Formation of a Dilute Glass

Despite the thermodynamic driving force to achieve the unfrozen water content corresponding to Wg', one must also consider the large kinetic factors which "overtake" the freezing process. At sub-zero temperatures, the formation of an amorphous state is time-dependent since the limiting factor of the process (water removal in the form of ice) becomes more difficult as concentration increases. The exponential effect of viscosity on mass transfer properties acts as the limiting factor for growth. In addition, under conditions where heat removal is rapid, a high level of undercooling at the interface will only add to a further decrease in propagation rate. The net result is that freezing becomes progressively slower as ice crystallization is hindered and consequently more time is required for lattice growth at each temperature.

Therefore the kinetic restriction imposed on the system can lead to a situation in which non-equilibrium freezing, resulting in a partial dilute glass, can occur. The typical pathway a system may follow during non-equilibrium freezing is shown in the figure on the previous page as the line leading to lower Tg (path E) than Tg' with a corresponding lower sucrose concentration in the glass (Cg) and higher water content in the glass (Wg) due to excess undercooled water plasticized within the glass. This is often referred to as a dilute glass. The magnitude of deviation from the equilibrium curve, and hence the actual path followed, may be regarded as a function of the degree of departure from equilibrium.

Systems possessing this undesirable structure may undergo various relaxation-recrystallization mechanisms in order to maximally freeze-concentrate and minimize the unfrozen water content. As a result, during warming, systems formed under these conditions may lead to one or more low temperature transitions, followed by an exothermic devitrification peak due to crystallization of immobilized water, and finally the onset of ice melting, Tm.

Ice crystallization, recrystallization and glass transitions are active areas of our research here at the University of Guelph. Please see my publications for more details of our research.