Micelle formation, discussion

The two distinct types of SDS headgroup packing indicated by the difference spectra are consistent with the spherocylindrical model of rod micelle formation discussed above. The increase in absorbance of the difference bands with increasing salt concentration indicates a continual increase in the number of relatively ordered SDS molecules packing into the cylindrical portion of the micelle, with the subtraction procedure simply eliminating the spectral contributions from the hemispherical endcaps. 

Fig. 6.2-4. Micelle formation and isodesmic aggregation. In the type of micelle formation discussed here, n monomers combine to form an n-mer. No other sizes are present. In isodesmic association, monomers add with equal facility to monomers or aggregates of any size.

A careful characterization of copolymers is quite time consuming and a combination of methods as discussed in Section 6.1 might be considered. In practice the situation is often complicated by an amphiphilic character of the copolymers, which leads additionally to micelle formation. 

A similar multiphase complication that should be kept in mind when discussing solutions at finite concentrations is possible micelle formation. It is well known that for many organic solutes in water, when the concentration exceeds a certain solute-dependent value, called the critical micelle concentration (cmc), the solute molecules are not distributed in a random uncorrelated way but rather aggregate into units (micelles) in which their distances of separation and orientations with respect to each other and to solvent molecules have strong correlations. Micelle formation, if it occurs, will clearly have a major effect on the apparent activity coefficient but the observation of the phenomenon requires more sophisticated analytical techniques than observation of, say, liquid-liquid phase separation. 

As introduced previously, type 2 ABC triblock copolymer micelles are formed by triblock copolymers containing an insoluble A block while the B and C blocks are soluble in the considered solvent. The insoluble blocks can be located either between the two soluble blocks (BAC structure) or at one end of the triblock (ABC or ACB structures). Micelles of the latter type were discussed above for, e.g., PS-P2VP-PEO pH-responsive micelles and are indeed considered as core-shell-corona, onion, or three-layer structures since the heterogeneity in the micellar corona is observed in the radial direction (Fig. 18). Micelles formed by BAC triblock copolymers are different from the previous case because they can give rise in principle to a heterogenous corona in the lateral dimension (Fig. 18). This could induce the formation of noncentrosymmetric micelles as discussed in Sect. 7.3. 

As an extension of the perspective of micelle formation by amphiphihc block copolymers the following part will focus on two other types of polymers. The micellar structures that will discussed are (i) micelles and inverse micelles based on a hyperbranched polymers and (ii) polysoaps, that are copolymers composed of hy-drophihc and amphiphihc or hydrophobic monomers. Whereas the first class of polymers is stiU very new and only few examples exist of the synthesis and appH-cation of such stracture in catalysis, the synthesis and aggregation characteristics of polysoaps has already been intensively discussed in the hterature. 

The most fascinating characteristic some amphiphile molecules exhibit is that, when mixed with water, they form self-assembly structures. This was already discussed in Chapter 2 on micelle formation. Since most of the biological lipids also exhibit self-assembly structure formation, this subject has been given much attention in the literature (Birdi, 1999). Lipid monolayer studies thus provide a very useful method to obtain information about SAM formation, both concerning technical systems and cell bilayer structures. 

The bile salts and their ability to form mixed micelles is discussed in some detail in order to foster a better understanding of their applications. It is highly important for the electrophoretic characterization of the micellar phase, and therefore for the calculation of the distribution coefficients, to have a thorough understanding of the mode of micelle formation and structural changes achieved by alteration of the surfactant concentration and micelle composition as well as to develop strategies for micelle optimization. 

The effect of using mixtures of surfactants on micelle formation, monolayer formation, solubilization, adsorption, precipitation, and cloud point phenomena is discussed. Mechanisms of surfactant interaction and some models useful in describing these phenomena are outlined. The use of surfactant mixtures to solve technological problems is also considered. 

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,... 

The higher the negative deviation from ideality in monolayer formation, the lower the concentration required to attain a given surface tension below the CMC. The higher this deviation for micelle formation, the lower the CMC. Since the CMC is where the surface tension approximately levels out at near a minimum value, the minimum surface tension in such a system represents the relative enhancement of monolayer formation over micelle formation. This relative favorability of aggregate formation is often an important factor in many applications, as will be further discussed in this article. 

The same effect is seen when a non—aromatic cationic surfactant/nonionic surfactant system is used. Since the nonideality of mixed micelle formation in this case is due almost entirely to the electrostatic effects and not to any specific interactions between the dissimilar hydrophilic groups, the geometrical effect just discussed will cause the EO groups to be less compactly structured... 

This brief review has attempted to discuss some of the important phenomena in which surfactant mixtures can be involved. Mechanistic aspects of surfactant interactions and some mathematical models to describe the processes have been outlined. The application of these principles to practical problems has been considered. For example, enhancement of solubilization or surface tension depression using mixtures has been discussed. However, in many cases, the various processes in which surfactants interact generally cannot be considered by themselves, because they occur simultaneously. The surfactant technologist can use this to advantage to accomplish certain objectives. For example, the enhancement of mixed micelle formation can lead to a reduced tendency for surfactant precipitation, reduced adsorption, and a reduced tendency for coacervate formation. The solution to a particular practical problem involving surfactants is rarely obvious because often the surfactants are involved in multiple steps in a process and optimization of a number of simultaneous properties may be involved. An example of this is detergency, where adsorption, solubilization, foaming, emulsion formation, and other phenomena are all important. In enhanced oil recovery. 

