Influence of ice formation on the dynamic and thermodynamic properties of aqueous solutions

Abstract

Water dynamics in solutions with biological or non-biological solutes has been intensely studied when both components (solvent and solute) are amorphous. Here, we apply broadband dielectric spectroscopy combined with calorimetric measurements to analyze the dynamics of the aqueous solutions tri-propylene glycol (3PG) and ε-poly (lysine) (ε-PLL), after their water becomes semi-crystalline. Various crystallization levels were explored by conducting experiments with different annealing times at temperatures above the glass transition temperature (Tg). We find that the amount of ice depends on both the time and temperature of the annealing, and that this, in turn, affects Tg and dynamics of the amorphous part of the samples. However, it should be noted that the observed differences are relatively small for the degrees of crystallinity we have studied (up to about 26 wt% of the water). This also implies that the dynamic crossover of the water relaxation from a high temperature non-Arrhenius behavior to a low temperature Arrhenius dependence is unaffected by the partial crystallization and still occurs as a single crossover at the calorimetric Tg. Thus, we cannot detect two different crossovers, as commonly observed for other types of two-component systems, such as two glass formers.