PREPARATION AND CHARACTERIZATION OF SALT HYDRATES ENCAPSULATED IN POLYAMIDE MEMBRANES

Thermodynamic water activity control is a common technique in organic- phase biocatalysis. This can be accomplished by using the transitions of salt hydrates between their various hydrated forms as a water buffe r. While this technique is well established, the use of free salt crys tals in the reaction mixture poses numerous problems such as difficult recovery and poisoning of the biocatalyst. This article outlines a no vel technique for the encapsulation of such materials which avoids the se difficulties. The characterization of the capsules and their use as water activity buffers has also been described. Hydrates of Na2HPO4 w ere encapsulated in a polyamide membrane by interfacial polycondensati on (IPC) of sebacoyl dichloride and diethylene triamine soaked onto th e surface of the salt crystals. This technique, non-aqueous interfacia l polycondensation (NAIPC) circumvents the need for the use of an aque ous phase to supply the polar reactant, the amine, thereby facilitatin g the encapsulation of water soluble materials. The coatings thus prod uced have an asymmetric membrane-like structure. A thin, non-porous, l ayer around the salt crystal supports a superstructure of porous polym er. This composite structure facilitates diffusion of material through the capsule wall and the use of hydrophilic polyamides for encapsulat ion promotes the transport of water. The capsules produced were betwee n 0.5 and 2.5 mm in size and were of adequate mechanical strength to w ithstand osmotic pressure differences upto 26 bar and resist attritive forces experienced during their use.