Solid core-liquid shell

FIGURE 11.1 Core-shell structure for (a) solid core-hairy shell, (b) solid core-liquid shell, and (e) double emulsions. S = solid, H = hairy shell, and L = liquid. 

Core/shell particles exhibit a number of features which make them very useful for controlled (sustained or triggered) release purposes. The release kinetics may be zero-order (constant rate) or first order (exponentially decreasing with time. In sustained release, the release kinetics is primarily determined by the structure of the core. Solid cores (i.e. of the active molecule itself) will give rise to zero-order kinetics during sustained release through a shell. Clearly, there must, in this case, be a reverse flow of liquid from the continuous phase back... 

Diagrams of the second type have been constructed for model systems with model sets of parameters but the only realistic system for which this approach has been applied is to argon clusters. The model parameters indicate that it should be possible to find cases in which pc increases at low temperatures and /> , only at higher temperatures. This would be the situation of a solid shell stable around a liquid core, in contrast to the usual case one would expect, of a liquid surface around a solid core. 

FIGURE 10.1 Different types of particles (from left to right) core-shell encapsulated particle with a solid or liquid core and a solid shell, core-shell encapsulated particle with a cell suspension inside, classical matrix encapsulated, respectively, granulated particle. 

Detailed study by use of optical and electron microscopy has shown that sulfate particles are often present in liquid or semi-liquid form. This feature has formed the basis for techniques by which the chemical "speciation" of atmospheric aerosol can be investigated (e.g. 18). It is now well accepted that acidic sulfate particles (which are typically composed of sulfuric acid or ammonium bisulfate) are hygroscopic and tend to leave impaction patterns indicative of a liquid shell, perhaps surrounding a solid core. 

Figure 4d represents in situ encapsulation processes (13,14), an example of which is presented in more detail in Figure 6 (14). The first step is to disperse a water-immiscible liquid or solid core material in an aqueous phase that contains urea, melamine, water-soluble urea-formaldehyde condensate, or water-soluble urea-melamine condensate. In many cases, the aqueous phase also contains a system modifier that enhances deposition of the aminoplast capsule shell (14). This is an anionic polymer or copoljuner (Fig. 6). Shell formation occurs once formaldehyde is added and the aqueous phase acidified, eg, pH 2-4.5. The system is heated for several hours at 40-60°C.

This results in the initiation of the crystallization, which increases the volume of the crystalline core of the nickel droplet at the expense of nickel from the liquid shell and as a consequence, decreases the volume of the liquid shell (Fig. 25 (c)). In this case, since the solubility of carbon in the nickel liquid phase is much higher than that in the solid phase, carbon remains in the liquid phase and its concentration in the phase sharply increases. Eventually, this results in a supersaturation of the liquid with carbon and the transition to the two-phase region L+C of the coexistence of crystalline carbon and liquid i.e., carbon precipitates as an individual crystalline phase in the form of nanotubes (Fig. 25(d)). 

The liquid shell nucleation (LSN) model [34] assumes that a liquid layer of thickness Kq is in equilibrium at the surface, which indicates that the surface melts before the core of the solid. 

The nature of bonding in the carbides is known to be a mixture of covalent and metallic with little ionic tendency. If solid solutions were formed, Ta from TaSi substimted the transition metal atoms in the carbide lattice. This may occur either by cations diffusion or by solntion-precipitation. Given the low diffusion coefficient of this class of materials, it is presumed that lattice diffusion can occur only at very high temperature. Indeed, solution re-precipitation seems to be the dominant mechanism, in light of the sintering behaviour characterized by a relatively low T, 1400-1600°C (Table 2). The well-defined boundary between core and shell and the morphology of the interface between them also put forward a re-precipitation from liquid phase over a diffusion process. 

Measuring static granular structures by MRI incurs some complications. The majority of NMR/MRI experiments to date use heterogeneous particles, specifically, solid particles with liquid cores. Thus, a close packed array of such particles would not image as close packed but would show gaps because the solid shells would yield weak or no NMR signal. 

Fig. 3. Comparison between prediction of typical shell and core model for partial reaction of a solid by a liquid or gas leading to solution or gasification with an unattacked solid residue.

Since this process depends on the stoichiometry of the reactants, sufficient amounts of EDA must be present to produce fully solidified polymer particles. Incomplete reactions yielded a polyurethane shell, which on the removal of unreacted liquid in the core by evaporation resulted in hollow particles (68). It would appear that the solid encapsulating polymer inhibits the diffusion of EDA into the rest of the original droplet. 

The core may be a solid or a liquid, or indeed a gas but this is unusual in the context of release applications. Hollow particles have found applications in other areas such as surface coatings, where they offer a high refractive index contrast with film itself, and therefore good light scattering properties. The shell... 

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