Micelle-only

Due to the very small (nanometric) size of a micelle, only one radical at a time can exist inside. 

This section gives tabulated examples of recent work on micellar effects upon chemical and photochemical reactions. In general the examples given in this section do not duplicate material covered elsewhere in the chapter for example micellar effects on some photochemical reactions and reactivity in reversed micelles are listed here although they are neglected in the body of the text. For many ionic reactions in aqueous micelles only overall rate effects have been reported, in many cases because the evidence did not permit estimation of the parameters which describe distribution of reactants between aqueous and micellar pseudophases. These reactions are, nevertheless, of considerable chemical importance, and they are briefly described here. 

Micelles only form above a crucial temperature known as the Krafft point temperature (also called the Krafft boundary or just Krafft temperature). Below the Krafft temperature, the solubility of the surfactant is too low to form micelles. As the temperature rises, the solubility increases slowly until, at the Krafft temperature 7k, the solubility of the surfactant is the same as the CMC. A relatively large amount of surfactant is then dispersed into solution in the form of micelles, causing a large increase in the solubility. For this reason, IUPAC defines the Krafft point as the temperature (or, more accurately, the narrow temperature range) above which the solubility of a surfactant rises sharply. 

The Krafft Point may be defined as the temperature above which the solubility of a surfactant increases steeply. At this temperature, the solubility of the surfactant becomes equal to the critical micelle concentration (Cj ) of the surfactant. Therefore, surfactant micelles only exist at temperatures above the Krafft Point. This point is a triple point at which the surfactant coexists in the monomeric, the micellar, and the hydrated solid state (, ). 

Figure 8. Comparison of the o, CH2 frequencies of CgAO from experimental spectra of 0.08 M and 0.40 M solutions, and synthetic micelle-only spectra.

As surfactant is added to the second system of Figure 10, however, the tieline that terminates at the CMC for formation of inverted micelles in the oleic phase is reached before the normal CMC is encountered. Once the former tieline is reached, further additions of surfactant merely increase the concentration of inverted micelles. Only with much larger additions of surfactant will normal micelles begin to form. If only aqueous phases were studied, this could lead to the belief that normal micelles and the associated wettability would be encountered in a flow experiment, when, in fact, inverted micelles and different wettabilities would actually occur. 

In water solutions, bile salt molecules associate to form micelles only when present in sufficient concentration. The critical concentrations for micelle formation are in the low millimolar range. The structures of these micelles are not particularly stable. They continually change in size and shape by fusing and blending with nearby micelles. 

Among all surfactants developed for experiments on solubilization of biomolecules into reversed micelles, only two have, up to now, received the greatest attention to achieve studies on extraction. 

To avoid agglomeration, one possibility is to use a saturated fatty acid instead of oleic acid. Also, the use of such surface-active agents that are both oxidation inhibitors and form micelles only at concentrations larger than the ones used is recommended. Examples of such surfactants are the polyoxyethylene fatty amines. Since the critical concentration for micelle formation increases with decreasing alkyl- and increasing ethylene oxide chain length, short alkyl and long polyoxyethylene chains are favorable, such as Ethomeen C-25. 

This analysis assumes that the added LMA does not significantly change the mixed micelle CMC. In previous measurements of nonionic-anionic mixed micelle CMC, the CMC for a 60 40 mixture is nearly that of the nonionic surfactant (29, 30). Therefore, the surfactant concentration in these solutions is probably more than 100 times greater than the CMC (the CMC of C12EO5 is 5.9 X 10 M at 25 °C, ref. 33). Assuming for the sake of argument that the surfactant concentration is only 10 times the CMC, then a decrease of the CMC to zero would increase the number of micelles only by 10%, not nearly enough to account for the deviation in PLMA MW in... 

As shown above, the quasichemical approach to the micellisation is based on the condition of aggregation equilibrium (5.21), which allows us to obtain the mass action law (5.22) for the micellisation process. The simplest situation arises when monodisperse micelles (only with aggregation number ni), composed only of the non-ionic surfactant molecules (component 1), are formed. The corresponding reaction can be represented by the following equation... 

Comparison of diagrams of Fig. 18a, b indicates that morphology i of aggregate formed by pH-sensitive block copolymers can be tuned by variations in both concentration of added salt, Oion and pH in solution. For example, when pH < pAa (e.g., at ttfc = 0.1, Fig. 18b), a copolymer with lengths of the blocks, Na = 50 and Nb = 125, retains the cylindrical (C) morphology at any salt concentration, Oion. In contrast, when pH = pAa (a, = 0.5, Fig. 18a), the same copolymer makes cylindrical (C) micelles only at low salt concentrations, and associates into spherical (S) micelles upon a further increase in Oion. 

