Micellization processes

Figure 6.9 Schematic illustration of the micellization process. Cutaway view of spherical micelle shows hydrocarbon interior with polar heads on surface.

Fluorescence quenching studies in micellar systems provide quantitative information not only on the aggregation number but also on counterion binding and on the effect of additives on the micellization process. The solubilizing process (partition coefficients between the aqueous phase and the micellar pseudo-phase, entry and exit rates of solutes) can also be characterized by fluorescence quenching. 

TABLE 8.3 Some Thermodynamic Properties for the Micellization Process at or Near 25 °C for Various Surfactants... 

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. 

The micellization process was found to be much more strongly dependent on the insoluble block molecular weight than the soluble block molecular weight, as for non-ionic block copolymers. These conclusions were supported by fluorescence measurements by Morishima et al. (1981,1982a,6). 

Junquera, E, Tardajos, G, andAicart, E. (1993). Effect of the preserlfeeyiflodexdtrin on micellization process of sodium dodecyl sulfate or sodium per uorooctanoate in kasHtgrpuir, 9, 1212-1219. 

The thermodynamics of the micellization process were studied and gave different results at low and high F127 concentrations. In the low F-127 concentration range (u 59ang/mL), AH =... 

Table 13.2 summarizes the effect of the chain architecture, and provides a detailed comparison of micellar parameters between 17R4 and L-64 micelles, including the thermodynamic quantities of the micellization process. 

The charged micelles give rise to strong local electrical fields in the solution, which will influence the distribution and motion of other ionic entities. The micelles also repel each other reducing the translational mobility. These electrostatic interactions influence the energetics of the micellization process substantially. This is seen from the comparatively high values of the CMC for ionic amphiphiles and by the fact that the addition of salt decreases the CMC. 

It is thus clear that a treatment of the micellization process of ionic amphiphiles must include a discussion of electrostatic effects. Furthermore, even for zwitterionic and nonionic surfactants, the electrostatic effects play a role. The favorable interaction between the polar groups of these amphiphiles and the solvent water is probably mainly of an electrostatic origin. 

The micellization process can be described by the stepwise association equilibrium... 

It is important to stress that the micellar parameters presented in Tables I and II are for the indicated detergent in water at ambient atmospheric pressure and room temperature ( 25°C). The quoted values can be altered (sometimes dramatically) by changes In the experimental conditions. For instance, temperature and pressure can impact the micellization process. Typically, plots of CMC vs. temperature exhibit a minimum somewhere between 20 - 30° C for charged ionic surfactants while for nonionic surfactants, only a limiting minimum is observed at ca. 40 - 50° C 06). The micellar CMC and N can also depend upon pressure (J 6). However, at the pressures under which most separation techniques are conducted ( 3.5 MPa), the changes in micellar parameters are such that this effect can be neglected in all but the most exacting work (64). 

This chapter will be concerned with the characterization of micellization processes, physical properties of micelles, the structure of self-aggregated... 

CMCs of these polyfluorinated surfactants are of the order of 10 5 m.47 Plots of the observed H chemical shifts versus surfactant concentration of cetyl trimethyl ammonium chloride, cetyl pyridinium chloride, cetyl dimethyl phenyl ammonium chloride, cetyl dimethyl benzyl ammonium chloride, cetyl dimethy 1-2-phenyl ethyl ammonium chloride, and cetyl dimethyl-3-phenyl propyl ammonium chloride, are sigmoidal and were fitted to a model based on the mass action. The H chemical shift-based CMC values are in excellent agreement with those determined by the surface tension method.48 The micellization processes of dodecyl trimethyl ammonium halides (chloride and bromide) studied by calorimetric titration show different behaviors at 298 K. However, these disappear at 313 K, while the results measured by the chemical shift versus surfactant concentration do not show this difference.49 The CMC of 3-aminopropyl triethoxy silane in toluene is ca. 0.47m, measured by H and l3C chemical shifts.50 The CMC of optically active potassium A -n-dodecanoyl alaminate measured by H and l3C chemical shifts is lower (11-15 him) in D20 than that in a mixed solvent of 1,4-dioxane and D20 (19mM).-51 The H chemical shift shows that the CMC of resorcinol-type calix[4] phosphoric esters having four alkyl side-chains, [4]Ar 5P-R-n, is insensitive to the length of the side-chains, n.52 The CMC values of a family of surfactants, the sodium cyclohexyl alkanoates, with different lengths of the alkanoate side-chains, were obtained from 13C chemical-shift measurements. The results show that these amphiphiles have high CMCs (0.12-1.02 m).-53... 

The nucleus 31P is useful in determining the micellization processes of phosphorus-containing surfactants. For the lithium ethyl (n-octyl) phosphate in D20, a critical micelle concentration was determined by 3IP NMR spectra.54 Three CMCs were detected by the inflection points in the concentration-dependent slopes of the 3IP chemical shift of a soybean phosphatidylcholine in n-butanol. They are 7.5%, 35% and 63% w/w soybean phosphatidylcholine.-55... 

The NMR self-diffusion coefficient measurements also provide a convenient tool for the study of the micellization process. They are based on the large... 

The quadrupolar 2H spin-lattice relaxation of specifically deuterated surfactants, such as the methyl deuterated n-alkyl phosphocholine surfactants, also shows a concentration dependence. The CMC of dodecyl phosphocholine was determined by exploiting the difference in the 2H spin-lattice relaxation times of the monomers versus the micellar state.68 Quadrupole splittings of 14N were also used to characterize the micellization process of short-chain perfluoro-carboxylic acid salts.69... 

This could include retinol, cholecalciferol/ergocalciferol, a-tocopherol, vitamin K from food, all of which are more readily absorbed when they can be part of a normal mixed micelle process that occurs with lipid digestion and absorption... 

To gain more insights into the micellization processes, and the competition between phase separation and self-assembly, here we report Monte Carlo results for surfactant molecules with asymmetric chemical structures. We start with pure T molecules, which phase separate from solvent but show... 

Figure 11 The preparation of nanoparticles by the inverse micelle process in which a chemical reaction between microemulsion or inverse micelles after collision and perhaps fusion converts the soluble salt into an insoluble metal or metal oxide as shown. Source From Ref. 75.

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