THERMAL ANALYSIS
11
economically feasible. Provided that the proportion of the eutectic mixture present is very
small compared with the rest of the frozen mass, any changes, that do occur on drying at
higher temperatures, may pass undetected.
The above technique, Whilst enabling the temperature of complete solidication to be
predicted, does not detect any higher eutectic temperatures. Let us now consider a time—= '
temperature plot of a'simple salt solution in water (gure 2). As the temperature is decreased
it is possible to supercool the solution below its freezing point. The solution is now said
to be in a metastable state. Eventually this state Will be ruptured and the crystallisation
of water to form ice induced. In so doing, the energy which was distributed within the
random motion of the water molecules in the liquid state is lost and as these molecules
become stabilised in the crystallised form the excess energy is given o' in the form of heat
——
namely the latent heat of fusion. This accounts for the plateau obtained on the timetemperature plot. Ice separates Out until the concentration of salt reaches a certain concen—
tration, the solution then begins to cool again, ice being progressively liberated, and eventually the eutectic temperature is reached. The eutectic mixture crystaises out and again
there is an exothermic reaction as the heat of mobility is liberated. Once this is complete
the sample again begins to cool and the process repeated for any further eutectic mixtures.
Alternatively the process may be studied on warming, whence at each eutectic temperature
an endothermic change occurs as heat is taken in to provide the energy for random motion
within the liquid. During the thawing cycle metastable states are never encountered and
for this reason eutectic temperatures are measured accurately only during warming.
If one compares the exothermic or endothermic changes occurring within a salt solution
in water as compared with those occurring in water under identical conditions, it is possible
to detect those thermal effects which are dependent upon the presence of eutectic mixtures
(gure 2 a, b, c). This briey summarises the technique of differential thermal analysis
whereby the thermal comparison between a solution and its solvent under identical Condi—
tions can be amplied. This technique is of common application in metallurgy and was
applied to mineral chemistry by Vuillard (1957). Rey (1960) developed this technique to
study the thermodynamic behaviour of frozen solutions. From experiments on the beha—
viour of aqueous solutions at low temperatures he was able to conclude that there is no
denite structure for a frozen solution at a given temperature, but that the structure is
highly dependent upon the freezing history. Studies using biological products and glycerol—
water systems showed that not all systems crystallise on freezing, but are instead capable
of forming a metastable or glassy state which may persist indenitely. A limited melting
of eutectic mixtures or a softening of these glassy structures makes drying difcult and also
results in denaturation.
Rey successfully avoided softening of such metastable states by a
thermal treatment, derived from his dierential thermal analysis and conductivity studies…
The technique involves a quick freeze to a low temperature in order to allow the glass to
develop, followed by rewarming slowly to the temperature of devitrication whence crys—
tallisation is induced.
The product may then be recooled and dried at the desired tempera—
ture previously determined from the position of the eutectic zone. This technique has
found useful application in the industrial drying of many delicate products.
The major breakthrough in the long term preservation of living cells at low temperatures
came With the discovery of the protective effect of glycerol (Rostand, 1946; Polge, Smith
& Parkes, 1949). To this success was later added the superior protective elfect obtained .
With dimethyl sulphoxide (Lovelock & Bishop, 1959), along With many reports of success
11
economically feasible. Provided that the proportion of the eutectic mixture present is very
small compared with the rest of the frozen mass, any changes, that do occur on drying at
higher temperatures, may pass undetected.
The above technique, Whilst enabling the temperature of complete solidication to be
predicted, does not detect any higher eutectic temperatures. Let us now consider a time—= '
temperature plot of a'simple salt solution in water (gure 2). As the temperature is decreased
it is possible to supercool the solution below its freezing point. The solution is now said
to be in a metastable state. Eventually this state Will be ruptured and the crystallisation
of water to form ice induced. In so doing, the energy which was distributed within the
random motion of the water molecules in the liquid state is lost and as these molecules
become stabilised in the crystallised form the excess energy is given o' in the form of heat
——
namely the latent heat of fusion. This accounts for the plateau obtained on the timetemperature plot. Ice separates Out until the concentration of salt reaches a certain concen—
tration, the solution then begins to cool again, ice being progressively liberated, and eventually the eutectic temperature is reached. The eutectic mixture crystaises out and again
there is an exothermic reaction as the heat of mobility is liberated. Once this is complete
the sample again begins to cool and the process repeated for any further eutectic mixtures.
Alternatively the process may be studied on warming, whence at each eutectic temperature
an endothermic change occurs as heat is taken in to provide the energy for random motion
within the liquid. During the thawing cycle metastable states are never encountered and
for this reason eutectic temperatures are measured accurately only during warming.
If one compares the exothermic or endothermic changes occurring within a salt solution
in water as compared with those occurring in water under identical conditions, it is possible
to detect those thermal effects which are dependent upon the presence of eutectic mixtures
(gure 2 a, b, c). This briey summarises the technique of differential thermal analysis
whereby the thermal comparison between a solution and its solvent under identical Condi—
tions can be amplied. This technique is of common application in metallurgy and was
applied to mineral chemistry by Vuillard (1957). Rey (1960) developed this technique to
study the thermodynamic behaviour of frozen solutions. From experiments on the beha—
viour of aqueous solutions at low temperatures he was able to conclude that there is no
denite structure for a frozen solution at a given temperature, but that the structure is
highly dependent upon the freezing history. Studies using biological products and glycerol—
water systems showed that not all systems crystallise on freezing, but are instead capable
of forming a metastable or glassy state which may persist indenitely. A limited melting
of eutectic mixtures or a softening of these glassy structures makes drying difcult and also
results in denaturation.
Rey successfully avoided softening of such metastable states by a
thermal treatment, derived from his dierential thermal analysis and conductivity studies…
The technique involves a quick freeze to a low temperature in order to allow the glass to
develop, followed by rewarming slowly to the temperature of devitrication whence crys—
tallisation is induced.
The product may then be recooled and dried at the desired tempera—
ture previously determined from the position of the eutectic zone. This technique has
found useful application in the industrial drying of many delicate products.
The major breakthrough in the long term preservation of living cells at low temperatures
came With the discovery of the protective effect of glycerol (Rostand, 1946; Polge, Smith
& Parkes, 1949). To this success was later added the superior protective elfect obtained .
With dimethyl sulphoxide (Lovelock & Bishop, 1959), along With many reports of success