Amphipathic surfactants, micellization

Once the dirty spot is removed from the substrate being laundered, it is important that it not be redeposited. Solubilization of the detached material in micelles of surfactant has been proposed as one mechanism that contributes to preventing the redeposition of foreign matter. Any process that promotes the stability of the detached dirt particles in the dispersed form will also facilitate this. We see in Chapter 11 how electrostatic effects promote colloidal stability. The adsorption of ions —especially amphipathic surfactant ions —onto the detached matter assists in blocking redeposition by stabilizing the dispersed particles. Materials such as carbox-ymethylcellulose are often added to washing preparations since these molecules also adsorb on the detached dirt particles and interfere with their redeposition. 

Effects of Surfactants on Solutions. A surfactant changes the properties of a solvent ia which it is dissolved to a much greater extent than is expected from its concentration effects. This marked effect is the result of adsorption at the solution s iaterfaces, orientation of the adsorbed surfactant ions or molecules, micelle formation ia the bulk of the solution, and orientation of the surfactant ions or molecules ia the micelles, which are caused by the amphipathic stmcture of a surfactant molecule. The magnitude of these effects depends to a large extent on the solubiUty balance of the molecule. An efficient surfactant is usually relatively iasoluble as iadividual ions or molecules ia the bulk of a solution, eg, 10 to mol/L. 

Although the notion of monomolecular surface layers is of fundamental importance to all phases of surface science, surfactant monolayers at the aqueous surface are so unique as virtually to constitute a special state of matter. For the many types of amphipathic molecules that meet the simple requirements for monolayer formation it is possible, using quite simple but elegant techniques over a century old, to obtain quantitative information on intermolecular forces and, furthermore, to manipulate them at will. The special driving force for self-assembly of surfactant molecules as monolayers, micelles, vesicles, or cell membranes (Fendler, 1982) when brought into contact with water is the hydrophobic effect. 

The impact of salt concentration on the formation of micelles has been reported and is in apparent accord with the interfacial tension model discussed in Sect. 4.1, where the CMC is lowered by the addition of simple electrolytes [ 19,65, 280,282]. The existence of a micellar phase in solution is important not only insofar as it describes the behavior of amphipathic organic chemicals in solution, but the existence of a nonpolar pseudophase can enhance the solubility of other hydrophobic chemicals in solution as they partition into the hydrophobic interior of the micelle. A general expression for the solubility enhancement of a solute by surfactants has been given by Kile and Chiou [253] in terms of the concentrations of monomers and micelles and the corresponding solute partition coefficients, giving... 

In considering the structure of micelles, we continue to base our discussion on aqueous, anionic surfactant solutions as prototypes of amphipathic systems. Cationic micelles are structured no differently from anionics, and nonionics are described parenthetically at appropriate places in the discussion. We summarize present thinking about the structure of micelles at surfactant concentrations equal to or only slightly above the CMC. We see that in nonaqueous systems (Section 8.8) and in concentrated aqueous systems (Section 8.6), the surfactant molecules are organized quite differently from the structure we describe here. 

The central core is predominantly hydrocarbon. The expulsion of the hydrophobic tails of the surfactant molecules from the polar medium is an important driving force behind micellization. The amphipathic molecules aggregate with their hydrocarbon tails pointing together toward the center of the sphere and their polar heads in the water at its surface. 

At relatively low concentrations of surfactant, the micelles are essentially the spherical structures we discussed above in this chapter. As the amount of surfactant and the extent of solubilization increase, these spheres become distorted into prolate or oblate ellipsoids and, eventually, into cylindrical rods or lamellar disks. Figure 8.8 schematically shows (a) spherical, (b) cylindrical, and (c) lamellar micelle structures. The structures shown in the three parts of the figure are called (a) the viscous isotropic phase, (b) the middle phase, and (c) the neat phase. Again, we emphasize that the orientation of the amphipathic molecules in these structures depends on the nature of the continuous and the solubilized components. 

Surfactant aggregation in an anhydrous, nonpolar medium differs in several important respects from aggregation in water. The most apparent of these differences is that the hydrophobic effect plays no role in the formation of reverse micelles. The amphipathic species are relatively passive in aqueous micellization, being squeezed out of solution by the water. In contrast, surfactant molecules play an active role in the formation of reverse micelles, which are held together by specific interactions between head groups in the micellar core. 

Surfactants are a class of amphipathic compounds that includes soaps, detergents, and emulsifiers. With the use of surfactants, hydrophobic compounds can be suspended in aqueous solution by forming micelles (see Fig. 2-7). A micelle has a hydrophobic core consisting of the hydrophobic compound and the hydrophobic tails of the surfactant the hydrophilic heads of the surfactant cover the surface of the micelle. A suspension of micelles is called an emulsion. The more hydrophilic the head group of the surfactant, the more powerful it is—that is, the greater its capacity to emulsify hydrophobic material. 

Photochemical processes in heterogeneous systems, and across micelle boundaries in particular, clearly has great potential. The photolysis of amphipathic alkylcobaloximes in mixed micelles shows a co-operative effect owing to structure.The photoreduction of anthraquinone in aqueous micellar solution has been compared with that in non-aqueous solution.The dimerization of 3-(n-butyl)cyclopentenone is solvent-dependent and the ratio of isomeric products depends on surfactant concentration. It is suggested that this can be used as a means of critical micellar concentration determination. 

VVe observe that surface-active substances that form micelles possess one common feature, namely they are amphipathic, which means that the molecule consists of two parts, one of which is highly soluble in the medium concerned and the other insoluble. We shall deal mainly with aqueous solutions, although it is important to realise that micellisation is not confined to water as solvent but can occur in many non-polar media. In the case of aqueous systems the surfactant molecule consists of a hydrophilic group (head group) to which is attached a hydrophobic hydrocarbon group (tail). 

The photoreaction of 2-substituted-1,4-naphthoquinones is accelerated by ionic surfactants but suppressed by cationic ones. Conversely, the flash photolysis of an amphipathic dodecylcarboxamide derivative of a pyridine—ruthenium(ii) complex gives the highest yield for photoreduction in cationic micelles.Anionic micelles increase electron transfer from iron(n) to iron(iii) by ca. 10 —and also the oxidation of ferrocenes by iron(in), but inhibit the oxidation by ferrocyanide. 

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