Membrane capsule

Y Okahata, H Noguchi, T Seki. Functional capsule membranes. 26. Permeability control of polymer-grafted capsule membranes responding to ambient pH changes. Macromolecules 20 15-21, 1987. 

Kono et al. [92] made use of this polyelectrolyte complexation between PAA and PEI in the formation of pH sensitive capsules that could release the contents contained in their aqueous centers as a function of pH. In this work, PAA-PEI complex capsule membranes were produced which contained 43.7 % PAA and 56.3% PEI (confirmed by elementary analysis). These capsules showed a minimum degree of swelling between a pH of 3.5 and 7.0 due to complex formation. Consequently, the permeability of phenylethylene glycol through the capsule membrane was lowest between pH 3.5 and 7.0 and increased above 7.0 and below 3.5. These investigators also used turbidimetric and potentiometric techniques to confirm the presence of these polyelectrolyte complexes. 

A related system is that of the lipid-bilayer corked capsule membranes which are formed from ultrathin (about 1 pm thick), spongy, 2.0- to 2.5-mm-diameter, more-or-less spherical nylon bags in which multiple bilayers are immobilized (Fig. 43) [343-345]. They were considered to combine the advantages of mechanical and chemical stabilities of polymeric membranes with the controllable permeabilities of surfactant vesicles. Polymerization of the bilayers, in situ,... 

Sherwood, J.D., Risso, F., Colle-Paillot, F., Edwards-Levy, F., and Levy, M.C. (2003) Rates of transport through a capsule membrane to attain Donnan equilibrium. J. Colloid Interface Sci. 263, 202-212. 

Fig. 9. Schematic diagram of an encapsulated adsorbent (Cb) concentration in bulk phase Cm, concentration in membrane phase Cc) concentration in capsule core Q, concentration in adsorbent pore C concentration in adsorbent surface R, radial distance in capsule R , radius of capsule membrane Ra radius of capsule core r, radial distance in adsorbent r0, radius of adsorbent) [18]...

When the relative volumes are known and the diffusion coefficients in the capsule core and capsule membrane can be estimated a priori in single component adsorption, the parameter to work with is the effective diffusivity in the adsorbent pore (Dn). Then, with the above estimated parameter values, the parameters of competitive adsorption are the maximum concentration at the solid phase of the adsorbent (CsmT), and the equilibrium constants of the target product (KS1) and byproduct (KS2)-... 

Plant cell cultures represent a potentially rich source of secondary metabolites of commercial importance and have been shown to produce them in higher concentrations than the related intact plants. However, plant cell cultures often produce metabolites in lower concentrations than desired and commonly store them intracellularly. These limitations can be overcome by product yield enhancement procedures, including immobilization of cultured cells, and permeabilization, or ideally using a combined immobilization/ permeabilization process with retained plant cell viability. Complex coacervate capsules consisting of chitosan and alginate or carrageenan proved to be effective biomaterials for entrapment, controlled permeabilization of cells and to allow control of capsule membrane diffusivity. 

Beaumont and Knorr (ID) described the detrimental effect of chitosan on cell viability of Apium graveolens. Later it was found that at chitosan concentrations <250 yg/mL (Beaumont, M. and Knorr, D., Univ. of Delaware, unpublished data), plant cell viability was retained. Development of complex coacervate capsules consisting of alginate chitosan (22) and kappa-carrageenan-chitosan (10) allowed the concurrent release of secondary metabolites while still maintaining reasonable cell viability. Chitosan comprised the outer layer of the gel capsule and chitosan diffusivity could be controlled via capsule membrane permeability. 

Okahata Y, Noguchi H, and Seki T. Thermoselective permeation from a polymer-grafted capsules membrane. Macromolecules 1986 19 493 94. 

Kono, K. Tabata, F. Takagishi, T. pH responsive permeability of poly(acrylic acid)-poly(ethylenimine) complex capsule membrane. J. Membrance Sci. 1993, 76 (2-3), 233-243. 

Fig. 26. Experimental ( H)- C cross-polarization spectra of an aqueous dispersion of poly-n-butylcyanoacrylate nanocapsules at different mixing periods tc. The spectra are measured at a resonance frequency of Wc 100 MHz after contact times of cp O.l, 0.25 and 0.5 ms under full proton decoupling. The wide lines derive from the polymer forming the capsule membrane while the narrow lines are assigned to the triglyceride oil. At cp = 0.25 ms, the broad signals of the polymer have almost developed to their full intensity, whereas the signals of the oil still gain amplitude. ...

Systematic studies of exchange rates under variation of the size of the tracer molecule are a potent tool for studies on the capsule membrane permeability. In the given case, the tracer molecules PEO 200 up to PEO 1500 are almost identical in... 

Similar to studies on the porosity of capsule membranes using series of tracer molecules of different size, one may use molecules of similar size which differ in a single other parameter like polarity, shape, flexibility, etc., to yield additional information about the membrane structure. As all these observations are performed in the state of equilibrium distribution, there are no restrictions in terms of the overall duration of the measurement. Overall, systematic studies on the membrane permeability could elucidate a variety of details on the capsule structure and the possible release properties. 

Okahata, Y. and Seki, T., pH-sensitive capsule membranes. Reversible permeability control from the dissociative bilayer-coated capsule membrane by an ambient pH change, J. Am. Chem. Soc., 106, 8065-8070, 1984. 

