if I evaporate a mixture of hexane:ethyl acetate in a rotary evaporator under vacuum, will the solvent collected from the condenser have approximately the same ratio of hexane:ethyl acetate? bp ethyl acetate 77.1C and bp hexanes 68C.
Depends. A mixture of hexane and ethyl-acetate forms an azeotrope around 65 mole% hexane. So it depends on your initial composition.
Here you can find more information on the vapor-liquid equilibrium of the hexane ethyl-acetate pair (Dortmund Databank). http://www.ddbst.com/en/EED/VLE/VLE%20Ethyl%20acetate%3BHexane.php
Leander Boer is correct about the azeotrope and the behaviour of the solvent.
In addtion to this, the settings and capability of the rotary evaporator will affec the recovery, and may compromise recovery of one component over another.
The basic physics of the situation is that the boiling point is lowered by the vacuum. The vacuum (pressure) in the system sets the boiling point of the solvent(s), it aslo sets the condensation point of the solvent(s). At this point the temperatuer of the coolant in the cold trap becomes important - it needs be lower than the boiling / condensing point of the solvent to be able to trap it.
No condenser is 100% effective, so surface area of the condenser coil is important, some solvent will be lost. Also, solvent will reflux in the collecting flask which leads to more condenser load and some loss.
Kind regards,
Rob
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