Micelles, 2+ dissociation

In highly diluted solutions the surfactants are monodispersed and are enriched by hydrophil-hydrophobe-oriented adsorption at the surface. If a certain concentration which is characteristic for each surfactant is exceeded, the surfactant molecules congregate to micelles. The inside of a micelle consists of hydrophobic groups whereas its surface consists of hydrophilic groups. Micelles are dynamic entities that are in equilibrium with their surrounded concentration. If the solution is diluted and remains under the characteristic concentration, micelles dissociate to single molecules. The concentration at which micelle formation starts is called critical micelle concentration (cmc). Its value is characteristic for each surfactant and depends on several parameters [189-191] ... 

Shiau, Y.F., R.J. Kelemen, and M.A. Reed. 1990. Acidic mucin layer facilitates micelle dissociation and fatty acid diffusion. Am J Physiol 259 G671. 

Griffin et al. (1988) reported that when the colloidal calcium phosphate was depleted, by addition of a EDTA solution to a micellar dispersion, there was essentially no selective dissociation of the individual caseins. This difference from the results of Holt et al. (1986) could reflect a difference of methodology. The method of Griffin et al. (1988) could bring about an almost complete and therefore non-selective disintegration of some micelles in the immediate vicinity of the added EDTA while leaving others virtually intact. In the dialysis method of Holt et al. (1986), the free Ca2+ concentrations is never depressed and hence micelles dissociate only because of the solubi-lazation of the colloidal calcium phosphate. 

At 100 MPa the turbidity has nearly vanished, since the particle size has been reduced to 20 nm. A large fraction of the casein micelles is decomposed into smaller fragments dominating the number average up to 200 MPa suggesting the existence of stable mini-micelles. Above 250 MPa the micelles dissociate further forming particles with d = 3 nm, most likely the casein monomers. Releasing the pressure induces limited reassociation to particles with d = 5 nm, the native micelle is not recovered. 

It is well known that block copolymers in a selective solvent (a good solvent for one block but a non-solvent for the other) form a micellar structure through the association of the insoluble segments. In contrast with micelles formed from low molecular weight surfactants, block copolymer micelles dissociate slowly to free polymeric chains. They have a greater capacity for solubilizing aromatic molecules and express lower CMCs. The AB block copolymers are considered useful vehicles for hydrophobic drugs. 

Micelles are loose aggregates of amphiphiles in water or organic solvents which form above a certain temperature (Krafft point) and concentration (critical micellar concentration, cmc). Below the Krafft temperature, clear micellar solutions become turbid and the amphiphile forms three-dimensional hydrated crystals. Below the cmc, micelles dissociate into monomers and small aggregates. Above the cmc, the micelles of an aggregation number n are formed n then remains stable over a wide concentration range . Table 1 gives some typical cmcs and three Krafft point values. 

An alternative strategy for design of dynamic, stimuli-responsive PE micelles is to use block copolymers with thermosensitive associating blocks, e.g., poly(A-isopropylacrylamide) [131, 134, 135] poly(A,iV-diethylacrylamide) [131-133], and poly(A,iV-dimethylacrylamide) [134]. In this case, reversible micellization-dissociation can be triggered by temperature variations that affect the solubility of the core-forming blocks. For example, in [131] it was shown that poly(acryUc acid)- Zock-poly(iV-isopropylacrylamide) copolymers can form micelles with poly(A-isopropylacrylamide) core and poly(acrylic acid) corona at pH 6 and T > 45°C, whereas at pH 4 and room temperature inverse micelles are formed. 

Periodic behavior is shown in Figure 11.3. Schork and Ray propose the following explanation. At 300 min, conversion increases rapidly because new polymer particles are formed and old ones grow. Additional surfactant adsorbs on the increased surface area of the particles. Micelles dissociate to contribute surfactant. 

Another crucial factor characterizing PM is their kinetic stability. The kinetic stability of micelles refers to the actual rate of micelle dissociation below the CMC. Thus, even below the CMC, PM may still be kinetically stable, provided that the dissociation into unimers proceeds slowly. Systems for which dissociation takes place over hours or even days have been reported, as opposed to low molecular weight surfactants that dissociate within milliseconds below their CMC. Several properties of the copolymer can be modified to improve the kinetic stability. These include the hydrophobic/hydrophilic balance, the physical state of the micelle core, the size of the hydrophobic block, and the incorporation of hydrophobic compounds (Figure 4.4). For instance, Creutz et showed that the rate of disassembly could be slowed down by increasing the hydrophobic/hydrophilic balance of the core-forming block. Increases in the hydrophobicity... 

