It is known that octane is a hydrocarbon and London dispersion forces are the main intermolecular forces which are present in it.
Whereas water molecules tend to form hydrogen bonding forces which are actually much stronger than dipole-dipole interactions. This means that more energy is necessary to separate water molecules from one another than to separate octane molecules from one another.
Hence, we can conclude that surface tension of octane is expected to be lower than that of water when the two are considered at the same temperature.
