Micellization thermodynamically stable solution

Association colloids Homogeneous, thermodynamically stable solutions of spontaneous self-assembled surfactant aggregates micelles, typically composed of single-tailed surfactants reversed micelles in oil with water pools, vesicles typically composed of twin-tailed surfactants and microemulsions composed of at least oil, water, and surfactant and also with alcohols, either aqueous (oil-in-water droplets), bicontinuous (no droplets), or reverse (water-inoil droplets). 

Generally, solubilization occurs spontaneously when the pure solubilizate contacts the solution of reversed micelles. Often, vigorous stirring consistently reduces the time necessary to obtain complete solubihzation and thermodynamically stable systems. 

The rates of multiphase reactions are often controlled by mass tran.sfer across the interface. An enlargement of the interfacial surface area can then speed up reactions and also affect selectivity. Formation of micelles (these are aggregates of surfactants, typically 400-800 nm in size, which can solubilize large quantities of hydrophobic substance) can lead to an enormous increase of the interfacial area, even at low concentrations. A qualitatively similar effect can be reached if microemulsions or hydrotropes are created. Microemulsions are colloidal dispersions that consist of monodisperse droplets of water-in-oil or oil-in-water, which are thermodynamically stable. Typically, droplets are 10 to 100 pm in diameter. Hydrotropes are substances like toluene/xylene/cumene sulphonic acids or their Na/K salts, glycol.s, urea, etc. These. substances are highly soluble in water and enormously increase the solubility of sparingly. soluble solutes. 

Microemulsions are thermodynamically stable, homogeneous, optically isotropic solutions comprised of a mixture of water, hydrocarbons and amphiphilic compoxmds. The microemulsions are usually four- or three-component systems consisting of surfactant and cosurfactant (termed as emulsifier), oil and water. The cosurfactants are either lower alkanols (like butanol, propanol and hexanol) or amines (Hke butylamine, hexylamine). Microemulsions are often called swollen micelles (Fig. 3) and swollen re-... 

An alternative to the injection method for importing enzymes into a microemulsion is the phase transfer method. In this method, a layer of an aqueous enzyme solution is located under a mixture of surfactant and oil. Upon gentle shaking, the enzyme is transferred into the reverse micelles of the hydrocarbon phase. Finally, the excess of water is removed and the hydrophobic substrates can be added. The main advantage of this method is that it ensures thermodynamically stable micro emulsions with maximum water concentrations. However, the method is very time consuming. The method is often applied in order to purify, concentrate or renaturate enzymes in the reverse micellar extraction process [54-58]. 

Aqueous micelles have diameters ranging typically from 0.5 to 5 nm, and being so small, do not scatter visible light and form transparent solutions. Figure 9.6 shows some basic parameters for aqueous micelles, relative to the well-known SDS (sodium dodecylsulfate). Micelles are thermodynamically stable, and this is a signihcant difference with respect to most large vesicle aggregates. 

In contrast to the above-described kinetic stability, colloids may also be thermodynamically stable. A stable macromolecular solution is an example we have already discussed. Formation of micelles beyond the critical micelle concentration is another example of the formation of a thermodynamically stable colloidal phase. However, when the concentration of the (say, initially spherical) micelles increases with addition of surfactants to the system, the spherical micelles may become thermodynamically unstable and may form other forms of (thermodynamically stable) surfactant assemblies of more complex shapes (such as cylindrical micelles, liquid-crystalline phases, bilayers, etc.). 

Above the CMC, a number of solutes that would normally be insoluble or only slightly soluble in water dissolve extensively in surfactant solutions. The process is called solubilization, the substance dissolved is called the solubilizate, and —in this context —the surfactant is called the solubilizer. The result is a thermodynamically stable, isotropic solution in which the solubilizate is somehow taken up by micelles since the enhancement of solubility begins at the CMC. This observation, in fact, provides one method for determining the CMC of a surfactant it... 

Micelles are large polymolecular aggregates in solutions. They are thermodynamically stable because of intermolecular interactions. Some... 

The primary mechanism for energy conservation is adsorption of surfactant molecules at various available interfaces. However, when, for instance, the water-air interface is saturated conservator may continue through other means (Figure 12.3). One such example is the crystallization or precipitation of the surfactant from solution, in other words, bulk phase separation. Another example is the formation of molecular aggregates or micelles that remain in solution as thermodynamically stable, dispersed species with properties distinct from those of an isotropic solution containing monomeric surfactant molecules (Myers, 1992). 

Solubilization is the formation of a thermodynamically stable, isotropic solution of a substance (the solubilizate), normally insoluble or slightly soluble in water, by the addition of a surfactant (the solublizer). The micelles of the surfactant cause solubilization of the substrate, producing an isotropic solution of the chemical. The solubilizate can be incorporated in the surfactant micelle in different ways, depending on the nature of the substrate and the surfactant micelles. For hydrophobic substrates, the molecules become incorporated in the hydrocarbon core of the micelle. With more polar substrates, the molecules may become incorporated in the hydrophilic PEO chains of the micelle or they may be simply adsorbed at the micelle surface. 

The characteristic effect of surfactants is their ability to adsorb onto surfaces and to modify the surface properties. Both at gas/liquid and at liquid/liquid interfaces, this leads to a reduction of the surface tension and the interfacial tension, respectively. Generally, nonionic surfactants have a lower surface tension than ionic surfactants for the same alkyl chain length and concentration. The reason for this is the repulsive interaction of ionic surfactants within the charged adsorption layer which leads to a lower surface coverage than for the non-ionic surfactants. In detergent formulations, this repulsive interaction can be reduced by the presence of electrolytes which compress the electrical double layer and therefore increase the adsorption density of the anionic surfactants. Beyond a certain concentration, termed the critical micelle concentration (cmc), the formation of thermodynamically stable micellar aggregates can be observed in the bulk phase. These micelles are thermodynamically stable and in equilibrium with the monomers in the solution. They are characteristic of the ability of surfactants to solubilise hydrophobic substances. 

