For any inorganic salts, there will always be a difference in the melting and solidification temperature which I have noted from some DSC results. What is the key concept behind this? In salt mixtures (combination of 2 or more inorganic salts), this difference is still higher. Is there any way to reduce this gap? Is there any research work currently going on related to this?
Phase transition are of two type:
1. First order phase transition
2.Second order phase transition
The system you study shows a first order phase transition and difference between the melting and solidification temperature is due to existence of solid superheating and liquid supercooling state.
To have a well understanding of first order phase transition please go through the article "Theory of first-order phase transitions" Rep. Prog. Phys. 50 (1987) 783-859. of Kurt Binder.
here Melting is liquefaction out of a super-heated solid and solidification is crystallization out of a super-cooled liquid.
Hi Hakeem, as already pointed by Syed, there are two types of phase transitions.
Usually in DSC signal the first order transitions are much more pronounced, as you have release of the latent heat. So your DSC signal will be sharp and well defined for melting and solidification transitions. The reason for different melting temperature upon heating and solidification temperature upon cooling is, as pointed by Syed, kinetics. To reduce this effect you can reduce the mass of the sample. Also try with different heating and cooling rates for example 1C/min and 5C/min. The slower heating rate will show smaller difference between solidification and melting temperature of the sample.
There is also influence of the contact between your sample and crucible. The first melting upon heating of the sample will produce different contact with crucible due the wetting, so it is better to do two or more cycles of heating and cooling without putting sample/crucible out of the DSC machine and use last run for estimating melting temperature. However, your sample has to be completely inert to crucible material, so
check the interface of the crucible/sample with SEM/EDX analysis after DSC experiment.
Hi Hakeem, at the melting point there is an equilibrium between the speed of dissolution of the solid and the condensation of the liquid, the problem is if you have dissolved anything and have no nuclei either from the substance itself or the containment you are working you have no solid in the system, thus you have to create nuclei of a certain size so that they can start to grow, for this you need some driving force which is given by the super cooling.
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