Solvent coring

Fig. 2 Schematic picture of a set of contrast variation experiments (A) hydrogenous hydrophobe and head group, deuterated solvent (drop contrast) (B) deuterated hydrophobe hydrogenous head group and deuterated solvent, (shell contrast) and (C) deuterated hydrophobe and hydrogenous head group and hydrogenous solvent (core contrast).

Polymer capsules Core-shell Flavour composition entrapped in capsule consisting of solvent core covered by protein shell Dry product or liquid dispersion Limited (oxidative) stability of encapsulated flavour Flavour can be loaded in empty particles Flexible particle size Relatively slow release/burst-like release... 

Deviations of the parameter a from the theoretical value may be due to small variations in local structural properties of the activated complex during the expulsion process. Here, factors like screening of solvent/core polymer contacts by the corona block and ill-defined core-corona interfaces might come into play. However, discrepancies could also arise from small uncertainties in the determination of polymer characteristics. However, these discrepancies are obviously system-specific and depend on selective solvent, type of block polymer, temperature, and degree of polymerization and are thus of minor relevance for the general understanding of equdibrium kinetics. From the experimental point of view, a more systematic study... 

This intricate scheme of interaction may be described in terms of sets of compatibility parameters easily calculated (or measured) for relevant binary systems (e.g., side chain/solvent, core/solvent, core/side chain) [49,50]. The knowledge of pairwise parameters is also necessary for a quantitative assessment of the temperature variation of solubility and demixing (two liquid phases or crystallization). According to an approximate treatment of binary solutions originally developed for mixtures of poorly interacting apolar polymers, a liquid-liquid phase separation is expected to occiu at a critical temperature T = 0 > 0 at which a balance of the enthalpy (k) and entropy i/r) components... 

Micellar structure has been a subject of much discussion [104]. Early proposals for spherical [159] and lamellar [160] micelles may both have merit. A schematic of a spherical micelle and a unilamellar vesicle is shown in Fig. Xni-11. In addition to the most common spherical micelles, scattering and microscopy experiments have shown the existence of rodlike [161, 162], disklike [163], threadlike [132] and even quadmple-helix [164] structures. Lattice models (see Fig. XIII-12) by Leermakers and Scheutjens have confirmed and characterized the properties of spherical and membrane like micelles [165]. Similar analyses exist for micelles formed by diblock copolymers in a selective solvent [166]. Other shapes proposed include ellipsoidal [167] and a sphere-to-cylinder transition [168]. Fluorescence depolarization and NMR studies both point to a rather fluid micellar core consistent with the disorder implied by Fig. Xm-12. 

As pointed out earlier, the contributions of the hard cores to the thennodynamic properties of the solution at high concentrations are not negligible. Using the CS equation of state, the osmotic coefficient of an uncharged hard sphere solute (in a continuum solvent) is given by... 

The majority of practical micellar systems of Tionnal micelles use water as tire main solvent. Reverse micelles use water immiscible organic solvents, altlrough tire cores of reverse micelles are usually hydrated and may contain considerable quantities of water. Polar solvents such as glycerol, etlrylene glycol, fonnamide and hydrazine are now being used instead of water to support regular micelles [10]. Critical fluids such as critical carbon dioxide are... 

Micelles are mainly important because they solubilize immiscible solvents in their cores. Nonnal micelles solubilize relatively large quantities of oil or hydrocarbon and reverse micelles solubilize large quantities of water. This is because the headgroups are water loving and the tailgroups are oil loving. These simple solubilization trends produce microemulsions (see section C2.3.11). 

