Calorimetric studies of dissolution of hydroxypropyl methylcellulose E5 (HPMC E5) in water.

The interaction of water with polymers is important because it directly or indirectly affects various properties such as rheological and transport properties. In this study, the hydration/dissolution of a hydrophilic polymer was investigated by calorimetric techniques. Hydroxypropyl methylcellulose E5 (Methocel, HPMC E5) was used as the model polymer. A water-insoluble, hydrophobic polymer, ethyl cellulose E4 (Ethocel, EC E4), was selected for comparison. The number of moles of non-freezing and freezing water were determined by differential scanning calorimetry. Moles of non-freezing water per polymer repeat unit were 6.2 +/- 1.3 and 1.6 +/- 0.3 for HPMC E5 and EC E4, respectively. The hydration/dissolution of HPMC E5 was exothermic, with a total heat of dissolution of -24.1 +/- 1.1 cal/g (n = 4). The heat of hydration of water-insoluble EC E4 was -2.6 cal/g (n = 2). The specific heat capacity value of HPMC E5 increased due to the addition of water. The heat of solution and an increase in specific heat capacity values were associated mainly with the addition of tightly bound water to the polymer. The dissolution of HPMC E5 in water is believed to have endothermic and exothermic components. In summary, the study provided insight into the dissolution of a representative hydrophilic polymer in water. The heat of solution and the specific heat capacity values were mainly dependent on the addition of tightly bound water.