Emulsions made by agitation of pure immiscible liquids are usually very unstable and break within a short time. Therefore, a surfactant, mostly termed emulsifier, is necessary for stabilisation. Emulsifiers reduce the interfacial tension and, hence, the total free energy of the interface between two immiscible phases. Furthermore, they initiate a steric or an electrostatic repulsion between the droplets and, thus, prevent coalescence. So-called macroemulsions are in general opaque and have a drop size > 400 nm. In specific cases, two immiscible liquids form transparent systems with submicroscopic droplets, and these are termed microemulsions. Generally speaking a microemulsion is formed when a micellar solution is in contact with hydrocarbon or another oil which is spontaneously solubilised. Then the micelles transform into microemulsion droplets which are thermodynamically stable and their typical size lies in the range of 5-50 nm. Furthermore bicontinuous microemulsions are also known and, sometimes, blue-white emulsions with an intermediate drop size are named miniemulsions. In certain cases they can have a quite uniform drop size distribution and only a small content of surfactant. An interesting application of this emulsion type is the encapsulation of active substances after a polymerisation step [25, 26]. 

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. 

The existence of micelles in solutions of large ions with hydrocarbon chains is responsible for the observation that certain substances, normally insoluble or only slightly soluble in a given solvent, dissolve very well on addition of a surfactant (detergent or tenside). This phenomenon is called solubilization and implies the formation of a thermodynamically stable isotropic solution of a normally slightly soluble substrate (the solubilizate) on the addition of a surfactant (the solubilizer) [128, 133], Non-ionic, nonpolar solubilizates such as hydrocarbons can be trapped in the hydrocarbon core of the micelle. Other amphiphilic solutes are incorporated alongside the principal amphiphile and oriented radially, and small ionic species can be adsorbed on the surface of the micelle. Two modes of solubilizate incorporation are illustrated in Fig. 2-13. 

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. 

Microemulsions are clear (transparent and translucent are also used in the literature), thermodynamically stable, isotropic liquid mixtures of oil, water, and surfactant, frequently in combination with a cosurfactant. The aqueous phase may contain salt(s) and/or other ingredients, and the oil may actually be a complex mixture of different hydrocarbons and olehns. In contrast to ordinary emulsions, microemulsions form upon simple mixing of the components and do not require high shear conditions generally used in the formation of ordinary emulsions. Microemulsions tend to appear clear due to the small size of the disperse phase. However, clear appearance (transparency) may not be a fundamental property. Sometimes microemulsion may not look clear to the naked eye in the case where dark viscous oil exists. The solution may not be purely transparent because it contains aggregates of micelles. Quite often, we still use these terms, even in this book. Probably we should simply use the term homogeneous solution. 

Thus, microemulsions have nothing in common with macroemulsions, and in many cases it is better to describe the microemulsion system as swollen micelles. The best definition of microemulsions is as follows [3] System of Water -I- Oil -I- Amphiphile that is a single Optically Isotropic and Thermodynamically Stable Liquid Solution. Amphiphiles refer to any molecule that consist of a hydrophobic and hydrophilic portions, for example surfactants and alcohols. 

Some colloidal systems such as polymer solutions and surfactant solutions containing micelles are thermodynamically stable and form spontaneously. These types of colloids are called lyophilic colloids. However, most systems encountered contain lyophobic colloids (particles insoluble in the solvent). In the preparation of such lyophobic colloidal dispersions, the presence of a stabilizing substance is essential. Because van der Waals forces usually tend to lead to agglomeration (flocculation) of the particles, stability of such colloids requires that the particles repel one another, either by carrying a net electrostatic charge or by being coated with an adsorbed layer of large molecules compatible with the solvent. 

One of the important properties of surfactants that is directly related to micelle formation is solubilization. Solubilization may be defined as the spontaneous dissolving of a substance (solid, liquid, or gas) by reversible interaction with the micelles of a surfactant in a solvent to form a thermodynamically stable isotropic solution with reduced thermodynamic activity of the solubilized material. Although both solvent-soluble and solvent-insoluble materials may be dissolved by the solubilization mechanism, the importance of the phenomenon from the practical point of view is that it makes possible the dissolving of substances in solvents in which they are normally insoluble. For example, although ethylbenzene is normally insoluble in water, almost 5 g of it may be dissolved in 100 mL of a 0.3 M aqueous solution of potassium hexadecanoate to yield a clear solution. 

IV. Detergents are surface active substances that have features of all three surfactant groups described above, and in addition they are able to spontaneously form thermodynamically stable colloidal systems (for micellization in surfactant solutions please refer to Chapter VI). The particles that are washed away may become incorporated into the nuclei or micelles, i.e. solubilization (See Chapter VI) takes place. Various anionic, cationic, and nonionic surfactants that are encountered further in this section are typically members of this surfactant group. 

The equilibrium between dispersed phase (i.e., micelles) and molecular solution of a surfactant (or the macroscopic phase, in case of saturation) exists in thermodynamically stable systems containing micelle-forming surfactants. One can, to a certain degree of approximation, describe the equilibrium between micelles consisting of m surfactant molecules and molecularly dissolved surfactant as a chemical reaction, namely [15,16]... 

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