Micellar formation

Quaternary ammonium salts are also known to promote nucleophilic substitution reactions in two-phase systems through the formation of micelles [15], but there is no evidence for micellar formation by bulky ammonium salts, such as tetra-n-buty-lammonium bromide, under liquidrliquid two-phase conditions [16]. 

Figure 2.4 Temperature responsive micellar formation in PNlPAM-based polymers.

Menger et al. synthesized a Ci4H29-attached copper(II) complex 3 that possessed a remarkable catalytic activity in the hydrolysis of diphenyl 4-nitrophenyl phosphate (DNP) and the nerve gas Soman (see Scheme 2) [21], When 3 was used in great excess (ca. 1.5 mM, which is more than the critical micelle concentration of 0.18 mM), the hydrolysis of DNP (0.04 mM) was more than 200 times faster than with an equivalent concentration of the nonmicellar homo-logue, the Cu2+-tetramethylethylenediamine complex 9, at 25°C and pH 6 (Scheme 4). The DNP half-life is calculated to be 17 sec with excess 1.5 mM 3 at 25°C and pH 6. The possible reasons for the rate acceleration with 3 were the enhanced electrophilicity of the micellized copper(II) ion or the acidity of the Cu2+-bound water and an intramolecular type of reaction due to the micellar formation. On the basis of the pH(6-8.3)-insensitive rates, Cu2+-OH species 3b (generated with pK3 < 6) was postulated to be an active catalytic species. In this study, the stability constants for 3 and 9 and the thermodynamic pvalue of the Cu2+-bound water for 3a —> 3b + H+ were not measured, probably because of complexity and/or instability of the metal compounds. Therefore, the question remains as to whether or not 3b is the only active species in the reaction solution. Despite the lack of a detailed reaction mechanism, 3 seems to be the best detoxifying reagent documented in the literature. 

Polymerisation Coacervation Micellar formation Interfacial Coacervation chilling Interfacial Polymerisation Coacervation In situ polymerisation Liposomes... 

Question What role can cholesterol play in micellar formation ... 

Cholesterol does not form micelles because (1) it is not amphiphilic and (2) its flat, rigid, fused-ring structure gives a solid rather than a liquid, mobile hydrocarbon phase necessary for micellar formation. Cholesterol forms mixed micelles with amphiphilic lipids and will enter monolayers. 

BILE ACID SEQUESTRANTS Cholestyramine, colestipol, and colesevalam effectively bind bile acids and some bacterial toxins. Cholestyramine is useful in the treatment of bile salt-induced diarrhea, as in patients with resection of the distal Ueum. In these patients, there is partial interruption of the normal enterohepatic circulation of bile salts, resulting in excessive concentrations reaching the colon and stimulating water and electrolyte secretion (see below). Patients with extensive ileal resection (usually >100 cm) eventually develop net bile salt depletion, which can produce steatorrhea because of inadequate micellar formation required for fat absorption. In such patients, the use of cholestyramine will aggravate the diarrhea. 

Diazo alkanes are sometimes involved as intermediates in deamination, especially in aprotic solvents . The counterion and micellar formation are other factors which can determine the products of deamination. 

Monolayer techniques were used to characterize the interfacial properties of the resultant Fractions. Fraction I contained highly cohesive complexes that did not unfold at the interface and had an average diameter of 9.1 nm. These particles are thought to represent submicelles, previously identified in micelle formation. Fraction II showed interfacial properties that are characteristic of spread casein monomers, and contained mainly a -casein. The results are discussed in relation to casein interactions and micellar formation. Mixed monolayers of sodium caseinate/glyceride monostearate (NaCas/GMS) were also examined at different composition ratios. The results show that for low surface pressures (0-20 mNm ), there is a condensation ascribable to hydrophobic interactions in the mixed film. At high surface pressures, the hydrophobic interaction is modified and the protein is expelled from the monolayer into the subphase. These results are discussed in relation to emulsion stability. 

The typical light scattering curves (Sjoblom, 1978) show the increase of scattered intensity characteristic of micellar formation first at relatively high water concentrations. The example in Fig. 5 is from microemulsions with 50% by weight hexadecane stabilized by potassium oleate and pentanol (Sjoblom, 1978). The scattering intensity below a water concentration of 30% by weight (counted on water, svirfactant. 

Nonionic surfactants with pronounced hydrophilic character behave like ionic surfactants they show a normal micellar formation in the aqueous phase with a hydrocarbon non-micellar phase in equilibrium. Higher surfactant concentrations give rise to liquid crystalline phases with a structure dependent on the length of the hydrophilic part of the surfactant. 

The real breakthrough in terms of kinetic theory was published in 1973 by Aniansson and Wall [80, 81], who provided much more applicable kinetic equations for stepwise micelle formation using a polydisperse model. In a substantial paper two years later they were able to predict the first-order rate constants for the dis-sociation/association of surfactant ions to and from micelles (and hence residence times/lifetimes of surfactant monomers within micelles) [82]. They found values for the association and dissociation of surfactants into/from micelles (Ar and k , respectively) for sodium dodecyl sulfate (SDS) as 1 x 10 s and 1.2 x 10 mok s". Their kinetic model still remains essentially unchanged as a basis for the kinetics of micellar formation and breakdown. Modifications made to existing theory also allowed them to offer a significant thermodynamic explanation for the low enthalpy change upon micellization. 

Figure 5.12 Schematic representation of the reversible process of micellar formation adapted from Pan and Firoozabadi, 1998b).

Matsni K., Nakazawa T., Morisaki H. Micellar formation of sodium dodecyl sulfate in sol-gel glasses probed by pyrene fluorescence. J. Phys. Chem. 1991 95 976-979 Matsni K., Sasaki K., Takahashi N. Luminescence oftris(2,2 -bipyridine)ruthenium(n) in sol-gel... 

The effect of surfactant and initiator concentrations on the particle size and particle number is well documented in the literature. It is well known that increasing the surfactant concentration in the reactor will induce micellar formation and increase the particle number in the reactor (Fig. 18.6 [34]) smaller polymer particles are thus obtained. 

Retsos et al. [55, 56] made an attempt to provide a semiquantitative analysis of the interfacial activity of block copolymers at the polymer-polymer interface the emphasis was on understanding the nonmonotonic dependence of the interfacial tension reduction on diblock molecular weight as well as the effects of macromo-lecular architecture and composition when graft copolymers were utilized as additives. The attempt was based on a modification of the analysis of Leibler [75], where the possibility of micellar formation was also taken into account. The thermodynamic equilibrium under consideration was, thus, that between copolymer chains adsorbed at the interface, chains homogeneously distributed in the bulk homopolymers, and chains at micelles formed within the homopolymer phases. 

Spherical micelles also share some of features of the p = 1 systems due to the characteristics of amphiphilic molecules (i.e., the nonpolar tails control micellar size) causing p" to reach a minimum at a finite value of N. Note, in fact, the similarity between the diagram in Fig. 25, illustrating helical polymerization, and the diagram illustrating micellar formation above CMC in isotropic solution (Fig. 4 in this volume. Chapter 2 or Fig. 16.5 of Ref. 26). 

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