Variation of crystal count and dimension with seeding temperature in case of DMSO

The variation of crystal count with seeding temperature is represented in Fig. 2. At each value of seeding temperature, the seeding was done four times, the range of which was at best within 0.4°C. The seeding temperature presented in the figure was actually the mean of the four temperature values. There were two slides, and in each slide, 10 random observations were made by the microscope. The modal value of the dimension of all crystals measured in 80 fields of view of the microscope at a particular seeding temperature is also presented in the same figure. The variation

Crystal Seeding
Fig. 2. Crystal count and modal value of crystal dimension against seeding temperature when DMSO is seeded.

of crystal count and the modal value of crystal dimension with seeding temperature have high degree of consistency. As already stated, the seeding temperature is varied from —22.0°C to 0°C. Both, the crystal count and the modal value of dimension have a sharp peak at —21°C, which is also the eutectic temperature of DMSO and water bivariate system. As Gibb's free energy becomes minimum at the eutectic point,27 there is a common tendency for freezing at that temperature. The count as well as the modal dimension show a sharp rise as the temperature is raised from — 22° C to —21°C. On the other hand, as the seeding temperature is raised above —21°C, both the crystal count and the modal dimension show a gradual fall. However, after a sharp fall in crystal count up to —19°C, the fall becomes more gradual. Except that, close to —14°C, there occurs a small peak in the crystal count, and at 0.5°C, both these quantities show insignificant values. From the nature of the curve, one can conclude that DMSO nuclei have no nucleation above 0°C.

On the other hand, in the case of mode of crystal dimension, a sharp fall in value occurs as the temperature rises from —21°C to —18°C. There are two small peaks in the modal value at —17°C and close to —14°C.

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