Structure of Micelles

Chevalier Y and Zemb T 1990 The structure of micelles and microemulsions Rep. Prog. Phys. 53 279-371... 

Figure 1 Schematic structures of micelle and liposome, their formation and loading with a contrast agent, (a) A micelle is formed spontaneously in aqueous media from an amphiphilic compound (1) that consists of distinct hydrophilic (2) and hydrophobic (3) moieties. Hydrophobic moieties form the micelle core (4). Contrast agent (asterisk gamma- or MR-active metal-loaded chelating group, or heavy element, such as iodine or bromine) can be directly coupled to the hydrophobic moiety within the micelle core (5), or incorporated into the micelle as an individual monomeric (6) or polymeric (7) amphiphilic unit, (b) A liposome can be prepared from individual phospholipid molecules (1) that consists of a bilayered membrane (2) and internal aqueous compartment (3). Contrast agent (asterisk) can be entrapped in the inner water space of the liposome as a soluble entity (4) or incorporated into the liposome membrane as a part of monomeric (5) or polymeric (6) amphiphilic unit (similar to that in case of micelle). Additionally, liposomes can be sterically protected by amphiphilic derivatization with PEG or PEG-like polymer (7) [1].

KP and v can, in contrast to kp, not be determined via the concentration gradient for binary and ternary mixed micelles, because for the calculation of the Nemstian distribution a constant CMC and an almost constant partial molar volume must be assumed. The calculation of aggregation constants of simple bile salt systems based on Eq. (4) yields similar results (Fig. 8b). Assuming the formation of several concurrent complexes, a brutto stability constant can be calculated. For each application of any tenside, suitable markers have to be found. The completeness of dissolution in the micellar phase is, among other parameters, dependent on the pH value and the ionic strength of the counterions. Therefore, the displacement method should be used, which is not dependent on the chemical solubilization properties of markers. For electrophoretic MACE studies, it is advantageous for the micellar constitution (structure of micelle, type of phase micellar or lamellar) to be known for the relevant range of concentrations (surfactant, lipids). 

The highly dynamic colloidal structures described in this chapter result in considerable complexity in behaviors. This complexity has resulted in relatively slow improvement in our understanding of colloidal systems despite the fact that the structure of micelles was in essence described almost a century ago already. Results from a series of relatively recent approaches to describe colloidal aggregates are now beginning to coalesce into a model of colloidal structures incorporating the dynamic and nonhomogeneous structures of these aggregates. 

Fig. 4. Illustrative structure of micell, bilayer membrane, and liposome (vesicle). O Ionic group v— Long alkyl group...

In polar solvents amphiphilic molecules, that is molecules with a polar head and hydrophobic tail , tend to form various aggregates. The structure of micelles is usually much more complicated than that schematically shown in Figure 1.4 (see the pertaining discussion in Section 2.3). Nevertheless, in water they can include nonpolar molecules into their voids acting like surfactants applied in toiletry [15]. Similarly to cyclodextrins such as 11 [6, 16] and liquid crystals [7] discussed in Section 2.6, surfactants are examples of few supramolecular systems which have found numerous practical applications. 

Excipients offer several possibilities and mechanisms. For microemulsions, Cremophor RH 40, Cremophor EL, and Solutol HS 15 act as surface active solubilizers in water and form the structures of micelles. The micelle that envelops the active substance is so small that it is invisible, or perhaps visible in the form of opalescence. Typical fields of application are oil-soluble vitamins, antimycotics of the miconazole type, mouth disinfectants (e.g., hexiditin), and etherian oils or fragrances. Solutol HS 15 is recommended for parenteral use of this solubilizing system and has been specially developed for this purpose. 

In considering the structure of micelles, we continue to base our discussion on aqueous, anionic surfactant solutions as prototypes of amphipathic systems. Cationic micelles are structured no differently from anionics, and nonionics are described parenthetically at appropriate places in the discussion. We summarize present thinking about the structure of micelles at surfactant concentrations equal to or only slightly above the CMC. We see that in nonaqueous systems (Section 8.8) and in concentrated aqueous systems (Section 8.6), the surfactant molecules are organized quite differently from the structure we describe here. 

PDADMABr gels was significantly higher than in the micelles of SDS in water. At the same time, for the more hydrophobic CTAB, the polarity of the microenvironment of the probe in micelles in the PMAA network is low in comparison to that of the micelles in an aqueous medium. Thus, the results obtained confirm the theoretical prediction that the CMC in charged networks is much lower than in the solution (see Sect. 2.5). At the same time, these results show that there is a significant difference between the structure of micelles which are formed in polyelectrolyte gels and in water. 

Cremophor RH 40, Cremophor EL, and Solutol HS 15 act as surface-active solubilizers in water and form the structures of micelles. The micelle that envelops the active substance is so small that it is invisible or perhaps visible in the form of an opalescence. 

FIGURE 13.5 Two models for the structure of micelles, (a) Uniform dissolution of I in B. (b) Stabilization of a microdroplet of I by putting Aon the interface. (Reproduced from Xing, L. and W L. Mattice. liBWjtjmuir 14 4074-4080. With permission from American Chemical Society.)... 

As with traditional surfactants, additives may inLuence the onset of micellization of polymeric surfactants and thus affect solubilization. These additives can include inorganic salts and sugars used to adjust isotonicity and even the solubilizate drug itself. In addition to micellization, these additives can inLuence the LCST or CP and even the structure of micelles formed. 

Xing, L. and W L. Mattice. 1998. Large internal structures of micelles of triblock copolymers with small insoluble molecules in their corelsangmuir14 4074-4080. 

The properties of surfactant at low concentration in water are similar to those of simple electrolytes except that the surface tension decreases sharply with increase in concentration. At a certain concentration, surfactant monomers assemble to form a closed aggregate (micelle) in which the hydrophobic tails are shielded from water while the hydrophilic heads face water. The critical aggregation concentration is called the critical micelle concentration (CMC) when micelles form in an aqueous medium. The CMC is a property of the surfactant. It indicates the point at which monolayer adsorption is complete and the surface active properties are at an optimum. Above the CMC, the concentrations of monomers are nearly constant. Hence, there are no significant changes in the surfactant properties of the solution since the monomers are the cause of the surface activity. Micelles have no surface activity and any increase in the surfactant concentration does not affect the number of monomers in the solution but affects the structure of micelles. 

We have seen evidence of a similar structuring of micelles in thin foam and emulsion films containing C AOS, in the form of stepwise transitions or stratification phenomena (see Figure 5). 

With this model, which is in fact consistent with the structuring of micelles in the bulk phase, an explanation may be offered for the existence of only one transition for C. A0S and two transitions for C. AOS, viz., the cmc for higher than that of C AOS... 

The original reaction mechanism for the growth of M41S and MCM-41 materials was proposed by Mobil researchers. 1 This proposed mechansim involved formation of rod-like structures of micelles and concomitant formation of a hexagonal array of rods, after which an inorganic species would encapsulate the rods and surround the surfactant species. Calcination of these composite materials led to the... 

Fig. 7 Chemical structures of micelle-forming block copolymers used in drug delivery.

Figure 5.9 Dependence of the shape and structure of micelles on the molecular architecture of the basic surfactant units. The packing possibilities, based on simple geometrical features of the surfactants, build the final shape of the micelles.

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