Water distribution studies within cellulose ethers using differential scanning calorimetry. 1. Effect of polymer molecular weight and drug addition.

Differential scanning calorimetry (DSC) was employed to characterize the distribution of water in gels produced from a series of hydroxypropylmethylcelluloses (HPMC, Methocel K-series) of different molecular weights (i.e., different viscosity grades). The presence of loosely bound water was characterized as pre-endothermic events occurring at temperatures below the main melting endotherm of free water. Both the magnitude and occurrence of these pre-endothermic events were affected by polymer molecular weight and gel storage time. In addition, the amount of water bound to the polymer depended on polymer molecular weight and gel storage time. The temperature at which frozen water melted within the gels was dependent on polymer concentration, with a depression of extrapolated endothermic melting peak onset occurring with an increase in polymer concentration. The addition of propranolol hydrochloride or diclofenac sodium, as model drugs, affected both the occurrence of pre-endothermic events and the distribution of water within the gels.