Glass Transition in Mixtures: Effect of Mutual Ionization and Quantitative Account with Modified Fox Equation

Amorphous mixtures have been studied where the components ionize each other by acid–base reactions, causing the glass transition temperature (Tg) to exceed the components' values. This phenomenon conflicts with most current theories where a mixture’s Tg is expressed as a weighted average of the components’ values. We show that the phenomenon can be quantitatively explained by treating the ionized portion as a new component with its distinct Tg and performing the usual weighted average. This model is fundamentally different from the previous thermodynamic models proposed to account for the Tg maxima. Amorphous mixtures in this work contained pharmaceutically important acids and bases, both small molecules and polymers. We tested our model using experimentally determined degrees of ionization, rather than treating them as fitting parameters. This work provides a foundation for predicting the Tg of salt-forming mixtures in designing amorphous materials with low mobility and high stability.