Core dissolution

Fig. 8 Hollow capsule fabrication by the polyelectrolyte LbL self-assembly. The core is alternately coated with polycation and polyanion, followed by core dissolution and capsule formation...

The mechanical properties of polyelectrolyte multilayer capsules have been subject of several studies using different methods. Baumler and co-workers [7] have used the micropipette technique and found that PMCs are not conserving their volume if pressure differences are applied between inside and outside of the shell. This is expected, since the shells can only be formed in first place because the membrane is permeable to low molecular weight species, the core dissolution products. They found no deformation up to a critical pressure followed by an irreversible collapse, showing that shells deform not elastically but plastically for large deformations. First quantitative estimates of the Young s modulus of the shell material were obtained by Gao and coworkers, using osmotic pressure differences between inside and outside of the shell [8,9], These authors monitored the onset of the buck-... 

Stable polyelectrolyte microcapsules were produced by means of the layer-by-layer adsorption of protamine and alginic acid on the surface of calcium carbonate microcores followed by the cores dissolution at low pH. The capsules obtained were investigated by atomic force microscopy and confocal laser scanning microscopy. 

In the last decade hollow spheres are extensively studied in the context of application as containers of prolonged action for substances of the different chemical nature dmgs, cosmetics, dye. A number of methods for preparation of microspheres with the sizes ranging from nanometers to micrometers and consisting of various materials are developed. Polyelectrolye capsules have been produced by sequential adsorption of oppositely charged polyelectrolytes, also known as Layer-by-Layer (LbL) assembly onto the surface of colloidal particles followed by core dissolution [1-2]. Most of the capsules applications imply their chemical or physicochemical modification by influence of the ionic strength [3], pH [3], temperature... 

Thus, overall, the details of core formation are not entirely clear, the exact nature of the core is still open to speculation, and the details of core dissolution need much more study. 

FIGURE 51.21 Electron micrograph of Au—Si02 (15 nm core, 10 nm shell) after exposure to 1 mM KCN at pH 10.5 in air. Hollow silica shells are obtained through complete core dissolution. Some non-dissolved cores are shown as well. 

Figure 8.11. TEM images of hollow (PAH/PSS)4 capsules obtained after core dissolution by THE (a) and (PAH/PSS)4 capsules loaded with silver nanoparticles (b). Taken with permission from [105], P. Rijiravanich, M. Somasundrum, and W. Surareungchai, Anal. Chem. 80, 3904-3909 (2008). American Chemical Society.

The curve is characterized by a delay due to hydration of the Elvanol coat (lag time), a linear portion corresponding to a constant concentration gradient across the Elvanol barrier and an exponential final portion after the complete core dissolution. 

Shell layer number is connected with capsules stability. In principle it is better to have capsules with as few layers as possible. The only low limit is connected with osmotic pressure rising inside the capsules during the core dissolution process. To avoid pressure values critical to the shells one needs to find new materials, to decrease the capsules diameter or to use some chemical reactions to strengthen capsule shells. 

The opportunity of inclusion of various substances inside microcapsules is the most interesting functionality of these structures. All variety of capsulation ways can be divided into two basic groups — capsulation during the formation of polyelectrolyte microcapsule shells and capsulation after core dissolution. 

Different colloidal cores can be decomposed after multilayers are assembled on their surface. If the products of core decomposition are small enough to expel out of polyelectrolyte multilayer the process of core dissolution leads to formation of hollow polyelectrolytes shells (Fig. 2.1, d-f). Up to now, various colloidal templates such as organic and inorganic cores, like MF-particles, organic crystals, carbonate particles and biological cells were used as templates for hollow capsule fabrication. Decomposition can be done by different means, such as low pH for MF- and carbonate particles [43], organic water miscible solvents for organic crystals [44] and... 

Micrometer-sized hollow spheres with metal NPs (10-30 nm) were obtained by photoreduction of Ag in polyelectrolyte multilayers comprising Ag -PSS layers immobilized onto submicrometer-sized PS particles [138]. Hollow capsules with metal NPs can be formed either via a reduction of Ag followed by a core dissolution or by a core dissolution with a subsequent reduction of Ag. The formed spherical nanocomposites with silver NPs were stable for long time (over 3 months). The silver-based core-shell particles and hollow spheres may find interesting applications in catalysis and molecular photoprinting. The PEI-Pd ... 

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