GLASS-TRANSITION OF FREEZE-CONCENTRATED AQUEOUS-SOLUTION OF ASCORBIC-ACID AS STUDIED BY ALTERNATING DIFFERENTIAL SCANNING CALORIMETRY

Aqueous solutions of ascorbic acid have been shown, using differential scanning calorimetry (DSC), to form glassy states during freezing. Al ternating DSC, as a new technique, further helped to assign five therm al events observed during heating of frozen ascorbic acid solutions to complete melting from below Tg: the temperature for maximal freeze co ncentration of ascorbic acid in water, as: (i) glass transition, (ii) molecular relaxation of the glass-forming matrix, (iii) devitrificatio n of the supercooled water, (iv) melting of small ice crystals trapped in the glass-forming matrix, and (v) melting of initially formed ice. Tg' depends somewhat on heating rate and was found to be -54.0 degree s C for 10 degrees C/min. On extrapolation to zero heating rate, Tg' w as -57.0 degrees C and on extrapolation to instant heating Tg' was -51 .0 degrees C, stressing the nonequilibrium nature and time dependency of glass transitions (C) 1998 Academic Press Limited.