Micellar assembly

Comparison of rate constants of reactions in micelles and in non-micellar assemblies... 

Phase separation controlled by diffusion exchange often results in a skin which is composed of a micellar assembly of nodules, as will be discussed below. When extremely hydrophobic polymers (e.g., modifled-PPO) are cast from dioxane into water (pg = p = p ) a dense polymer layer is formed at the solution s interface that somewhat resembles the type of layer formed by Interfacial polymerization. There is almost no inward contraction of the interfacial skin, and the coagulation process is controlled by diffusion through the dense, interfacial thin film. These result in an anisotropic membrane with a very fine "coral" structure (Figures 9 and 10). 

Similar to micellar assemblies in water, reverse micelles have also been utilized to bring about nonspecific binding interactions in organic solvents. Akiyoshi et al. (2002) have synthesized an amphiphilic block copolymer containing PEO and an amylase chain as receptor for methyl orange (MO Chart 2.2). Amylases are insoluble and methoxy-PEO (MPEO) is soluble in chloroform. Hence, an MPEO-amylase block copolymer forms reverse micelles in chloroform. Akiyoshi et al. established the capability of the buried receptors to extract the complementary analyte by studying the ultraviolet visible (UV-vis) spectra. A solution of polymer was shaken... 

Figure 2.5 Formation of micellar assemblies induced by pH complexation in ionic block...

Basu et al. (2004) reported a series of amphiphilic homopolymers in which the hydrophUic moiety is a carboxylic acid moiety and the hydrophobic moiety can be based on either an alkyl chain or an aromatic ring (Chart 2.7). These polymers self-assemble into either micelles or reverse micelles, depending on the solvent environment. Figure 2.7 represents the formation of a micellar assembly in a polar... 

Figure 2.7 Formation of micellar and inverted micellar assemblies from amphiphilic homopolymer.

Becker ML, Liu JQ, Wooley KL. Functionahzed micellar assemblies prepared via block copolymers synthesized by hving free radical polymerization upon peptide-loaded resins. Biomacromolecules 2005 6 220-228. 

Poly(3inido amine) (PAMAM) Dendrimers. These dendrimers have a hydro-phihc polyamide interior that is relatively open to solvent, in contrast to a micellar assembly. Yet, the picture of a unimolecular micelle also emerged for a generation 4.5 PAMAM dendrimers having sodium carboxylate terminal groups (Fig. 11.4). 

Accordingly, the morphology of the micellar assemblies transitioned from large objects with low curvature at a low dendron generation to smaller vesicular/spherical structures with high curvature at higher generations. 

These different casein monomers combine with calcium phosphate to form discrete particles on the nano-size scale. The phosphoserines of the caseins are seemingly clustered for the purpose of linking within the micelle to putative calcium phosphate microcrystallites, also known as nanoclusters (Holt, 1992 Home, 1998, 2002, 2003, 2006 Holt et al., 2003 Home et al., 2007). Structural evidence for the existence of such nanoclusters has come from neutron and X-ray scattering (de Kruif and Holt, 2003 Holt et al., 2003 Pignon et al., 2004 Marchin et al., 2007). The presence of nanoclusters allows native casein micelles to be effective natural suppliers of essential calcium salts in the human diet in a readily assimilated functional form. Protein-nanocluster interactions are the central concept of the cross-linking mechanism in Holt s model of casein micellar assembly (Holt et al., 2003 de Kruif and Holt, 2003). Any analogy with conventional soap-like micelles is considered to be... 

Micellar ocular DDS should be based on nontoxic and nonirritant materials and should be stable enough to achieve a reasonable shelf life. Attention should be paid to the CMC of detergents used for micellar assembly, since high CMC often renders them toxic and irritant to ocular tissues. 

FIGURE 7.44. Transmission electron micrographs of the micellar assemblies formed by the aggregation of a lipase-polystyrene giant amphiphile in water. Expansion reveals a single micellar fiber with a diameter of 20-30 nm. Schematic representation of the micellar rod which possesses a polystyrene core. 

Aqueous solutions of neutral (i.e., nonionic or zwitterionic) surfactants can form micellar assemblies in which a certain number of surfactant molecules aggregate to form an assembly possessing a central core region comprised of long alkyl (or alkylaryl) hydrocarbon chains with their more polar polyethyleneoxide (or zwitterionic) headgroups extending outward and interacting with the bulk water.213... 

More importantly, the use of heavy metal anionic micellar media has been shown to allow for observation of analytically useful room-temperature liquid phosphorescence (RTLP) (7.484.487). There are several examples in which phosphorescence has been employed as a LC detector with the required micellar assembly being present as part of the LC mobile phase (482) or added post column (485). More recently, metal ions have been determined in a coacervate scum by utilizing the micellar-stabilized RTLP approach (498). Thus, the future should see further development in RTLP detection of metal ions in separation science applications. 

In aqueous solutions the micellar assembly structure allows sparingly soluble or water-insoluble chemical species to be solubilized, because they can associate and bind to the micelles. The interaction between surfactant and analyte can be electrostatic, hydrophobic, or a combination of both [76]. The solubilization site varies with the nature of the solubilized species and surfactant [77]. Micelles of nonionic surfactants demonstrate the greatest ability for solubilization of a wide group of various compounds for example, it is possible to solubilize hydrocarbons or metal complexes in aqueous solutions or polar compounds in nonpolar organic solutions. As the temperature of an aqueous nonionic surfactant solution is increased, the solution turns cloudy and phase separation occurs to give a surfactant-rich phase (SRP) of small volume containing the analyte trapped in micelle structures and a bulk diluted aqueous phase. The temperature at which phase separation occurs is known as the cloud point. Both CMC and cloud point depend on the structure of the surfactant and the presence of additives. Table 6.10 gives the values of CMC and cloud point for the surfactants most frequently applied in the CPE process. 

Sharma, P.K. Bhatia, S.R. Effect of anti-inflammatories on Pluroinc FI27 micellar assembly gelation and partitioning. International Journal of Pharmaceutics 2004, 278, 361-377. 

Fig. 4.10. Schematics of light-induced charge separation by functional micellar assemblies...

Polydispersity of simple bile salt micelles can only be assessed by modem QLS techniques employing the 2nd cumulant analysis of the time decay of the autocorrelation function [146,161]. These studies have shown, in the cases of the 4 taurine conjugates in 10 g/dl concentrations in both 0.15 M and 0.6 M NaCl, that the distribution in the polydispersity index (V) varies from 20% for small n values to 50% for large n values [6,146]. Others [112] have foimd much smaller V values (2-10%) for the unconjugated bile salts in 5% (w/v) solutions. Recently, the significance of QLS-derived polydispersities have been questioned on the basis of the rapid fluctuation in n of micellar assemblies hence V may not actually represent a micellar size distribution [167-169]. This argument is specious, since a micellar size distribution and fast fluctuations in aggregation number are identical quantities on the QLS time scale (jusec-msec) [94]. 

Moroi, Y, Infelta, R R, Gratzel, M. (1979). Light initiated redox reactions in functional micellar assemblies. 2. Dynamics in europium(III) surfactant solutions, J. Am. Chem. Soc., 101 573. 

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