Hydrogen bonds.

Attraction between the positive part of a polar Moietyin a Functional group (like Hydroxyl) of one molecule and an electronegative atom (like N, or O) in another functional group.

Hydrogen when covalently bonded to a more electronegative atom (like Oxygen) will develop a positive charge, while the electronegative atom will develop a negative charge. The hydrogen atom in some molecule can then attract an electronegative atom in another molecule. $\sim 1\text{ to }5 k_B T$.