Preparation and characterization of salt hydrates encapsulated in polyamide membranes

Abstract

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 buffer. While this technique is well established, the use of free salt crystals in the reaction mixture poses numerous problems such as difficult recovery and poisoning of the biocatalyst. This article outlines a novel technique for the encapsulation of such materials which avoids these difficulties. The characterization of the capsules and their use as water activity buffers has also been described. Hydrates of Na2HPO4 were encapsulated in a polyamide membrane by interfacial polycondensation (IPC) of sebacoyl dichloride and diethylene triamine soaked onto the surface of the salt crystals. This technique, non-aqueous interfacial polycondensation (NAIPC) circumvents the need for the use of an aqueous phase to supply the polar reactant, the amine, thereby facilitating the encapsulation of water soluble materials. The coatings thus produced have an asymmetric membrane-like structure. A thin, non-porous, layer around the salt crystal supports a superstructure of porous polymer. This composite structure facilitates diffusion of material through the capsule wall and the use of hydrophilic polyamides for encapsulation promotes the transport of water. The capsules produced were between 0.5 and 2.5 mm in size and were of adequate mechanical strength to withstand osmotic pressure differences upto 26 bar and resist attritive forces experienced during their use.