Mixed micelles electron microscopy

Figure 12 Transmission electron microscopy study of protein desorption in icecream mix containing emulsifiers and hydrocolloids, (a) Immediately after homogenization the fat globules (0 are stabilized by adsorbed partially dissociated casein micelles (arrows), (b) During ageing the mix at 5°C, the previously adsorbed protein film is released in the form of coherent protein layers (arrows) into the water phase (w). (c) After mechanical treatment in the ice cream freezer, desorbed protein layers are seen more often in the water phase without association to fat giobules (arrows). From reference 48, courtesy of Dr. W.Buchheim, Kiel, Germany.

Monoglycerides and mono-diglycerides have low HLB values and cannot form micelles. They build up a multi-layer at the surface, resulting in a constantly decreasing surface tension as their concentration increases. However, in systems with proteins such as fat-free ice cream mixes, these emulsifiers behave as if they have a CMC. A possible explanation for this observation is that the unbound emulsifier in the fat-free mix is in equilibrium with the protein-bound emulsifier. Above a certain concentration of emulsifier in the mix, any surplus of emulsifier will adhere to the protein in the water phase after the surface has been saturated. The unadsorbed emulsifier is seen as very small crystals less than 200 nm by electron microscopy analysis4. ... 

The most thorough study of the formation of artificial casein micelles is that of Schmidt and co-workers (1977 1979 Schmidt and Koops, 1977 Schmidt and Both, 1982 Schmidt and Poll, 1989), who not only studied the properties of the casein aggregates but also attempted to relate them to the solution conditions under which they were formed. In the precipitation of calcium phosphate from solution, the means by which solutions are mixed together is of crucial importance Schmidt et al. (1977) described a method in which four solutions were pumped simultaneously into a reaction vessel while keeping the pH constant. As a result of careful, slow mixing, the reproducibility of the size distributions of particles, measured by electron microscopy on freeze-fractured and freeze-etched specimens, was very good. In the first series of experiments, the objective was to produce milk like concentrations of the most important ions while... 

Figure 12.2. The structure of ice cream mix and ice cream. (A). Fat globules (F) in mix with crystalline fat within the globule and adsorbed casein micelles (C), as viewed by thin section transmission electron microscopy. (B). Close-up of an air bubble (A) with adsorbed fat, as viewed by low temperature scanning electron microscopy. (C). Air bubble (A) with adsorbed fat cluster (FC) that extends into the unfrozen phase, as viewed by thin section transmission electron microscopy with freeze substitution and low temperature embedding.

Fig. 7. The effect of adsorbed protein on structure of ice-cream mix, ice cream, and melted ice cream. A-B, ice-cream mix with no surfactant and with added surfactant, respectively, as viewed by thin-section transmission electron microscopy. f= fat globule, c = casein micelle, arrow = crystalline fat, bar = 0.5 pm. See Reference 24 for methodology. C-D, ice cream with no surfactant and with added surfactant, respectively, as viewed by low-temperature scanning electron microscopy, a = air bubble, f = fat globule, bar = 4 pm. See Reference 34 for methodology. E-F, ice cream with no surfactant and with added surfactant respectively, as viewed by thin-section transmission electron microscopy with freeze substitution and low-temperature embedding. a = air bubble, f= fat globule, c = casein micelle, fc = fat cluster, bar = 1 pm. See Reference 13 for methodology. G-H, melted ice cream with no surfactant and with added surfactant respectively, as viewed by thin-section transmission electron microscopy. f= fat globule, c = casein micelle, fn = fat network, bar = 1 pm in G and 5 pm in H. See Reference 24 for methodology.

Schematic models for the expanded structure of bile acid-phosphatidylcholine mixed micelles are shown in Fig. 2B. The original model was proposed by Small in 1967 (S36). In this model the mixed micelle consisted of a phospholipid bilayer disk surrounded on its perimeter by bile acid molecules, which were oriented with their hydrophilic surhices in contact with aqueous solvent and their hydrophobic sur ces interacting with the hydrocarbon chains of the phosphohpid molecules. This model has recently been revised, based on further studies of mixed micelles using quasi-elastic light scattering spectroscopy (M20). In a new model for the molecular structure of bile acid-phospholipid mixed micelles. Mazer et al. (M20) propose a mixed disk, in which bile acids are found not only on the perimeter of phospholipid bilayers, but also incorporated within their interior in high concentrations (Fig. 2B). The size of these mixed micelles was estimated to be as high as 200 to 400 A in radius in some solutions, and disk-shaped particles in this size range were observed by transmission electron microscopy (M20). Micellar aggregates similar in size and structure to those found in model bile solutions have been demonstrated in dog bile (M22).

Electron microscopy showed that gigantic rod-like micelles were formed in viscoelastic solutions in the presence of several aromatic compounds as shown in Figure 4.1(a) [48-50]. It was very surprising that single crystals, mixed with the gigantic micelles, were found to co-exist in these viscoelastic solutions, as shown in Figure 4.1(b) [51]. From the aqueous solution composed of hexadecyltrimethylammonium bromide and several phenolic derivatives, similar crystalline materials were obtained. These were assumed to be complexes composed of the surfactant and the respective phenolic derivative based on elementary and thermal analyses [52]. The crystal structiu e was analysed by X-rays and a variety of complexes with similar structures were obtained [53]. 

A second class of lipid derivatives that contained two hydrophobic chains and a NTA polar head group was synthesized the double lipidic chain contained 12 (DC-12), 14 (DC-14), 16 (DC-16), and 18 (DC-18) carbon atoms. Unlike the single-chained (sc) class of molecules, no supramolecular assembly was observed for the double-chained (dc) reagents upon direct sonication with CNTs. Moreover, unlike the SC class, these reagents were not water soluble and formed vesicles in aqueous solutions. To ascertain whether micelles were required for supramolecular assembly formation, MWNTs were sonicated in the presence of DC compounds and a 1% concentration of the surfactant SDS. A mixed micelle consisting of DC/SDS formed in the aqueous solution SDS was subsequently removed through dialysis. TEM (see Transmission Electron Microscopy (TEM), Techniques) showed the formation of half-cylinder striations that were approximately 5.5-7.5nm, which was in agreement with the size of the different lipidic chains the striations were >4.5 nm, which implied that they were not due to SDS. Therefore, the formation of micelles appears to be the key step for the formation of noncovalent supramolecular assemblies on CNTs. 

It was proposed that the monomer/dopant salts form micelles which then serve as the soft-template for the formation of tubular structures. For the dopant-free or simplified method in which only aniline and ammonium persulfate are mixed, the initial formation of spherical micelles was observed by freeze-fracture transmission electron microscopy from which tubular structures were obtained. ... 

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