Figure 4a represents interfacial polymerisation encapsulation processes in which shell formation occurs at the core material—continuous phase interface due to reactants in each phase diffusing and rapidly reacting there to produce a capsule shell (10,11). The continuous phase normally contains a dispersing agent in order to faciUtate formation of the dispersion. The dispersed core phase encapsulated can be water, or a water-immiscible solvent. The reactant(s) and coreactant(s) in such processes generally are various multihmctional acid chlorides, isocyanates, amines, and alcohols. For water-immiscible core materials, a multihmctional acid chloride, isocyanate or a combination of these reactants, is dissolved in the core and a multihmctional amine(s) or alcohol(s) is dissolved in the aqueous phase used to disperse the core material. For water or water-miscible core materials, the multihmctional amine(s) or alcohol(s) is dissolved in the core and a multihmctional acid chloride(s) or isocyanate(s) is dissolved in the continuous phase. Both cases have been used to produce capsules. 

Figure 4c illustrates interfacial polymerisation encapsulation processes in which the reactant(s) that polymerise to form the capsule shell is transported exclusively from the continuous phase of the system to the dispersed phase—continuous phase interface where polymerisation occurs and a capsule shell is produced. This type of encapsulation process has been carried out at Hquid—Hquid and soHd—Hquid interfaces. An example of the Hquid—Hquid case is the spontaneous polymerisation reaction of cyanoacrylate monomers at the water—solvent interface formed by dispersing water in a continuous solvent phase (14). The poly(alkyl cyanoacrylate) produced by this spontaneous reaction encapsulates the dispersed water droplets. An example of the soHd—Hquid process is where a core material is dispersed in aqueous media that contains a water-immiscible surfactant along with a controUed amount of surfactant. A water-immiscible monomer that polymerises by free-radical polymerisation is added to the system and free-radical polymerisation localised at the core material—aqueous phase interface is initiated thereby generating a capsule sheU (15). 

Solvent Evaporation. This encapsulation technology involves removing a volatile solvent from either an oil-in-water, oil-in-oil, or water-in-oH-in-water emulsion (19,20). In most cases, the shell material is dissolved in a volatile solvent such as methylene chloride or ethyl acetate. The active agent to be encapsulated is either dissolved, dispersed, or emulsified into this solution. Water-soluble core materials like hormonal polypeptides are dissolved in water that contains a thickening agent before dispersion in the volatile solvent phase that contains the shell material. This dispersed aqueous phase is gelled thermally to entrap the polypeptide in the dispersed aqueous phase before solvent evaporation occurs (21). 

Spray Drying. Spray-dry encapsulation processes (Fig. 7) consist of spraying an intimate mixture of core and shell material into a heated chamber where rapid desolvation occurs to thereby produce microcapsules (24,25). The first step in such processes is to form a concentrated solution of the carrier or shell material in the solvent from which spray drying is to be done. Any water- or solvent-soluble film-forming shell material can, in principle, be used. Water-soluble polymers such as gum arable, modified starch, and hydrolyzed gelatin are used most often. Solutions of these shell materials at 50 wt % soHds have sufficiently low viscosities that they stiU can be atomized without difficulty. It is not unusual to blend gum arable and modified starch with maltodextrins, sucrose, or sorbitol. 

Air-Suspension Coa.ting. The Wurster process utilizes a cylindrical chamber in which the cores are suspended in a controlled stream of air. Film coatings are appHed by introducing the coating solution into the airstream, where the solvent evaporates quickly. The process is much quicker than film coating however, care must be taken to avoid destmction of the cores by attrition in the air stream. 

Water-borne adhesives are preferred because of restrictions on the use of solvents. Low viscosity prepolymers are emulsified in water, followed by chain extension with water-soluble glycols or diamines. As cross-linker PMDI can be used, which has a shelf life of 5 to 6 h in water. Water-borne polyurethane coatings are used for vacuum forming of PVC sheeting to ABS shells in automotive interior door panels, for the lamination of ABS/PVC film to treated polypropylene foam for use in automotive instmment panels, as metal primers for steering wheels, in flexible packaging lamination, as shoe sole adhesive, and as tie coats for polyurethane-coated fabrics. PMDI is also used as a binder for reconstituted wood products and as a foundry core binder. 

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