Amphiphiles critical micelle concentration

The Krafft point can be defined as the temperature T above which the amphiphile (surfactant) solubility in water greatly increases [3], The reason is that the water solubility of the amphiphile, which increases with temperature, reaches the amphiphile critical micelle concentration (Cm in Figure 3.6). When the solubility curve is above Cm the dissolved amphiphile forms micelles and the amphiphile... 

The pioneering work on amphiphilic polyelectrolytes goes back to 1951, when Strauss et al. [25] first synthesized amphiphilic polycations by quaternization of poly(2-vinylpyridine) with n-dodecyl bromide. They revealed that the long alkyl side chains attached to partially quaternized poly(vinylpyridine)s tended to aggregate in aqueous solution so that the polymers assumed a compact conformation when the mole fraction of the hydrophobic side chains exceeded a certain critical value. Thus, Strauss et al. became the first to show experimentally the intramolecular micellation of amphiphilic polymers and the existence of a critical content of hydrophobic residues which may be compared to the critical micelle concentration of ordinary surfactants. They called such amphiphilic polyelectrolytes polysoaps [25],... 

Micelles the mostly spherical nanoscale aggregates formed by amphiphilic compounds above their critical micelle concentration in aqueous solution have a narrow size distribution and are dynamic, because there is a fast exchange of amphiphiles in solution and those incorporated in micelles. However, micelles are defined as self-assembled structures, since the structure is in thermodynamical equilibrium. 

Oheme and co-workers investigated335 in an aqueous micellar system the asymmetric hydrogenation of a-amino acid precursors using optically active rhodium-phosphine complexes. Surfactants of different types significantly enhance both activity and enantioselectivity provided that the concentration of the surfactants is above the critical micelle concentration. The application of amphiphilized polymers and polymerized micelles as surfactants facilitates the phase separation after the reaction. Table 2 shows selected hydrogenation results with and without amphiphiles and with amphiphilized polymers for the reaction in Scheme 61.335... 

An amphiphilic molecule has a single positively charged head group and is in solution with a concentration of 10 mM. If the critical micelle concentration of the molecule is 25 mM and the Krafts temperature is 25°C ... 

Fig. 6.1 Self-organization of amphiphilic block polymers in water (cmc = critical micelle concentration).

It turned out that for all the polymeric amphiphiles of the (EO) -(PO)m-(EO) type there was an increase in enantioselectivity compared with the reaction without amphiphile. Moreover, the ratio of the length of the (PO) block compared with the (EO) block seemed to determine enantioselectivity and activity and not the cmc (critical micelle concentration). A (PO) block length of 56 units works best with different length of the (EO)n block in this type of hydrogenation [30]. for the work-up of the experiments, G. Oehme et al. used the extraction method, but initial experiments failed and the catalyst could not be recycled that way. To solve this problem the authors applied a membrane reactor in combination with the amphiphile (EO)37-(PO)5g-(EO)37 (Tab. 6.1, entry 9) [31]. By doing so, the poly-mer/Rh-catalyst was retained and could be reused several times without loss of activity and enantioselectivity by more than 99%. 

A structure formed by the reversible association of am-phiphiles in apolar solvents. In inverted micelles, the polar portion of the amphiphile is concentrated in the interior of the macrostructure. Such association usually occurs with aggregation and is not typically characterized by a definite nucleation stage. Thus, inverted micelles (also referred to as inverse or reverse micelles) often fail to exhibit critical micelle concentration behavior. See Micelle... 

Perfluorocarbons bearing a polar hydrophilic head are very active surfactants. Indeed, the presence of fluorine atoms strongly lowers the critical micelle concentration (CMC) of an amphiphilic compound. Moreover, fluorination generally has important effects on micellization phenomena, especially on the size and shape of formed micelles. 

An emulsifier is a molecule that possesses both polar and nonpolar moieties, i.e., it is amphiphilic. In very dilute water solutions, emulsifiers dissolve and exist as monomers, but when their concentration exceeds a certain minimum, the so-called critical micelle concentration (CMC), they associate spontaneously to form aggregates - micelles. Micelles are responsible for many of the processes such as enhancement of the solubility of organic compounds in water, catalysis of many reactions, alteration of reaction pathways, rates and equilibria, reaction loci for the production of polymers, etc. 

