MEASUREMENT OF GLASS-TRANSITION TEMPERATURES IN FREEZE CONCENTRATED-SOLUTIONS OF NONELECTROLYTES BY ELECTRICAL THERMAL-ANALYSIS

The electrical resistance (R) of frozen aqueous solutions was measured as a function of temperature in order to determine whether this techn ique can be applied for determination of glass transition temperatures of maximally freeze concentrated solutions (T-g') of non-electrolytes which do not crystallize during freezing. Electrical thermal analysis (ETA) thermograms of frozen solutions containing the solute alone sho w a gradual change in slope over the temperature range of interest, wi th no inflection point which corresponds to T-g'. However, addition of low levels (about 0.1%) of electrolyte changes the shape of the therm ogram into a biexponential function where the intersection of the two linear portions of the log (R) vs. T plot corresponds to the glass tra nsition region. The total change in log (R) over the temperature range studied increases as the ionic radius of the reporter ion increases. The sharpest inflection points in the log (R) vs T curves, and the bes t correlation with DSC results, were obtained with ammonium salts. T-g ' values measured by ETA were compared with Values measured by DSC. DS C thermograms of solutes with and without electrolyte (0.1%) show that the electrolyte decreases T-g' by about 0.5 to 1.0 degrees C. However , T-g' values measured by ETA are somewhat higher than those measured by DSC, and difference between the two methods seems to increase as T- g' decreases. T-g' as measured by ETA is less heating rate dependent t han DSC analysis, and ETA is a more sensitive method than DSC at low s olute concentrations and at low heating rates. Results of electrical t hermal analysis of frozen solutions are compared and contrasted with t he electrical resistance vs. temperature behavior of polymerelectrolyt es. ETA appears to be a useful complementary technique to DSC far char acterizing formulations intended for freeze drying.