Solubilized systems definition

As discussed in Chapter 2, liquid crystalline regions such as the neat and middle phases formed in concentrated surfactant solutions are capable of incorporation of solubilizates. The ternary systems so formed represent a type of solubilized system although such systems, being anisotropic, are not strictly in accordance with the definition of a solubilized system proposed above. Relatively few studies of these systems have been reported and we shall in this chapter concentrate exclusively on solubilization within the micellar, L, and the reverse micellar, L2, isotropic regions. 

Figure 10 is a ternary diagram for the systems Triolein/S/ Methanol, where S is respectively 4/1 molar ratios of 2-octanol to bis(2-ethylhexyl) sodium sulfosuccinate, triethylammonium linoleate or tetradecyldimethyl ammonium linoleate at 25°C. Not much difference is noted between phase areas for the triethylammonium linoleate and bis(2-ethylhexyl) sodium sulfosuccinate systems. Both are definitely inferior to the tetradecyldimethylammonium linoleate which shows the greatest solubilized area of methanol in triolein at 25°C. 

Solubilization can be defined as the preparation of a thermodynamically stable isotropic solution of a substance normally insoluble or very slightly soluble in a given solvent by the introduction of an additional amphiphilic component or components. The amphiphilic components (surfactants) must be introduced at a concentration at or above their critical micelle concentrations. Simple micellar systems (and reverse micellar) as well as liquid crystalline phases and vesicles referred to above are all capable of solubilization. In liquid crystalline phases and vesicles, a ternary system is formed on incorporation of the solubilizate and thus these anisotropic systems are not strictly in accordance with the definition given above. 

Because aqueous micelles have a hydrophobic core they can, in effect, act as a second, nonaqueous phase in a system and greatly enhance the apparent water solubility of relatively insoluble hydrophobic organic compounds (HOC). Because this solubility enhancement is only observed at, or after, the onset of micelle formation, it is a criterion for identifying the formation of a micelle (3). The coincidence of the onset of a constant surface tension and the abrupt solubilization of a HCX is a definitive test for micelle formation. 

The solubilization process seems to be well understood on a qualitative basis. Quantitatively, however, there appears to be less agreement. First, in reporting the extent of solubilization different authors may use different definitions and concentration units, as we discuss later. Second, for a three-component system both the concentrations of the surfactant and the solute can be varied. This means that we rarely find data that are directly comparable due to variation in concentrations. Often solubilization is reported as single points along the concentration profiles of surfactant and solute. In some cases the method of measurement sets the limits. 

As the driving force for the precipitation in the GAS process is the antisolvent effect caused by the solubilization of CO2 in the liquid phase, and the solvent power of a liquid is often proportional to its density it has been found that it is possible to select the optimum thermodynamic conditions for this process by studying the volumetric expansion in the solvent caused by CO2. De la Fuente et al. presented a definition of the volumetric expansion based on the variation of the partial molar volume of the solvent, which allows an optimum selection of the solvent and the operating pressure and temperature for a particular application. These authors also showed that a study of the solubility of the solute in the solvent-C02 mixtures allows predicting if the GAS process can yield satisfactory results in systems in which there is a sharp decrease in solubility at some concentration of CO2, the performance of the GAS process will be optimum, since in this case, the precipitation will take place very quickly and homogenously upon reaching this region systems that show a slow decrease in solubility as the CO2 amount increases are likely to yield worse results, since in this case, the precipitation will take place continuously and relatively slowly as CO2 is fed to the precipitator. 

Whenever a system has a composition that lies in the polyphasic region, it will generally separate (at equilibrium) into two phases, the representative points of which are located at the two extremes of the tie-line (see Fig. 3). In most cases the tie-hne is inchned i.e., one of the phases is rich in surfactant because it is located relatively near A or far from the OW side. If it is also located far from the AW and AO sides, then it contains both W and O in sizable amounts and fits the definition of a microemulsion (shaded region). Near the upper end of the tie-line in Fig. 3, it is an O/W type microemulsion. The other extreme of the tie-line is located near the OW side and near one of the component vertices (O in Rg. 3) and thus contains essentially one of the components. It is called an excess phase, in this case an oil excess phase. In most cases, particularly with ionic surfactant, the excess phase does contain a very small concentration of amphiphile, about the critical micelle concentration (cmc). In other words, the excess phase does not contain micelles, and as a consequence no micellar solubilization of the other phase can occur in the excess phase, an important feature when the mass balance is to be discussed. 

Microemulsions are systems consisting of water, oil, and amphiphile(s) that constitute a single optically isotropic and thermodynamically stable liquid solution [6]. Using this definition of microemulsions, it follows that solutions of micelles or reverse micelles with solubilized oil and water, respectively, should also be referred to as microemulsions, and these systems are therefore included in the present chapter. 

There is some disagreement within the surfactant literature as to the exact definition of solubilization, particularly as the ratio of surfactant to additive decreases, and one approaches the nebulous frontier between swollen micellar systems and the micro- and macroemulsion regions. For present purposes, solubilization will be defined as the preparation of a thermodynamically stable, isotropic solution of a substance (the additive ) normally insoluble or only slightly soluble in a given solvent by the addition of one or more amphiphilic compounds at or above their critical micelle concentration. By the use of such a definition, a broad area can be covered that includes both dilute and concentrated surfactant solutions, aqueous and nonaqueous solvents, all classes of surfactants and additives, and the effects of complex interactions such as mixed micelle formation and hydrotropes. It does not, however, limit the phenomenon to any single mechanism of action. 

One problem with that definition is that it says nothing about the relative amounts of surfactant and additive in the system. That question will arise again in the context of microemulsions. For present purposes, we will say that in solubilization, the ratio of additive to surfactant will generally be less than two. The reasons for that limitation will be discussed a bit more later. 

If the system is a strictly ideal dilute solution with respect to all solutes, then, by definition W a l) must be equal to W a w), and there will be no solubilization. This is what is observed in the premicellar region, where the solubility of a in the solution / is almost constant and equal to the solubility in pure w up to the CMC. 

In the previous relationship, the solubilization parameters SP refer to the Vo/Vs and Vw/l s ratios of the volume of oil and water solubilized in the microemulsion phase (the surfactant-rich phase) per volume of surfactant. If the surfactant is a solid, and as a more general definition, the solubilization parameters may be taken as Vo/nts and V ulfn, where ms is the mass of surfactant. The volumes can be deduced firom the measurement of the microemulsion-phase volume at equilibrium mid the amount of surfactant, oil, and water that were added to the system in the first place. It is helpful to note that in two- or three-phase systems, the excess phase(s) contain(s) at most the critical micelle concentration of the surfactant, which is, in general, a negligible amount in the mass balance. Thus, all the surfactants can be assumed to be in the microemulsion phase, and the excess phase can be assumed to be a pure component. 

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