In aqueous solution, amphiphilic molecules aggregate into micelles above the critical micelle concentration. Such solutions have been the object of research for many years, with special interest in shape and size of these micellar aggregates [37]. Size and shape (spherical, wormlike, or disklike micelles) depend strongly on the molecular structure of the amphiphilic molecule. 

Block or graft copolymers in a selective solvent can form structures due to their amphiphilic nature. Above the critical micelle concentration (CMC), the free energy of the system is lower if the block copolymers associate into micelles rather than remain dispersed as single chains. Often the micelles are spherical, with a compact core of insoluble polymer chains surrounded by a corona of soluble chains (blocks) [56]. Addition of a solvent compatible with the insoluble blocks (chains) and immiscible with the continuous phase leads to the formation of swollen micelles or polymeric micro emulsion. The presence of insoluble polymer can be responsible for anomalous micelles. 

Micelles are colloidal dispersions that form spontaneously, under certain concentrations, from amphiphilic or surface-active agents (surfactants), molecules of which consist of two distinct regions with opposite afL nities toward a given solvent such as water (Torchilin, 2007). Micelles form when the concentration of these amphiphiles is above the critical micelle concentration (CMC). They consist of an inner core of assembled hydrophobic segments and an outer hydrophilic shell serving as a stabilizing interface between the hydrophobic core and the external aqueous environment. Micelles solubilize molecules of poorly soluble nonpolar pharmaceuticals within the micelle core, while polar molecules could be adsorbed on the micelle surface, and substances with intermediate polarity distributed along surfactant molecules in intermediate positions. 

It was found that unloaded PEBPBLA micelles cause no hemolysis, even at a level of 0.70 mg/mL. PECb-PBLA has an extremely low critical micelle concentration (Kwon et al., 1993), and thus, there is little monomeric PBEPBLA for the lysis of lipid bilayer membranes. In addition, PEO-bPBLA micelles may break apart slowly to monomers. The lack of hemolytic activity of PEO-bPBLA contrasts strongly with other amphiphiles used for drug solubilization and intravenous drug administration. Sodium deoxycholate causes 100% hemolysis at a level of 0.32 mg/mL. This is due to disruption of lipid bilayer membranes of red blood cells. 

The molecular structure of retinoic acid is typical for an amphiphilic compound that is concentrated at interfaces. Further, the carboxylic acid groups allow such compounds to adjust their amphiphilic character by the degree of their dissociation. Surface tension measurements were carried out in order to determine the surface activity of retinoic acid [179]. The surface tension with respect to the concentration at pH 5 decreases more strongly than at pH 9. This reflects the fact that the protonated form of retinoic acid is more efficient in its surface activity than the deprotonated form. The critical micelle concentrations are 3.7 0.5 mg/L (pH 5) and 19 2 mg/L (pH 9). The limiting surface tension values in both curves is about 35 mN/m. Due to the precipitation of retinoic acid, the highest concentration in the surface tension curve at a pH of 5 was 20 mg/L. By contrast the solubility at pH 9 is at least 1 g/L. In order to verify the results from the FTIR measurements, films of the complexes were immersed in a solution of 0.15 mol/L sodium... 

Additives are usually amphiphilic in nature, and thus are either ionic or neutral surfactants or even polymers. The role of surfactants in solvent extraction is ambiguous. Usually, they should be avoided as they lower the interfacial tension, which may lead to emulsion formation in an agitated extractor. However, every metal-loaded ion exchanger is amphiphilic, and can adsorb at the interface or aggregate in the bulk phase. This occurrence is well known with sodium or other metals [17], and above a critical surfactant concentration (cmc, critical micelle concentration) micellar aggregates are formed. A dimensionless geometric parameter is decisive for the structure of the associates, according to Fig. 10.6 ... 

Another study on these variegated cells depicting an amphiphile revealed a temperature effect on the critical micelle concentration (cmc) that was minimal at about PB(W) = 0.25. Experimentally, the minimal cmc value occurs at about 25 °C.64 The onset of the cmc was also modeled and shown to be dependent on a modestly polar fragment of the amphiphile. 

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