Résumé
Polyurethane foams are interesting enzyme supports for reactions in organic media. This study investigated the effects of: (i) support hydrophilicity; (ii) presence of immobilized lipase within the foams; and (iii) hydrophilic substrate concentration on water activity and on the partitioning of reactants between the microenvironment of the biocatalyst and the bulk organic phase. Two foams were used with different hydrophilicities. The organic phase was ethanol and butyric acid in n-hexane. The system contained water remaining from the polymerisation reaction. Experiments were carried out following a central composite rotatable design as a function of butyric acid concentration and ethanol/butyric acid molar ratio. Water activity was estimated from global medium composition by the UNIFAC-LLE group contribution method. UNIFAC calculations were also used to compare experimental bulk medium compositions with the theoretical composition of a monophasic or a biphasic system. For most experimental conditions, the organic phase composition was consistent with the presence of a water phase with no influence of the presence of enzyme in the foams. The influence of foam hydrophilicity was only significant for low water content systems (<0.05%, v/v). The system behaved as a reverse emulsion with hexane as the continuous phase and water droplets trapped within the foam matrix