Aggregate micelle-like

As has been described in Chapter 4, random copolymers of styrene (St) and 2-(acrylamido)-2-methylpropanesulfonic acid (AMPS) form a micelle-like microphase structure in aqueous solution [29]. The intramolecular hydrophobic aggregation of the St residues occurs when the St content in the copolymer is higher than ca. 50 mol%. When a small mole fraction of the phenanthrene (Phen) residues is covalently incorporated into such an amphiphilic polyelectrolyte, the Phen residues are hydrophobically encapsulated in the aggregate of the St residues. This kind of polymer system (poly(A/St/Phen), 29) can be prepared by free radical ter-polymerization of AMPS, St, and a small mole fraction of 9-vinylphenanthrene [119]. 

While CMC is assumed to be an observable and definite value in the case of surfactant monomers, there are frequent reports in the literature of the formation of aggregates or micelle-like associations in solutions of organic solutes so dilute as to preclude apparently the formation of micelles [208, 267-269, 272, 275,278]. Work with different types of commercial surfactants has indicated that molecularly non-homogeneous surfactants do not display the sharp inflection in surface tension associated with CMC in molecularly homogeneous monomers, but rather the onset of aggregation is broad and indistinct [253,267,268]. The lack of well-defined CMCs for non-homogeneous surfactants is speculated to result from the successive micellization of the heterogeneous monomers at different stoichiometric concentrations of the surfactant, which results in a breadth of the monomeric-micelle transition zone. 

Some surfactants aggregate at the solid-liquid interface to form micelle-like structures, which are popularly known as hemimicelles or in general solloids (surface colloids) [23-26]. There is evidence in favor of the formation of these two-dimensional surfactant aggregates of ionic surfactants at the alumina-water surface and that of nonionic surfactants at the silica-water interface [23-26]. 

Figure 29 shows the dependence of the equilibrium concentration of the cetylpyridinium cations in the anionic gel of SMA-MAA, Cg, on the contents of CPB in the surrounding solution. At low Cs, the Cg values are higher by a factor of 104 than the value of CPB concentration in the solution and they are much higher than the critical micelle concentration (CMC) of CPB in water (3 x 10 4mol/l). One can assume that CPB cations in the gel should form micelle-like aggregates. 

This model represents the most radical treatment among the three most frequently discussed approaches to describe micelle formation it simple postulates ab initio the micellization to be a phase transition. This is justified all the more if large aggregation numbers (like those often encountered in aqueous surfactant solutions) are considered. 

A more common situation is the effect of adsorption on the stability of a suspension. This may be favourable (by modifying, for example, the surface charge of the particles, or by increasing the distance between particles) or unfavourable. Such an example of the latter effect was found with non-ionic surfactant Triton TX-100 and silica suspensions. It was found that the adsorption produced micelle-like aggregates which underwent flocculation by a bridging mechanism (Giordano-Palmino et al.,... 

Hydrophobically modified polymers can associate in aqueous media to form micelle-like structures above their critical association concentrations (CACs). The nanosized self-aggregates were prepared using modified natural polysaccharides such as pullulan, curdlan, and glycol chitosan. The modified polysaccharides provide excellent biocompatibility, biodegradability, low immunogenicity, and biological activities. 

The two-stage growth process of the pellicle is of importance to the structure and function of the pellicle. The first step has been explained by an initial adsorption of discrete proteins to the enamel surface, whereas the second step is the adsorption of salivary protein aggregates in the form of micelle-like structures that move more slowly towards the interfaces, and hence give a stepwise increase in the pellicle thickness [47]. The latter structures are believed to account for the globular surface morphology of acquired salivary pellicle. 

Nam, Y. S. Kang, H. S. Park, J. Y. Park, T. G. Han, S.-H. Chang, I.-S. New micelle-like polymer aggregates made from PEl-PLGA diblock copolymers micellar characteristics and cellular uptake. Biomateiials 2003, 24, 2053-2059. 

Since surfactant molecules can aggregate into micelle-like structures on a solid surface, the enthalpies of micellization in the bulk phase are also listed in this table. They will... 

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