 10. In most cases, < < cb reaches a value comparable with 0cb only if the degree of polymerization of the homopolymer is much smaller than that of the copolymer block, for example, Ahb % Acb/10. When the two have comparable degrees of polymerization, /Vhb — Nqb, the solubilization limit is very small, with... 

Figure 10 Solubilization limits of homopolymer calculated using a simple mean field model [143], These calculations are specifically for PS-PEO copolymers forming micelles in a PS matrix, swollen by PEO homopolymer, but results are similar for other systems.

Nonionic ethoxylated surfactants can also be used to produce isotropic W/O microemulsions. A low HLB number surfactant may be dissolved in an oil, and such a solution can solubilize water to a certain extent, depending on surfactant concentration. If water is added above the solubilization limit, the system separates into two phases W/O solubilized - - water. If the temperature of such a two-phase system is reduced an isotropic W/O microemulsion is formed below the solubilization temperature. If the temperature is then further reduced below the haze point, sep-... 

Therefore, maximum solubilization is obtained with the optimal value of the interfacial curvature and of the elasticity where the bending stress and the attractive forces between droplets are minimized. This is illustrated in Figure 9.6 (32). The left side of the figure (a) represents a region where microemuisions with highly curved and rigid interfaces are in equilibrium with water in excess at the solubilization limit. The de-mixing is... 

In this section, we will discuss the characterization of a droplet microemulsion in more detail and in particular we will focus on spherical droplets. It turns out that spherical droplets are present in particular regions of the phase diagram, namely near the phase boundary, or solubilization limit, where the microemulsion is in equilibrium with the dispersed solvent (i.e. oil in the case of oil-in-water droplets). 

The generalization that antifoams must be present as undissolved entities has, however, occasionally been challenged [6,9,10]. A number of authors in fact report experimental results that purport to show antifoam effects due to additives that are solubilized in the foaming solution [11-13]. Thus, Ross and Haak [11], for example, identify two types of antifoam behavior associated with the effect of oils like tributyl phosphate and methyl isobutyl carbinol on the foam behavior of aqueous micellar solutions of surfactants such as sodium dodecylsulfate and sodium oleate. Wherever the oil concentration exceeds the solubility limit, emulsified drops of oil contribute to an effective antifoam action. However, it is claimed [11,14] that a weak antifoam effect is associated with the presence of such oils even when solubilized in micelles. The consequences of all this behavior are revealed if, for example, tributyl phosphate is added to micellar solutions of sodium oleate [11] at concentrations below the solubilization limit. A marked decrease in foamability is found immediately after dispersing the oil. As the oil becomes slowly solubilized, the foamability increases. However, even after the oil is completely solubilized, the foamability is still apparently less than that intrinsic to the uncontaminated surfactant solution [11]. By contrast, Arnaudov et al. [7] have more recently shown that the significant antifoam effect of n-heptanol on aqueous micellar solutions of sodium dodecylbenzene sulfonate (in the presence of NaCl) is almost completely eliminated after solubilization. 

Figure 8.9 Shift of optimum emulsification peak by addition of lauryl alcohol (emulsions contain 30 % oil phase, 65 % deionized water, and 5 % surfactant mixtures. Surfactant mixtures consist of hydrophilic Tween 20 and lipophilic Span 20 at ratios and corresponding HLB values indicated by abscissa). Dotted lines represent data for pure mineral oil systems. Solid lines represent data for oil mixture consisting of 8 parts mineral oil and 2 parts lauryl alcohol. O mean droplet size A solubilization limit. From Lin et al [44] with permission.

Saturated monoglycerides in amounts over the solubilization limit tend to precipitate as a network between fat or sugar crystals, which causes bulky sediments and results in better stability against oiling-out. 

Figure 11.2 Apparent order parameter, S, and isotropic hypeifine splitting, a, of 5-doxylstearic acid as a function of water content in water/C,2EOf cyclohexane and water/CnEOfcyclohexane systems, a and b denote the solubilization limits in CnEOs and Ci EO systems, respectively. The oil/surfactant ratio in the C EOs system is 1.5 and in the C16EO4 system is 1.63 (Reproduced by permission of the American Chemical Society from ref. 27)...

The results shown in Table 12.4 demonstrate that for an extractant/soil mass ratio of six the extracted amounts of pyrene from the contaminated soil material is nearly the same for a — 0.5 and a = 0.3. Even for the lower oil content the pyrene concentration is far from the solubilization limit. It is worth noting that the results for the extraction with microemulsions at extraction temperatures much lower than the boiling point of toluene are higher than for the extraction with hot toluene. 

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