T. Nakagawa, Critical examination of published data concerning the rate of micelle dissociation and proposal of a new interpretation. Colloid Polym. Sci., 252, 56-64 (1974). 

Ultra pH-sensitive (UPS) fluorescent nanoprobes with a 355-fold signal amplification in tumor relative to blood after systemic injection were recently established [40]. UPS probes possess an ultra pH-sensitive core copolymer (poly (ethylene glycol)-b-poly (2-(hexamethyleneimino) ethyl methacrylate) with micellar transitions within 0.25 pH units, fluorophores and cRGD for broad targeting of the tumor vasculature. The pH responsiveness is based on pH-dependent micellization of the core, micellization induced self-quenching of conjugated fluorophores (homoFRET), whereas micelle dissociation in acidic conditions resulted in an increased fluorescence signal. Systemic... 

Addition of soluble inorganic salts can also induce the precipitation from aqueous solutions of crystalline or amorphous crystalline surfactant precipitates. Consider then, for example, the increase in the Krafft temperature of SDS caused by the addition of a common ion, Na" [32-34]. This follows from simple mass action because the degree of micelle dissociation < 1 (i.e., the number of bound Na+ counterions is less than the micelle aggregation number) [35]. Peck [25] has shown, in measurements at 20°C, that the foamability of SDS declines markedly in the presence of added salt at concentrations > 0.3 M. The Krafft temperature of SDS under these conditions is >25°C [34], which means that crystalline SDS particles should be present as indicated by the onset of turbidity. Filtration to remove the turbidity partially restores the foamability [25] to a significant extent, which implies that the crystalline SDS particles exhibit some antifoam behavior. A combination of slow transport of surfactant to air-water surfaces and antifoam action by the crystalline surfactant would account for the almost total loss of foamability in the case of 0.01 M SDS in the presence of >0.3 M NaCl solution [25]. Antifoam action by crystalline particles... 

The approach of enzymatically sensitive polymers is also widely used in the case of polymeric micelles that physically encapsulate their cargo and release it upon enzymatic degradation of the polymer of which they are composed. One example of this is the work of Mao and co-workers (Mao and Gan, 2009). They synthesized amphiphilic diblock copolymers based on poly(glyddol-WocA - -caprolactone) (PG-h-PCL) with well-controlled structure and pendant hydroxyl groups along the hydrophilic block. These copolymers formed 74-95 nm micelles that demonstrated enzymatically triggered release of the encapsulated dye (pyrene) in the presence of lipase, due to degradation of the PCL block, which resulted in micelle dissociation. 

In Equation 3.26, T is the equilibrium surface excess, C the bulk concentration, t the time, and D the surfactant monomer diffusion coefficient. Eastoe et al. have measured the time dependence of the DST and the relaxation time %2 for solutions of many surfactants nonionic, dimeric, and zwitterionic. In all instances the fitting of the data to Equation 3.26 with the experimentally determined value of %2 was poor. The authors concluded that the micelle dissociation may have an effect on the measured DST only if the concentration of monomeric surfactant in the subsurface diffusion layer is limiting or when the micelle lifetimes are extremely long. No surfactant for which this last condition is fulfilled was evidenced by the authors. They also concluded that the rapid dissociation of monomers from micelles present in the subsurface was not likely to limit the surfactant adsorption and thus the DST. 

Micelles are not entities composed of fixed numbers of molecules having a fixed geometrical shape. They must be regarded as statistical in nature, in equilibrium with the surrounding amphiphilic molecules, and fluctuating constantly in size and shape in response to temperature. On dilution of the mixture, micelles dissociate rapidly, while on concentrating the solution, more extended micellar structures appear, eventually forming the many different lyotropic liquid-crystal phases. 

Micellar colloids are in a dynamic association-dissociation equilibrium, and the kinetics of micelle formation have been investigated for a long time. " In 1974, a reasonable explanation of the experimental results was proposed by Aniansson and Wall, " and this conception has been accepted and used ever since. The rate of micelle dissociation can be studied by several techniques, such as stopped flow, pressure jump, temperature jump, ultrasonic absorption, NMR, and ESR. The first three methods depend on tracing the process from a nonequilibrium state brought about by a sudden perturbation to a new equilibrium state— the relaxation process. The last two methods, on the other hand, make use of the spectral change caused by changes in the exchange rate of surfactant molecules between micelle and intermicellar bulk phase. 

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