<P> The van der Waals forces arise from interaction between uncharged atoms or molecules, leading not only to such phenomena as the cohesion of condensed phases and physical adsorption of gases, but also to a universal force of attraction between macroscopic bodies . </P> <P> The first contribution to van der Waals forces is due to electrostatic interactions between charges (in molecular ions), dipoles (for polar molecules), quadrupoles (all molecules with symmetry lower than cubic), and permanent multipoles . It is termed Keesom interactions, named after Willem Hendrik Keesom . These forces originate from the attraction between permanent dipoles (dipolar molecules) and are temperature dependent . </P> <P> They consist of attractive interactions between dipoles that are ensemble averaged over different rotational orientations of the dipoles . It is assumed that the molecules are constantly rotating and never get locked into place . This is a good assumption, but at some point molecules do get locked into place . The energy of a Keesom interaction depends on the inverse sixth power of the distance, unlike the interaction energy of two spatially fixed dipoles, which depends on the inverse third power of the distance . The Keesom interaction can only occur among molecules that possess permanent dipole moments, i.e., two polar molecules . Also Keesom interactions are very weak van der Waals interactions and do not occur in aqueous solutions that contain electrolytes . The angle averaged interaction is given by the following equation: </P> <P> − m 1 2 m 2 2 24 π 2 ε o 2 ε r 2 k b T r 6 = V (\ displaystyle (\ frac (- m_ (1) ^ (2) m_ (2) ^ (2)) (24 \ pi ^ (2) \ varepsilon _ (o) ^ (2) \ varepsilon _ (r) ^ (2) k_ (b) Tr ^ (6))) = V) </P>

Intermolecular attractive forces include which of the following