A Model System for Differentiating Between Water Activity and Glass Transition Effects on Solid State Chemical Reactions

Recent debates have emerged on whether water activity (aw) or the state of the system as dictated by the glass transition temperature (Tg) controls the rates of chemical reactions in reduced‐moisture solid systems. Previously, model systems in which the effects of water activity and glass transition on chemical reactions could be evaluated independently did not exist. The use of polyvinyl‐pyrrolidone (PVP) of different molecular weights allows the water activity and moisture content of the systems to be kept virtually constant while the glass transition temperature varies. At a given water activity and temperature, the equilibrium moisture content for any molecular weight of PVP differed by only 1–2%, while the Tg values differed by 20–30C. Using PVP of different molecular weights as a model system will allow the effects of water activity and glass transition on chemical reactions to be studied independently and at a constant temperature.