Van der Waals Forces.

All atoms have a weak tendency to stick together,
and because of this even helium liquifies at a low
enough temperature. This is a result of the van der
Waals or “London dispersion forces” which act strongly only at a very short distance. These forces arise from electrostatic attraction between the positively charged nucleus of one atom and the negatively charged electrons of the other. Because nuclei are screened by the electron clouds surrounding them, the force is weak. The energy (enthalpy) of binding of one methylene (–CH2 –) unit into a monomolecular layer of a fatty acid is about –∆H° = 1.7 kJ/mol. 22 Although this is a small quantity, when summed over the 16 or more carbon atoms of a typical fatty acid the binding energy is substantial. When a methylene group is completely surrounded in a crystalline hydrocarbon, its van der Waals energy, as estimated from the heat of sublimation, is 8.4 kJ/mol; that of H2O in liquid water at the melting point of ice is 15 kJ/mol.
While van der Waals forces between individual atoms act over very short distances, they can be felt at surprisingly great distances when exerted by large molecules or molecular aggregates. Forces between very smooth surfaces have been measured experimentally at distances as great as 10 nm and even to 300 nm. However, these “long-range van der Waals forces” probably depend upon layers of oriented water molecules on the plates.