N. Murase, P. Echlin, F. Franks
Aug 1, 1991
Citations
2
Influential Citations
18
Citations
Journal
Cryobiology
Abstract
The behavior of phosphates concentrated during freezing was investigated because of its importance in the cryopreservation or freeze-drying of biological materials. The appearance of the freeze-dried phosphates was observed by scanning electron microscopy (SEM), while the eutectic formation and melting of the salt/ice mixtures was examined by differential scanning calorimetry (DSC). There was a reasonably close relationship between the two sets of results. Of the four sodium and potassium phosphates, the monosodium salt did not exhibit the equilibrium phase behavior under the experimental conditions. Instead, it formed supersaturated solutions which turned into an amorphous state without precipitation; this was confirmed by SEM. The disodium and monopotassium salts, on the other hand, readily precipitated when the initial solution concentration was >0.2 M. A regular structure, indicating crystal formation, was observed by SEM of the freeze-dried salts. In the case of the dipotassium salt, no crystals were detected by SEM when the initial concentration was <0.2 M. Although an endotherm due to eutectic melting was still observed by DSC, it was very small, an indication of very incomplete precipitation. With more dilute solutions, e.g., 10 mM, salt precipitation was not observed with any of the phosphates. The salt concentration dependence of the degree of precipitation is discussed in terms of the sizes of the solution domains in the freeze-concentrates.