Micellar-type aggregates

Chemical modification by simple copolymerization, for example, creates a new class of polyelectrolyte [20,27-43,129] allowing exploitation of their amphiphilic nature the ability of such polymers to form macromolecular aggregates with a micellar-type structure has been recognized [28,33,128] and their capacity to solubilize organic species in aqueous media [31,32,38,40,44] is of importance in consideration of potential applications which range from controlled release of materials [165,181-183] to photochemical conversion and storage [31,32,36,38,39,44,45]. 

Phases built up of discrete aggregates include the normal and reversed micellar solutions, micellar-type microemulsions, and certain (micellar-type) normal and reversed cubic phases. However, discrete self-assemblies are also important in other contexts. Adsorbed surfactant layers at solid or liquid surfaces may involve micellar-type structures and the same applies to mixed polymer-surfactant solutions. 

Behavior of metal complexes of phthalocyanines in supramolecular systems with TX-lOO (Fig. 13.8) is very different from other systems of micellar carriers. Tetra-substituted ZrL Pc, as in other supramolecular systems, has an absorption band with a wavelength of 678 nm corresponding to monomeric of molecules in the micelles. In micellar solutions of SiCl Pc absorption maximum at wavelength of 676 nm is observed only, indicating a completely isolated metal complex in this conditions. Unlike other systems, V=OPc has a well-defined absorption band at 678 mn, which corresponds to the presence of monomers in the system, the band bathochromic shifted to 830 nm, which corresponds to J-aggregates and the absorption band hypsochromic shifted to 630 nm, which corresponds to H-aggregates. Hence, V=OPc molecules interaction with TX-lOO micelles form three different type aggregation states of the molecules. 

Micellization is a second-order or continuous type phase transition. Therefore, one observes continuous changes over the course of micelle fonnation. Many experimental teclmiques are particularly well suited for examining properties of micelles and micellar solutions. Important micellar properties include micelle size and aggregation number, self-diffusion coefficient, molecular packing of surfactant in the micelle, extent of surfactant ionization and counterion binding affinity, micelle collision rates, and many others. 

Polar lipids form different kinds of aggregates in water, which in turn give rise to several phases, such as micellar and liquid crystalline phases. Among the latter, the lamellar phase (La) has received the far greatest attention from a pharmaceutical point of view. The lamellar phase is the origin of liposomes and helps in stabilizing oil-in-water (O/W) emulsions. The lamellar structure has also been utilized in creams. We have focused our interest on another type of liquid crystalline phase - the cubic phase... 

What characterizes surfactants is their ability to adsorb onto surfaces and to modify the surface properties. At the gas/liquid interface this leads to a reduction in surface tension. Fig. 4.1 shows the dependence of surface tension on the concentration for different surfactant types [39]. It is obvious from this figure that the nonionic surfactants have a lower surface tension for the same alkyl chain length and concentration than the ionic surfactants. The second effect which can be seen from Fig. 4.1 is the discontinuity of the surface tension-concentration curves with a constant value for the surface tension above this point. The breakpoint of the curves can be correlated to the critical micelle concentration (cmc) above which the formation of micellar aggregates can be observed in the bulk phase. These micelles are characteristic for the ability of surfactants to solubilize hydrophobic substances in aqueous solution. So the concentration of surfactant in the washing liquor has at least to be right above the cmc. 

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