In this paper, we report the solution properties of sodium dodecyl sulfate (SDS)-alkyl poly(oxyethylene) ether (CjjPOEjj) mixed systems with addition of azo oil dyes (4-NH2, 4-OH). The 4-NH2 dye interacts with anionic surfactants such as SDS (11,12), while 4-OH dye Interacts with nonionic surfactants such as C jPOEn (13). However, 4-NH2 is dependent on the molecular characteristics of the nonionic surfactant in the anlonlc-nonlonic mixed surfactant systems, while in the case of 4-OH, the fading phenomena of the dye is observed in the solubilized solution. This fading rate is dependent on the molecular characteristics of nonionic surfactant as well as mixed micelle formation. We discuss the differences in solution properles of azo oil dyes in the different mixed surfactant systems. 

The conditions for synergism in surface tension reduction efficiency, mixed micelle formation, and Surface tension reduction effectiveness in aqueous solution have been derived mathematically together with the properties of the surfactant mixture at the point of maximum synergism. This treatment has been extended to liquid-liquid (aqueous solution/hydrocarbon) systems at low surfactant concentrations.) The effect of chemical structure and molecular environment on the value of B is demonstrated and discussed. 

Model Development. There is vast opportunity for development of fundamentally based models to describe the thermodynamics of mixed micelle formation. As discussed in Chapter 1, regular solution theory has yielded useful relations to describe monomer—mi cel 1e equilibrium. 

The lack of certain critical data for these systems, as already discussed, has hampered development of improved theories. Models of mixed micelle formation need to be based on the fundamental forces causing nonidealities of mixing. Some of these have been discussed in Chapter 1. Chapter 2 Schechter is an example of the... 

The same thermodynamic quantities needed for mixed micelle formation (already discussed) are also needed for mixed admicelle formation. Luckily, the monomer-admicelle equilibrium data can be fairly easily and unambiguously obtained (e.g., see Chapter 15). This should be combined with calorimetric data for a more complete thermodynamic picture of the mixed admicelle. As with micelles, counterion bindings on mixed admicelles also need to be obtained in order to account for electrostatic forces properly. Only one study has measured counterion binding on single-component admicelles (3 .), with none reported for mixed admicelles. 

The break in curve 3 in Figure 7.14 is characteristic of this type of plot for soluble amphipathic molecules. Note that it appears in the experimental curves of Figure 7.15 also. The break is understood to indicate the threshold of micelle formation (see Chapter 1, Section 1.3a), known as the critical micelle concentration (see Chapter 8). We do not discuss this phenomenon any further since the next chapter is devoted entirely to micelles and related structures. 

In the second item above, the presence of bound and free water molecules was noted. Both bound ions and ionic surfactant groups are hydrated to about the same extent in the micelle as would be observed for the independent ions. The dehydration of these ionic species is an endothermic process, and this would contribute significantly to the AH of micellization if ion dehydration occurred. In the next section we discuss the thermodynamics of micellization, but it can be noted for now that there is no evidence of a dehydration contribution to the AH of micelle formation. The extent of micellar hydration can be estimated from viscosity... 

We conclude this section with a brief discussion of the relatively large, positive values of AS°,C, which we have seen are primarily responsible for the spontaneous formation of micelles. At first glance it may be surprising that AS for Reaction (A) is positive after all, the number of independent kinetic units decreases in this representation of the micellization process. Since such a decrease results in a negative AS value, it is apparent that Reaction (A) is incomplete as a description of micelle formation. What is not shown in Reaction (A) is the aqueous medium and what happens to the water as micelles form. The water must experience an increase in entropy to account for the observed positive values for AS °,c. 

Micelle formation in solutions of an AB diblock in low-molecular-weight A homopolymer has been considered by Leibler et al. (1983), using Flory-Huggins theory to determine the free energy of mixing of micelles. This model is discussed in detail in Section 3.4.2. 

The work was planned on the basis of a model of a dispersed solid particle onto which one type of sequences of a BG copolymer is adsorbed selectively while the other type sequence is dissolved in the dispersion medium. A sketch of this model is shown in Figure 1. The model is the result of applying the same arguments which had been advanced (12) in discussing the mechanism of stabilization of polymeric oil-in-oil emulsions by BG copolymers to the problem of stabilization of dispersions of solid particles in organic media. Previously, essentially the same arguments had led to the demonstration of micelle formation of styrene-butadiene block copolymers in organic media under certain conditions (15). 

Micelle formation is opposed by thermal agitation and c.m.c. s would thus be expected to increase with increasing temperature. This is usually, but not always, the case, as discussed on page 93. 

This model represents the most radical treatment among the three most frequently discussed approaches to describe micelle formation it simple postulates ab initio the micellization to be a phase transition. This is justified all the more if large aggregation numbers (like those often encountered in aqueous surfactant solutions) are considered. 

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