Shell formation

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. 

A key feature of encapsulation processes (Figs. 4a and 5) is that the reagents for the interfacial polymerisation reaction responsible for shell formation are present in two mutually immiscible Hquids. They must diffuse to the interface in order to react. Once reaction is initiated, the capsule shell that forms becomes a barrier to diffusion and ultimately begins to limit the rate of the interfacial polymerisation reaction. This, in turn, influences morphology and uniformity of thickness of the capsule shell. Kinetic analyses of the process have been pubHshed (12). A drawback to the technology for some apphcations is that aggressive or highly reactive molecules must be dissolved in the core material in order to produce microcapsules. Such molecules can react with sensitive core materials. 

Figure 4b represents the case where a reactant dissolved in the dispersed phase reacts with the continuous phase to produce a co-reactant. The co-reactant and any remaining unreacted original reactant left in the dispersed phase then proceed to react with each other at the dispersed phase side of the interface and produce a capsule shell. Capsule shell formation occurs entirely because of reaction of reactants present in the droplets of dispersed phase. No reactant is added to the aqueous phase. As in the case of the process described by Figure 4a, a reactive species must be dissolved in the core material in order to produce a capsule shell. 

Apart from gastropods, harmful effects of TBT have also been demonstrated in oysters (Environmental Health Criteria 116, Thain and Waldock 1986). Early work established that adult Pacific oysters (Crassostrea gigas) showed shell thickening caused by the development of gel centers when exposed to 0.2 pg/L of TBT fluoride (Alzieu et al. 1982). Subsequent work established the no observable effect level (NOEL) for shell thickening in this, the most sensitive of the tested species, at about 20 ng/L. It has been suggested that shell thickening is a consequence of the effect of TBT on mitochondrial oxidative phosphorylation (Alzieu et al. 1982). Reduced ATP production may retard the function of Ca++ ATPase, which is responsible for the Ca++ transport that leads to CaCOj deposition during the course of shell formation. Abnormal calcification causes distortion of the shell layers. 

Fig. 3.5 Representation of a scheme of an experiment (upper set of drawings) and the obtained experimental results presented as AFM images (middle part) and cross-sectional profiles (bottom) that provides evidence of silica nucleation and shell formation on biopolymer macromolecules. Scheme of experiment. This includes the following main steps. 1. Protection of the mica surface against silica precipitation. It was covered with a fatty (ara-chidic) acid monolayer transferred from a water substrate with the Langmuir-Blodgett technique. This made the mica surface hydrophobic because of the orientation of the acid molecules with their hydrocarbon chains pointing outwards. 2. Adsorption of carbohydrate macromolecules. Hydrophobically modified cationic hydroxyethylcellulose was adsorbed from an aqueous solution. Hydrocarbon chains of polysaccharide served as anchors to fix the biomacromolecules firmly onto the acid monolayer. 3. Surface treatment by silica precursor. The mica covered with an acid mono-...

Machado, J., J. Coimbra, F. Castilho, and C. Sa. 1990. Effects of diflubenzuron on shell formation of the freshwater clam, Anodonta cygnea. Arch. Environ. Contam. Toxicol. 19 35-39. 

SCREW SENSE OF METAL-NUCLEOTIDE COMPLEXES Sea shell formation, BIOMINERALIZATION SECOND... 

It is also possible to generate microcapsules through interfacial polymerization using only one monomer to form the shell. In this class of encapsulations, polymerization must be performed with a surface-active catalyst, a temperature increase, or some other surface chemistry. Herbert Scher of Zeneca Ag Products (formerly Stauffer Chemical Company) developed an excellent example of the latter class of shell formation (Scher 1981 Scher et al. 1998). He used monomers featuring isocyanate groups, like poly(methylene)-poly(phenylisocyanate) (PMPPI), where the isocyanate reacts with water to reveal a free primary amine. Dissolved in the oil-dispersed phase of an oil-in-water emulsion, this monomer contacts water only at the phase boundary. The primary amine can then react with isocyanates to form a polyurea shell. Scher used this technique to encapsulate pesticides, which in their free state would be too volatile or toxic, and to control the rate of pesticide release. 

A number of suggestions have been made that calcium may be transported because it is coupled to the movement of other ions or because it moves passively down an electrochemical gradient established by the movement of some other ions600. Thus, a sodium-induced potential has been found which was sufficient to account for the passive movement of calcium into the shell gland of the domestic fowl during egg shell formation. In the mollusc, the shell side of the mantle is normally positive relative to blood and a potential of this type would, of course, tend to move calcium away from the extrapallial fluid. A potential of this orientation could be produced by the movement of an anion into the animal (mollusc) and the low chloride concentration of the extrapallial fluid could be accounted for on this basis. 

There is a good deal of support for such an origin of the carbonate ion. Certainly, in invertebrates there are conclusive kinetic data based on radioisotope studies to show that, although the blood supplies Ca2+ for egg shell formation, the plasma bicarbonate plays no direct role602). 

A variation of the proton removal hypothesis of shell formation has been proposed603, 604. It has been shown that in molluscs and birds the sites of shell formation contain high concentrations of ammonia and they suggest that this may be an initiating phenomenon for calcification, i. e. 

Weiner, S., Hood, L. Soluble protein of the organic matrix of mollusc shells A potential template for shell formation. Science 190, 987 (1975)... 

Wilbur, K. M. Shell formation in molluscs. Chemical Zoology 7, 103 (1972)... 

Figure 3.25 Cyclopentane hydrate formation from a water droplet (a) initial contact, (b) hydrate shell formation around the water droplet, (c) depressions formed on the hydrate shell, (d) conversion of interior water to hydrate, indicated by darkening, (e) almost completely converted hydrate. (From Taylor, C.J., Adhesion Force between Hydrate Particles andMacroscopic Investigation of Hydrate Film Growth at the Hydrocarbon/Water Interface, Master s Thesis, Colorado School of Mines, Golden, CO, (2006). With permission.)...

Smith, J.D. and Clegg, J.A. (1959) Egg shell formation in trematodes and cestodes. Experimental Parasitology 8, 286-323. 

Wilbur, K. M., and Jodrey, L. H. Studies on shell formation. V. The inhibition of shell formation by carbonic anhydrase inhibitors. Biol. Bull. 108, 359-365 (1955). 

Coombs, J., and Volcani, B. E. Studies on the biochemistry and fine structure of silica-shell formation in diatoms. Chemical changes in the wall of Navicula pelliculosa during its formation. Planta (Berl.) 82, 280-292 (1968). 

Mechanical problems thermal expansion, concentric stress cracks, displacements, deformation of the kiln shell, formation of grooves, forces originating from retaining rings... 

Onuma N Masuda F Hirano M. and Woda K. (1979) Crystal structure control on trace element partition in molluscan shell formation. Geochemical Jour. 13, 187-189. 

It is well known that well-ordered (3-chitin (a polysaccharide) associated with a less ordered protein in the (3-sheet conformation is the main component of nacreous organic matrix in shell. The amino acid sequence of such proteins is very similar to those of silk fibroins. Indeed, the amino acid sequence of a major protein from the nacreous shell layer of the pearl oyster resembles that of spidroin (Sudo et al., 1997 Weiner and Traub, 1980). The question of whether silk-like proteins play an important role in shell formation is raised. When Falini et al. (1996) did the experiment with the proteins from the shell, they assembled a substrate in vitro that contained (3-chitin and natural silk fibroin and concluded that the silk fibroin may influence ion diffusion or the accessibility to the chi tin surface or both. Furthermore, cryo-TEM study of the structure of the Atrina shell nacreous organic matrix without dehydration... 

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