Hydrogen bonding occurs when hydrogen is bound to a strongly electronegative element, #O# or #F# or #N#. The heteroatom polarizes electron density towards itself, and acquires a partial negative charge. Conversely, the hydrogen acquires a partial positive charge because the heteroatom has denuded it (slightly!) of electron density. This is, in a nutshell, the phenomenon of hydrogen bonding, which is responsible for the exceptionally high boiling points of #OH_2#, and #HF#, and #NH_3#.
Thus between molecules, there is an additional electrostatic interaction. The dipoles line up, positive to negative. Ethylene glycol, #HOH_2C-CH_2OH# has two polar hydroxyl groups, and the dipoles line up from molecule to molecule. This constitutes a potent intermolecular force, which accounts for the high boiling point of ethylene glycol, #197.3# #""^@C#, and also its viscosity.
