Posted on November 28, 2016 in
The rate of chemical reactions in multi-component, solid systems such as drug products depends exponentially on the water activity (equilibrium relative humidity). It is proposed that this dependence is directly linked to an exponential increase in water content as a function of relative humidity at amorphous (non-crystalline) regions. Such exponential sorption is attributed to cooperative water binding where water-water interactions are predominant. This condensation tendency as a function of relative humidity adjusts for temperature based on the change in water partial pressure in air at saturation. The result is that the exponential condensation (sorption) as a function of water activity is relatively temperature independent. Reaction rates overall increase proportionately to the amount of water condensed due to an increased amount of active in its more reactive dissolved, semi-solid and potentially admixed state. Since the dissolved state itself is independent of RH in character, though not in amount, the activation energy is independent of RH. Temperature effects on solubility, which themselves tend to be exponential for organic materials, are suggested to contribute to the activation energy for the degradation.