And we distinguish between "intermolecular force", the force between molecules, and "intramolecular force", the force between atoms WITHIN molecules.
For pentane, for any of its three structural isomers (which are?), the "intramolecular force" is STRONG. That is C-H and C-C bonds are thermodynamically strong; they are not as strong as C=O, and O-H bonds, which is why we use hydrocarbons as a source of energy.
On the other hand, the "intermolecular force", relies on dispersion force between the pentane strands....longer strands generate greater intermolecular force, and this is nicely indicated by the normal boiling points, "n-pentane", 36.1 ""^@C; versus "isopentane", 27.7 ""^@C; "neopentane", 9.5 ""^@C. The more branching the LESS the degree of intermolecular interaction.
And as regards solubility in water, there is little for water to work on as a solvent. The pentanes are molecular, and certainly they are non-polar, and hence addition of "n-pentane" to water gives rise to two immiscible layers, with the LESS dense pentane floating on top of the funnel. The reduced surface tension of pentane, can also be attributed to the lack of intermolecular interaction.
And contrast this with the solubility of ethanol and methanol in water. What has an alcohol got that an alkane ain't got?
