Micelle temperature range

Figure 4.2. Solubility increase above the micelle temperature range depending on the aggregation number of micelle (n). (a) n = 25 (b) n = 50 (c) n = 100 (d) n = 150 (e) n = 200. (Reproduced with permission of Academic Press.)...

In the previous chapters, the dissolution and micellization of surfactants in aqueous solutions were discussed from the standpoint of the degrees of freedom as given by the phase rule. The mass-action model for micelle formation was found to be better for explaining the phenomena of surfactant solutions than the phase-separation model. Two models have similarly been used to explain the Krafft point, one postulating a phase transition at the Krafft point and the other a solubility increase up to the CMC at the Krafft point. The most recent version of the first approach is a melting-point model for a hydrated surfactant solid. The most direct approach to the second model of the Krafft point rests entirely on measurements of the solubility and CMC of surfactants with temperature. From these mesurements the concept of the Krafft point can be made clear. This chapter first reviews the concepts used to relate the dissolution of surfactants to their micellization, and then shows that the concept of a micelle temperature range (MTR) can be used to elucidate various phenomena concerning dissolution... 

Figure 6.1. Changes of the solubility and CMC with temperature (a) Solubility curve, (b) CMC curve (AC, narrow concentration range of CMC AT, micelle temperature range). (Reproduced with permission of Academic Press.)...

Table 6.1. Micelle Temperature Range (MTR) and Heat of Dissolution (All ) of 1, T-[l,

The term micelle temperature range expresses the relation between solubility and micelle formation better than Krafft point... 

FIGURE 22.5 OTHdC study on the temperature range of the dissociation of the micelle of a styrene-isoprene two-block polymer in n-decane. Column 3.70 fim x 300 cm. (Reprinted with permission from Ref. 14. Copyright 1989 American Chemical Society.)... 

A slight increase in the turbidity upon heating of aqueous solutions of the s-fractions of the NVCl/NVIAz-copolymers obtained from the feeds with initial comonomer molar ratios of 75 25 (Tcp 65 °C) and 80 20 (Tcp 66 °C) could be due to the micellization phenomena, although the absence of DSC peaks over the same temperature range testified to the non-cooperative character of the process. This could indicate that the chains of these s-type copolymers had, nevertheless, a certain amount of oligoNVCl blocks non-buried by the hydrophilic microenvironment sufficiently well and thus capable of participating in the hydrophobically-induced associative intermolecular processes at elevated temperatures. At the same time, the sequence of monomer units in the s-copolymers obtained from the feeds with the initial comonomer ratios of 85 15 and 90 10 (mole/mole) corresponded to the block-copolymers of another type. The basis for such a conclusion is the lack of macroscopic heat-induced phase separation at elevated temperatures (Fig. 3 a and b) and, simultaneously, the transi-... 

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. 

A critical micelle temperature or CMT is a very useful value for PEO-PPO-PEO copolymers. This arises from the fact that micellization in these copolymers is due to the dehydration of the PPO block with increasing temperature. The value of the CMT ranges from 20 to 50 °C in commercially available PEO-PPO-PEO copolymers. The CMT increases whenever the copolymer concentration is increased [19]. 

At all temperatures, asl-CN B and C are insoluble in calcium-containing solutions and form a coarse precipitate at Ca2+ concentrations greater than about 4 mM. asl-CN A, from which the very hydrophobic sequence, residues 13-26, is deleted, is soluble at [Ca2+] up to 0.4 M in the temperature range 1-33°C. Above 33°C, it precipitates but redissolves on cooling to 28°C. The presence of asl-CN A modifies the behaviour of asl-CN B so that an equimolar mixture of the two is soluble in 0.4 M Ca2+ at 1°C asl-CN B precipitates from the mixture at 18°C and both asl-CN A and B precipitate at 33°C. aBl-CN A does not form normal micelles with K-casein. Since asl-CN A occurs at very low frequency, these abnormalities are of little consequence in dairy processing but may become important if the frequency of asl-CN A increases as a result of breeding practices. 

Phase diagrams of water, hydrocarbon, and nonionic surfactants (polyoxyethylene alkyl ethers) are presented, and their general features are related to the PIT value or HLB temperature. The pronounced solubilization changes in the isotropic liquid phases which have been observed in the HLB temperature range were limited to the association of the surfactant into micelles. The solubility of water in a liquid surfactant and the regions of liquid crystals obtained from water-surfactant interaction varied only slightly in the HLB temperature range. 

Evans et al. also showed that the 1 1 mixture of BAN and (3, y-distearoyl-phos-photidylcholine (DSPC) gives a smectic A texture in the temperature range of 57.3 to 100°C [21]. This is the first notice of lyotropic lamellar liquid crystals formed in the ionic medium. Additionally, Seddon et al. [28] and Neve et al. [29] have described the long-chained A-alkylpyridinium or l-methyl-3-alkylimidazolium ions to display smectic liquid-crystalline phases above their melting points, when Cl or tetrachloro-metal anions like CoCl " and CuCl " are used as the counter ions. Lin et al. have also noted the liquid crystal behavior of 1-alkylimidazolium salts and the effect on the stereoselectivity of Diels-Alder reactions [30]. However, liquid crystals are classified as ionic liquid crystals (ILCs), and they are distinguished from liquid crystals that are dispersed in ionic liquids. Although the formation of micelles and liquid crystal phases in ionic liquids have been thus reported, there has been no mention of the self-assembly of developed nano-assemblies that are stably dispersed in ionic liquids. In the next section the formation of bilayer membranes and vesicles in ionic liquids is discussed. 

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