When I conduct a synthesis, after some workups, I usually evaporate large amount of solvent using rotovap, recover compounds sticked to the wall of the flask to the glass vial using small amount of solvent, and finally evaporate all residual solvent using vacuum oven (usually at low temperature about 20~30 ℃ and apply vacuum gradually overnight). After the solvent is completely evaporated, I usually get gooey gel-like compound that is sticked to the bottom of the vial.
I have two questions:
I have never obtained powder-like sample after evaporating the solvent. But I have a few experience in organic synthesis, so does the formation of gel-like form rather than powder form depend on the characteristics of the compounds, not the evaporating method? Actually, I have once obtained gel-like sample containing large amount of unreacted starting materials, even though that material was weighed as powder form at the beginning. How can I get the powder form rather than gel-like form when I evaporate the solvent? Does the evaporating method matter?
After I evaporate all the solvent, I try to redissolve the sample with the same solvent, but it is not readily redissolved although I use similar amount. Usually it takes some time and heat. But why? It was readily dissolved in the workup stages. Is this because that the sample is sticked to the bottom of the vial?