Boiling Point
Silanes experience inter-molecular van der Waals forces. Stronger inter-molecular van der Waals forces give rise to greater boiling points of silanes.
There are two determinants for the strength of the van der Waals forces:
- the number of electrons surrounding the molecule, which increases with the silane's molecular weight
- the surface area of the molecule
Under standard conditions, SiH4 and Si2H6 silanes are gaseous; Si3H8 and Si4H10 are liquids. As the boiling point of silanes is primarily determined by weight, it should not be a surprise that the boiling point has almost a linear relationship with the size (molecular weight) of the molecule.
A straight-chain silane will have a boiling point higher than a branched-chain silane due to the greater surface area in contact, thus the greater van der Waals forces, between adjacent molecules.
On the other hand, cyclosilane tend to have higher boiling points than their linear counterparts due to the locked conformations of the molecules, which give a plane of intermolecular contact.
Read more about this topic: Silanes, Physical Properties
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