In our computer studies of the conformational behavior of the shell-forming chains, we used MC simulations [91, 95] on a simple cubic lattice and studied the shell behavior of a single micelle only. Because we modeled the behavior of shells of kinetically frozen micelles, we simulated a spherical polymer brush tethered to the surface of a hydrophobic spherical core. The association number was taken from the experiment. The size of the core, lattice constant (i.e., the size of the lattice Kuhn segment ) and the effective chain length were recalculated from experimental values on the basis of the coarse graining parameterization [95]. 

Starting from reservoir I where the micelles only contain proteated chains (H) and reservoir II with only deuterated chains (D), we define the initial conditions as /i = 1 and /n = 0 where the subscripts denote micelle I (i = I) or micelle... 

Rcorona Fmean cxpccted for Star-like micelles. The results thus show that the micelles grow like weU-defined star-like micellar entities. No regime where the size grows faster than the molecular weight, which was observed on a less segregated system by Honda et al. [169], were observed in this case. This probably reflects the fact that the initial time range of the micellization in this is too fast to be captured (t < 2-3 ms). Also, for star-like micelles only a few chains are necessary to achieve a star-like structure [38]. 

Reverse micelles have an inverted structure in comparison to the conventional normal micelles in aqueous systems. Therefore, they are often known as inverse or inverted micelles. In reverse micelles, the micellar cores consist of a hydrophilic polar component and the shells consist of lipophilic nonpolar part of the surfactant molecules. The dipole-dipole interaction between the hydrophilic headgroups acts as one of the driving forces for the formation of reverse micelles in organic solvents. Reverse micelles are mostly observed in the ternary mixtures of surfactant/ water/oil, mostly in oil-rich regions [1-3]. Furthermore, reverse micelles have also been observed in aqueous systems of UpophUic surfactant in surfactant-rich regions [4, 5]. In most of the studies carried out in the past, water was regarded as an essential component in the formulation of reverse micelles. Only a few reports exist in the literature of surfactant science that describe the formation of reverse micelles in organic solvents without water addition [6-10]. 

According to Israelachvili, a surfactant will be able to form spherical micelles only if the radius of the incipient micelle, R, is less than or equal to the 4 so that... 

A phenomena very similar to that seen with potassium oleate is seen with 1-monoolein. Even small amounts of 1-monoolein, roughly 1 molecule per micelle, swell the taurocholate micelle appreciably. As the weight proportion of monoolein increases, the micellar weight also increases. Again, poly-dispersity was not suggested by the schlieren sedimentation traces. As the number of molecules of 1-monoolein increases from 1 to 6 in the micelle only a small increase in bile salts (from 10 to 14) occurs. 

Micellar Efllecls on Inorganic Reactions.—Electron transfer between ferric ion and phenothiazine is inhibited by cationic micelles and accelerated by up to 10 -fold in anionic micelles of sodium lauryl sulphate. " In both cases the rate-surfactant concentration profile can be simulated accurately. Anionic micelles only cause a small effect on the reactivity of ruthenium(iii) tris(bipyridyl) with molybdenum(iv) octacyanide but accelerate the reaction between ferrous ion and tris(tetramethylphenanthroline)iron(ra). In the latter case a plot of surfactant concentration is linear with the reciprocal of the observed rate constant. Fast outer-sphere electron-transfer reactions may decrease in rate constant by up to four orders of magnitude when one of the reactants is solubilized in an anionic micelle. When this partner is neutral the inhibition is reduced somewhat by added salt, but when it is cationic the effect may be attenuated by competitive binding of Na or HsO and exclusion of reactant from the micelle. Oxidation of diethyl sulphide is catalysed by micelles of sodium lauryl sulphate containing carboxylate-ions by the mechanism shown. (Scheme 3). The rate advantage is quantitatively accounted for by the entropy term, and hexanoate is forty-fold more effective than acetate. Electron-transfer between the anionic trans-1,2-diaminocyclohexane... 

For triblock copolymers of the Pluronics type, the interaction is now well established. These BCPs behave very similarly when mixed with different surfactants. The behavior is qualitatively independent of the block lengths and also independent of the type of surfactant used. At high surfactant concentration, a complete decomposition of the polymer micelles is usually observed. In the final state, the mixed micelles only contain a single polymer molecule. This behavior is caused by the interaction of the hydrophobic PPO block with the alkyl tails of the different surfactants. It is also a common feature of these mixed systems that low surfactant concentrations lead to a decrease of the cmt of the different Pluronics. However, for other triblocks the available number of studies is rather small and it is not necessarily possible to transfer the outcome of the studies on the behavior of Pluronics to other systems. 

Sugars are a commonly used source for amphiphilic liquid crystals [10]. These materials show lamellar, columnar, and cubic phases, but chiral phases are very rarely observed. Thermotropic cholesteric phases are never observed and lyotropic cholesteric phases based on asymmetric micelles only in a few cases [11]. The bicontinuous cubic phase of these glycolipids may have macroscopic chiral ordering, but this has not been resolved hitherto [12], [13]. Thus, alkylated sugars are chiral compounds, but not effective... 

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