Relaxation and antiplasticization measurements in trehalose–glycerol mixtures – A model formulation for protein preservation

We utilize dielectric relaxation spectroscopy for the quantitative characterization of antiplasticization of glassy-trehalose by glycerol. The high frequency Johari–Goldstein relaxation time (τ) was obtained by analyzing the complex permittivity data in terms of the distribution function of relaxation times and a regularization technique. We analyzed the dielectric spectrum without prior assumptions about a spectral function and the number of the relaxation processes. The ratio of τ values for the mixture and pure trehalose, an antiplasticization factor (Θ), is found to provide a useful measure of the extent of antiplasticization. We observe that increasing the glycerol mass fraction (xw) at fixed temperature increases τ, extending antiplasticization until a temperature dependent critical plasticization concentration is reached. At a fixed concentration, we find an antiplasticization temperature at which antiplasticization first occurs upon cooling. At a temperature of 293 K the antiplasticization factor peak value is about 1.6 when xw of glycerol is about 0.24. At 323 K a mild antiplasticization maximum occurs when xw decrease to about 0.05. Above 323 K, Θ < 1, glycerol plasticizes trehalose, thus the antiplasticizing effect apparently no longer exists. The antiplasticization factor that we describe in terms of Arrhenius functions is a convenient predictive model to characterize antiplasticization in glassy sugar formulations and other glass-formers.