Wall polymer. Wall polymers include acacia, alginate, carboxymethyl-cellulose, gelatin, polyethylene glycol, poly (vinyl alcohol), albumin, car-bupol and pectin. Appropriate polymers may be used singly or in pairs as described below.
Cores - water-soluble. In general, water-soluble solids or liquids are not encapsulated to a great extent when water-soluble polymers are used because the core will be distributed between the aqueous polymer-rich phase and the aqueous polymer-poor phase. There are, however, techniques that may be used to encapsulate water-soluble compounds with water-soluble polymers (Harris, 1981).
1. Make the water-soluble core such as KC1 insoluble in water through the use of waxes such as carnauba.
2. While the control of pH is important for water-soluble polymers such as gelatin, it also may be used to alter the solubility of many drugs which are weak acids or bases. Thus, the solubility of salicylic acid is decreased in acid solution and may be a candidate for encapsulation by water-soluble polymers.
3. Preparing water-insoluble cores by first making microcapsules using a water-insoluble polymer and then providing a second coat with a water-soluble coating.
Cores-solid. The encapsulation of a number of solid core materials by means of simple coacervation has been studied in an organized manner by Okada etal. (1985a). The ability of different core particles to be encapsulated with gelatin was studied as a function of different miscible non-solvents, and other manufacturing parameters. The effect of solubility, zeta potential and the adsorption of gelatin was related to the ability of the product to be encapsulated. Low solubility, high gelatin adsorption and zeta potential play a significant role in the ability of the process to encapsulate the core.
Cores - liquid. Research has also been carried out on the encapsulation of liquids with water-soluble polymers. Gelatin-acacia microcapsules containing oils or oils containing a drug were prepared by employing polyethylene glycol or polyethylene oxide as the incompatible or non-wall-forming polymer. After cooling, the microcapsules formed were cross-linked with glutaraldehyde (Jizomoto, 1984). Jizomoto also showed that the minimum concentration of the polymer polyethylene glycol (or polyethylene oxide) necessary for complex coacervation depended upon the molecular weight. The molecular weight may be related to the chemical potential and the excluded volume of the polymer used to effect coacervation (Jizomoto, 1985).
Process variables. The total polymer concentration has been shown to be related to droplet size. The mean diameter of the coacervate droplets of gelatin-Carbopol 941 microspheres increased from 50 /xm to 135 /xm as the concentration of polymers increased fivefold (El Gindy and El Egakey, 1981a,b). Similar results have been described by Mortada etal. (1987a) for the gelatin-Gantrez system.
El Gindy and El Egakey (1981a) showed that the droplet size decreased as the speed of rotation increased for the gelatin-Carbopol system; at a speed of 600-650 r.p.m. the mode was about 35 /¿m, while at 100-150 r.p.m. the mode was approximately 85 /zm.
In order to minimize aggregation in the complex coacervate system, gelatin-acacia, Maierson (1969) added a surfactant to the prepared microcapsules, and as a result of steric and charge effects, microcapsules were kept apart and aggregation minimized.
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