Microcapsules with adsorbed

A polyion layer adsorbed onto a capsule membrane, as depicted in Figure 1, would result in a resistance to mass transfer. Thus, a study of the permeation of low molecular-weight solutes through such a capsule membrane would be useful for obtaining information on pH-induced changes in the configuration of the adsorbed polyion. In this section, the effects of pH on the permeability of microcapsules with adsorbed polyelectrolytes are investigated and compared with results from the viscometric and electrophoretic studies. 

Application of Microcapsules with Adsorbed Polyelectrolytes in the On/Off Control of Enzyme Reactions... 

Capsule membranes with an adsorbed polyelectrolyte layer through which the permeation of solutes is dramatically altered in response to small changes in pH would be useful in constructing a functional encapsulated enzyme system in which the initiation/termination of an enzymatic reaction could be controlled. PSt microcapsules with copoly(MA, St) or PIE seem to be well suited for this purpose, since their permeability alters rapidly over a very narrow pH range as a result of changes in the configuration of the adsorbed polyion. The present section describes the on/off control of an enzyme reaction using pH-sensitive PSt microcapsules with copoly(MA, St). 

Figure 5. Curves of permeability constant (P) against pH for PSt microcapsules (a) without adsorbed polyelectrolyte layer with adsorbed layers of (b) copoly(MA, MVE), (c) copoly(MA, St), and (d) PIE. (Adapted from refs. 5 and 6)...

Similar pH-sensitive on/off control of the hydrolysis of maltotriose can be performed using p-amylase-loaded PSt microcapsules with a surface layer of adsorbed PIE (6). Therefore, pH-sensitive microcapsules, the outer surfaces of which are covered with the adsorbed polyions undergoing rapid changes in configuration, enable the control of enzymatic processes through smdl pH changes in the outer medium. 

Microcapsules with hydrophobic core oils could also be obtained by interface complexation between the maleic copolymer adsorbed on the surface of the organic phase drops and the gelatin from the aqueous continuous phase, followed by the crosslinking of the gelatin [191]. In this case, the maleic comonomer should be hydrophobic in order to orientate itself to the organic solvent and to form a thin copolymer layer... 

Complex Coacervation. This process occurs ia aqueous media and is used primarily to encapsulate water-iminiscible Hquids or water-iasoluble soHds (7). In the complex coacervation of gelatin with gum arabic (Eig. 2), a water-iasoluble core material is dispersed to a desired drop size ia a warm gelatin solution. After gum arabic and water are added to this emulsion, pH of the aqueous phase is typically adjusted to pH 4.0—4.5. This causes a Hquid complex coacervate of gelatin, gum arabic, and water to form. When the coacervate adsorbs on the surface of the core material, a Hquid complex coacervate film surrounds the dispersed core material thereby forming embryo microcapsules. The system is cooled, often below 10°C, ia order to gel the Hquid coacervate sheU. Glutaraldehyde is added and allowed to chemically cross-link the capsule sheU. After treatment with glutaraldehyde, the capsules are either coated onto a substrate or dried to a free-flow powder. 

An unusual type of derivative is the complex that forms between urease and bentonite in acid medium (61). The adsorbed form was found catalytically active. Similarly, urease immobilized in a polyacrylamide gel matrix has been used to prepare a urea-specific enzyme electrode (62). Yet another active water-insoluble derivative has been prepared (63) by allowing p-chloromercuribenzoate-treated urease to react with a diazotized copolymer of p-amino-D,L-Phe and L-Leu. Urease has been found to retain about 20% of its original activity when encapsulated in 100 n microcapsules of benzalkonium-heparin in collodion (64). 

Figure 6. Hydrolysis of sucrose in aqueous suspension of PSt microcapsules without ( , o) and with (A, A) the adsorbed layer of copoly(MA, St) at (, A) pH 4.5 and (o, A) pH 5.5. The arrows show pH adjustment by quick addition of a small amount of 2M HCl or NaOH to the capsule suspension. (Adapted from ref. 5)...

It can be concluded from the above-mentioned differential capacitance curve and cathodic polarization curve analyses that the mechanistic model of electrolytic codeposition of Hquid microcapsules is associated with the stable chelation of—OH and -O groups of the wall material (e.g., PVA, gelatin) with metal ions (e.g., Ni ", Cu +), and this gives rise to positively charged microcapsules. This in turn helps to accelerate the electrophoretic migration of microcapsules in the plating solution. The liquid microcapsules were also adsorbed onto the electrode due to the presence of a surfactant. Consequently, it is feasible for microcapsules to enter the electrical double layer at the interface and to become embedded in the co-deposited coating. 

Solid acids are utilized as one component of pressure sensitive recording paper. The principle of the carbonless paper is indicator color change by adsorption on solid acids. The model construction of carbonless paper consists of a set of two types of paper as schematically shown in Fig. 6.4. The top sheet is coated with indicators encapsulated in microcapsules, and the bottom sheet is coated with solid acids. Applying pressure to the top sheet destroys the microcapsule, and the indicator is released and transferred to the solid acids on the bottom sheet to be adsorbed. 

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