Freeze concentrated glass

The Cg value, the concentration of the maximally freeze concentrated glass (see Glossary), for most carbohydrate solutions has a similar value (see Table 9.3). Highly viscous solutions do not reach equilibrium, and a cooling curve will follow the lower trajectory on graph 1.14 (see Chapter 1). This means that the concentration of water in the glass is actually above the equilibrium content, and subsequent heating will increase mobility such that the excess water can recrystallise to form ice. Both Tg and Cg will thus be difficult to obtain from simple measurements. 

Freeze-concentrated glass A glassy material resulting from a lyophilisation (fi"eeze-drying)... 

Glass transition temperature of the maximally freeze-concentrated glass Transmission electron microscopy Thermogravimetric analysis/analyser... 

Her, L., Deras, M., Nail, S. Electrolyte-induced changes in glass transition temperatures of freeze-concentrated solutes. Pharm Res 12 768-772 (1995). 

Tg versus degree of polymerization was linear.254 Similarly, for a series of commercial starch hydrolysis products (polydisperse solutes), there was a linear relationship between Tg (Tg of the maximally freeze-concentrated solute glass) and 1/MW.270... 

Figure 5.4. A schematic state diagram showing water plasticization at increasing water weight fraction towards glass transition of water at -135°C. Relaxation times decrease rapidly above the glass transition as a result of thermal or water plasticization. Maximally freeze-concentrated solutes show glass transition at Tg and onset of ice melting at TJ. Equilibrium melting is described by the T curve.

Figure 5.5. State diagram of sucrose with typical experimental data at high concentrations and in the maximally freeze-concentrated state. Dynamic mechanical and dielectric measurements show glass transition-related relaxations.

Figure 7.3 is another example of a typical state diagram, developed for maltose. Maltose solutions are in glassy state below Tg curve. T g (onset of glass transition) and (onset of ice melting) show constant values for the maximally freeze concentrated solutions, where maximum ice formation occurs between T and Tg, and T is at the end point region of Tg (Figure 7.3) (Roos and Karel 1991b). State diagram for sucrose (Figure 7.4) also shows similar characteristics (Roos and Karel 1991a).

Her, L.-M. Nail, S.L. Measurement of glass transition temperatures of freeze-concentrated solutes by differential scanning calorimetry. Pharm. Res. 1994, 11, 54-59. 

---