Capsule membrane PTC systems are more amenable to a mechanistic analysis than typical triphase systems where the mechanism of interaction between the aqueous and organic phases with the catalytic sites is complex and not understood. A mechanism for capsule membrane PTC involving mass transfer and smface reaction for both PTC and IPTC reactions has been developed by Yadav and Mehta(1993), Yadav and Mistry (1995). A Langmuir-Hinshelwood type model with the anchored quaternary-nucleophile complex as the active site was assumed to govern the overall rate of reaction... 

Although capsule membrane PTC is not suitable for direct scale-up to industrial level due to the inconveniences of working with capsules, the principles can be exploited in membrane reactors, with the PT catalyst immobilized on the membrane surface. This would not only enable easy recovery of both aqueous and organic phases after reaction without any problems of emulsification, but also ensure that the PT catalyst does not contaminate the product in the organic phase. Using a membrane reactor will also ensure high mass-transfer rates due to high interfacial areas per unit volume of reactor. More importantly, it will open up possibilities for continuous operation. 

Okahata, Y., and K. Ariga, A New Type of Phase-Transfer Catalysts (PTC) Reaction of Substrates in the Inner Organic Phase with the Outer Aqueous Anions Catalyzed by PTC Grafted on the Capsule Membrane, J. Org. Chem., 51, 5064 (1986). 

Yadav, G. D., and C. KMistiy, A New Model of Capsule Membrane Phase Transfer Catalysis for Oxidation of Benzyl Chloride to Benzaldehyde with Hydrogen Peroxide, J. Mol.Catal, 102, 67 (1995). 

Alginate-polylysine has been used to encapsulate hepatocytes (32-34), parathyroid cells (35) and growth hormone transfected fibroblasts (36). Poly (acryl-onitrile/vinyl chloride) (PAN/PVC) macrocapsules have been used with PC12 (37, 38), embryonic mesencephalon tissue (39), thymic epithelial cells (40), adrenal chromaffin cells (41) and islets (25) using preformed hollow fibers or more recently coextrusion techniques (41) similar to those we have developed microcapsules cannot be made since DMSO is used as the solvent. All these studies have concluded from the maintenance of viability of the islets or cells that immunoprotection provided by the capsule membrane was compatible with... 

The capsule membrane appeared to consist of an outer skin, a thin macropo-rous layer and a very thick dense membrane, with an overall (mean) thickness of 90 pm (Fig. 12). The capsule diameter was 900 pm after 7 days. Smaller capsules can be made with a variation of this method in which the needle is held stationary and the hexadecane pumped past the end of the needle the hexa-decane is recirculated and care must be exercised to ensure that capsules are not entrained with the recirculated hexadecane (Fig. 13). The effect of hexadecane flowrate on capsule size (as it leaves the needle) in an early prototype is shown in Fig. 14 capsules shrink to approximately half their initial diameter as solvent is extracted. Note that capsules as small as 300 pm can be produced. As a consequence of the thin skin it is possible to damage the capsules through mishandling forceps with serrated edges or forcing capsules through narrow bore needles are avoided as a result. 

Capsule membrane phase transfer catalysis selective alkaline hydrolysis and oxidation of benzyl chloride to benzyl alcohol and benzaldehyde... 

Selectivity of multiphase reactions catalysed by phase transfer catalysts can be greatly improved by the use of the so called capsule membrane - PTC (CM-PTC) technique. We report here the theoretical and experimental analysis of the CM-PTC and Inverse CM-PTC for exclusively selective formation of benzyl alcohol and benzaldehyde from the alkaline hydrolysis and oxidation of benzyl chloride, respectively. The theoretical analysis shows that it is possible to simultaneously measure rate constant and equilibrium constant under certain conditions. The effects of speed of agitation, catalyst concentration, substrate concentration, nature of catalyst cation, membrane structure, nucleophile concentration, surface area for mass transfer and temperature on the rate of reaction are discussed. 

Selectivity engineering is a new term that is coined with the engineering aspects of multiphase reactions that could be manipulated through the use of several techniques such as use of an additional immiscible liquid phase, porous inert solids, particles smaller than diffusion film thickness, etc. in order not only to intensify the rates of reaction but also to improve greatly the selectivity of the desired product. We are particularly concerned in this paper with the selectivity engineering aspects of the capsule membrane phase transfer catalysis, which has interesting attributes, for the preparation of benzyl alcohol and benzaldehyde by selective alkaline hydrolysis and oxidation, respectively, of benzyl chloride. 

Inverse capsule membrane phase transfer catalysis (ICM-PTC)... 

There is a merit in having the aqueous phase nucleophile inside the capsule and the organic phase substrate as the bulk outside phase. This way, the capsule can be reused several times and the process can be made economical. The aqueous phase byproduct salt could be washed easily with water, or digested with fresh aqueous solution of the substrate. We have named this process as inverse capsule membrane phase transfer catalysis (ICM-PTC) wherein the locale of the reaction is likely to be outer surface of the capsule. Some aspects of ICM-PTC are also reported in this paper. 

Okahata and Ariga [2] have presented a mechanistic model for CM-PTC wherein the organic phase reactant resides inside the capsule membrane and aqueous phase reactants outside. This model has been modified by Yadav and Mehta [4] and is shown in Figure 1. 

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