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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.

The distribution of water within gels composed of a range of cellulose ether polymers of similar molecular weights (viscosity grades of 4000-6000 cP) but varying substitution types and levels was assessed by differential scanning calorimetry (DSC). Water loosely bound to the polymer was detected as one or more events appearing at the low-temperature side of the main endotherm for the melting of free water in DSC scans. Polymer substitution types and levels, and added drugs (50 mM propranolol hydrochloride or 50 mM diclofenac sodium) influenced the appearance of these melting events. Hydroxypropylcellulose (HPC) and hydroxypropylmethylcellulose (HPMC F4M) gels showed behavior different to that of the other polymers studied. It is thought that any water binding to HPC gels is tightly attached and is not visible as pre-endothermic events on DSC scans. The amount of water bound per polymer repeating unit (PRU) was influenced by and related to the degree of hydrophilic and hydrophobic substitution on the polymer backbone and by the inclusion of either drug. HPC gels had the highest bound water content after 96 h and this was probably related to the high percentage of hydrophilic hydroxypropoxyl substitutions in this polymer. In contrast, methylcellulose (MC A4M) had the lowest bound water content after 96 h storage, and this was explained by the lack of hydrophilic hydroxypropoxyl substitutions in the polymer.

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