Investigation of PEG crystallization in frozen PEG-sucrose-water solutions: II. Characterization of the equilibrium behavior during freeze-thawing

Our objective was to characterize, by DSC and XRD, the equilibrium thermal behavior of frozen aqueous solutions containing polyethylene glycol (PEG) and sucrose. Aqueous solutions of (i) PEG (2.5-50% w/w), (ii) sucrose (10% w/v) with different concentrations of PEG (1-20% w/v), and (iii) PEG (2% or 10% w/v) with different concentrations of sucrose (2-20% w/v), were cooled to -70°C at 5°C/min and heated to 25°C at 2°C/min in a DSC. Annealing was performed for 2 or 6 h at temperatures, ranging from -50 to -20°C. Experiments under similar conditions, on select compositions, were also performed in a powder X-ray diffractometer. Two endotherms, observed during heating of a frozen PEG solution (10% w/v), were attributed to PEG-ice eutectic melting and ice melting, and were confirmed by XRD. At higher PEG concentrations (≥37.5% w/w), only the endotherm attributed to the PEG-ice eutectic melting was observed. Inclusion of sucrose decreased both PEG-ice melting and ice melting temperatures. In unannealed systems with a fixed sucrose concentration (10% w/v), the PEG-ice melting event was not observed at PEG concentration ≤5% w/v. Annealing for 2-6 h facilitated PEG crystallization. In unannealed systems with a fixed PEG concentration (10% w/v), an increase in the sucrose concentration increased the devitrification but decreased the PEG-ice melting temperature. The PEG-ice melting temperatures obtained by DSC and XRD were in good agreement. In ternary systems at a fixed PEG to sucrose ratio, the $T_{\rm g}^\prime$ as well as the PEG-ice melting temperature were unaffected by the total solute concentration. XRD confirmed the absence of a PEG-sucrose-ice ternary eutectic. When the PEG to sucrose ratio was systematically varied, the PEG-ice and ice melting temperatures decreased with an increase in the sucrose concentration. However, at a fixed PEG to sucrose ratio, the PEG-ice melting temperature, was unaffected by the total solute concentration.