Measurement and numerical analysis of freezing in solutions enclosed in a small container

Abstract

The latent heat of fusion, L, of the cryobiological media (a solute laden aqueous solution) is a crucial parameter in the cryopreservation process and has often been approximated to that of pure water (335 mJ/mg). This study experimentally determines the magnitude and dynamics of latent heat during freezing of fourteen different pre-nucleated solute laden aqueous systems using a Differential Scanning Calorimeter (DSC). These solutions include NaCl-H_20, Phosphate Buffered Saline (PBS), serum free cell culture media (RPMI), glycerol and Anti Freeze Protein (AFP) in 1x PBS solutions. The latent heat of the solutions studied is found to be significantly less than that of pure water and is dependent on both the amount and type of solutes (or solids) in solution. DSC experiments are also performed at 1, 5 and 20 C/min in five representative cryobiological media to determine the kinetics of ice crystallization. The total magnitude of the latent heat release L is found to be independent of the cooling rate. However, the experimental data shows that at a fixed temperature, the fraction of heat released at higher cooling rates (5 and 20 C/min) is lower than at 1 C/min for all the solutions studied. We present a model to predict the experimentally measured behavior based on the full set of heat and mass transport equations during the freezing process in a DSC sample pan. Analysis of the parameters relevant to the transport processes reveals that the heat transport occurs much more rapidly than mass transport. The model also reveals the important physical parameters controlling the mass transport at the freezing interface and further elucidates the measured temperature and time dependence of the latent heat release.