Micelle lyophilization

Enzymes are suspended in hydrated microemulsion surrounded by a monolayer of surfactant molecules dispersed in an apolar solvent [53-60,135] [Fig. 1(b)]. Micelles ( 2 nm sphere) are formed when lyophilized or aqueous preparation of enzymes are introduced with stirring or shaking into a solution of synthetic or natural surfactant in an organic solvent. 

Micelle solutions of PlPAAm-Ci8H35 was prepared by direct dissolution of the polymer in cold water (4°C) due to its good water solubility [23]. Each solution of PIPAAm-PSt, PlPAAm-PBMA, and PIPAAm-PLA was prepared by dissolving each copolymer in DMF, A-ethylacetamide, and DMAc, respectively. The solutions were put into a dialysis bag (MWCO = 13,000) and dialyzed against distilled water at 10°C, 20°C, and 4°C, respectively, for 24 hours. The micelles were purified with ultrafiltration membrane of 200,000 molecular weight cut off at 4°C. The aqueous solution was lyophilized to leave a white powder of micelles. 

As shown in Figure 6.4, the water-insoluble monomer (M) is attracted to the lyophilic ends in the micelles, causing the micelles to swell. The number of swollen micelles per milliliter of water is on the order of 10. However, at the initial stages of polymerization (phase I) most of the monomer is present as globules that resemble those observed in suspension polymerization. 

Above we used the words continuous phase and dispersed phase to refer to the medium and to the particles, respectively, in the colloidal size range. It should be understood that these are solvent and solute in lyophilic systems. In micellar systems, the micelles are dispersed in an aqueous continuous phase. Furthermore, the system as a whole is generally called a dispersion when we wish to emphasize the colloidal nature of the dispersed particles. This terminology is by no means universal. Lyophilic dispersions are true solutions and may be called such, although this term ignores the colloidal size of the solute molecules. 

La et al. (1996) used the oil-in-water emulsion method to prepare IMC-incorporated micelles. Sixty mg of PEO-PBLA without drug was dissolved in 120 mL of distilled water and homogenized by sonication for 30 s. To the solution of PB PBLA micelle in distilled water, a chloroform solution of IMC (6 mg in 1.8 mL) was added drop wise under vigorous stirring at room temperature. The chloroform was removed by evaporation from the open system. The solution was then Lltered, using an Amicon YM-30 ultraLltration membrane (MWCO 50,000) to remove unbound IMC and low molecular weight contaminants, and lyophilized. 

Critical micelle concentrations depend almost entirely on the nature of the lyophobic part of the surfactant. If micelle structure involved some kind of crystal lattice arrangement, the nature of the lyophilic head group would also be expected to be important. 

Microemulsions, like micelles, are considered to be lyophilic, stable, colloidal dispersions. In some systems the addition of a fourth component, a co-surfactant, to an oil/water/surfactant system can cause the interfacial tension to drop to near-zero values, easily on the order of 10-3 - 10-4 mN/m, allowing spontaneous or nearly spontaneous emulsification to very small drop sizes, typically about 10-100 nm, or smaller [223]. The droplets can be so small that they scatter little light, so the emulsions appear to be transparent. Unlike coarse emulsions, microemulsions are thought to be thermodynamically stable they do not break on standing or centrifuging. The thermodynamic stability is frequently attributed to a combination of ultra-low interfacial tensions, interfacial turbulence, and possibly transient negative interfacial tensions, but this remains an area of continued research [224,225],... 

This micelle is comprised of surfactant molecules consisting of long hydrocarbon tails attached to an anionic lyophilic group. Typically, there are 50-100 molecules in the micelle. Some counterions in the medium are adsorbed on the aggregate, whereas others form the diffuse ionic environment. Some workers believe that there is considerable penetration of the medium into the micelle. Micelles are important for detergent action, with oily dirt particles dissolved in the hydrocarbon interior of the micelle. 

Most kinds of emulsions that will be encountered in practice are lyophobic, metastable emillsions. However, there remain some grey areas in which the distinction between lyophilic and lyophobic dispersions is not completely clear. A special class of aggregated surfactant molecules termed micelles and the microemulsions of extremely small droplet size are usually but not always considered to be lyophilic, stable, colloidal dispersions and will be discussed separately. 

Colloids can be broadly classified as those that are lyophobic (solvent-hating) and those that are lyophilic and hydrophilic. Surfactant molecules, because of their dual affinity for water and oil and their consequent tendency to associate into micelles, form hydrophilic colloidal dispersions in water. Proteins and gums also form lyophilic colloidal systems. Hydrophilic systems are dealt with in Chapters 8 and 11. Water-insoluble drugs in fine dispersion or clays and oily phases will form lyophobic dispersions, the principal subject of this chapter. While lyophilic dispersions (such as phospholipid vesicles and micelles) are inherently stable, lyophobic colloidal dispersions have a tendency to coalesce because they are thermodynamically unstable as a result of their high surface energy. 

Some colloidal systems such as polymer solutions and surfactant solutions containing micelles are thermodynamically stable and form spontaneously. These types of colloids are called lyophilic colloids. However, most systems encountered contain lyophobic colloids (particles insoluble in the solvent). In the preparation of such lyophobic colloidal dispersions, the presence of a stabilizing substance is essential. Because van der Waals forces usually tend to lead to agglomeration (flocculation) of the particles, stability of such colloids requires that the particles repel one another, either by carrying a net electrostatic charge or by being coated with an adsorbed layer of large molecules compatible with the solvent. 

Peak fractions containing PEG-PE micelles eluted in the void volume are pooled, dialyzed against water, and lyophilized. 

The formation of lyophilic colloidal systems in broad temperature and concentration ranges is typical if molecules of one of the components are of a strong diphilic nature. Such are the surfactant molecules with either ionic or large non-ionic polar group and long hydrocarbon chain. The ability of these surfactants to spontaneously form equilibrium colloid systems, referred to as micelles, stipulates their broad use in various applications [1]. 

Micellar dispersions, which contain micelles along with individual surfactant molecules, are the typical examples of lyophilic colloidal systems. Micelles are the associates of surfactant molecules with the degree of association, represented by aggregation number, i.e. the number of molecules in associate, of 20 to 100 and even more [1,13,14]. When such micelles are formed in a polar solvent (e.g. water), the hydrocarbon chains of surfactant molecules combine into a compact hydrocarbon core, while the hydrated polar groups facing aqueous phase make the hydrophilic shell. Due to the hydrophilic nature of the outer shell that screens hydrocarbon core from contact with water, the surface tension at the micelle - dispersion medium interface is lowered to the values othermodynamic stability of micellar systems with respect to macroscopic surfactant phases. 

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