Van der Waals Forces

Van der Waals forces are weak attractive forces between electrically neutral atoms or molecules. They are much weaker than the electrostatic forces which bind charged atoms or molecules (ions) of opposite sign or the covalent forces that bond neighboring atoms by sharing electrons. These forces develop because the rapid shifting of electrons within molecules causes some parts of the molecule to become momentarily charged, either positively or negatively. For this reason, weak, transient forces of attraction can develop between particles that are actually neutral. The magnitude of the forces is dependent on the distance between neighboring molecules. Van der Walls forces cause gas molecules to condense first to a liquid and finally to a solid as the gas is cooled.

Several groups of forces are included within Van der Walls force. These groups include London forces (forces that deal with distinct changes in electron cloud distributions); Keesom forces (forces that involve fixed or angle averaged dipoles); and Debye forces (forces that involve free or rotation dipoles).

The forces are named for Dutch physicist Johannes Diederik van der Waals (1837–1930). The discovery of these forces evolved from van der Waals’s research on the mathematical equations describing the gaseous and liquid states of matter. These equations are generally known as gas laws and relate the temperature, pressures and volume of gases. Originally derived for an idealized gas, these equations assumed that gas molecules had zero volume and that there were no attractive forces between them. In 1881, van der Waals proposed an empirical gas law that included two parameters to account for molecular size and attraction. This more accurate model was the first serious attempt to formulate gas laws for real gases, and it earned van der Waals the Nobel Prize for physics in 1910.

All neighboring molecules in liquids and solids attract each other. The nature and strength of these interactions depend on the types of atom groups or functional groups that comprise the molecules. Some molecules are polar and some have hydrogen bonds. These relatively strong intermolecular interactions require specific structural features. Polar interactions require a non-symmetric arrangement of bonds with atoms of different electronegativity—what are called polar bonds. Hydrogen bonding requires that one species possess a hydrogen atom bonded to a highly electronegative atom such as fluorine, oxygen, or nitrogen. The other species must possess a highly electronegative atom without a hydrogen atom bonded to it. However, all molecules interact with other molecules through Van der Waals interactions. Van der Walls interactions are especially noticeable in noble gases because such elements are stable; that is, they do not react very readily.

Van der Waals force is the group of attractive forces of one transient dipole for another. A transient dipole is a temporary imbalance of positive and negative charge. At particular instants, even atoms that are spherical on average, such as those of the noble gases, will have greater electrondensity on one side of the atom than another. At that instant, the atom will possess a temporary dipole with a negative charge concentration at the side of the atom with greater electron density. If this happens in the case of an argon atom in liquid argon, for example, the argon atoms next to the one with temporary dipole would feel the effect of the dipole. An atom near the negative end of the dipole would have its own electrons slightly repelled from the negative concentration of charge, developing a dipole with its positive end near the negative charge of the original atom. An argon atom on the other side of the original temporary dipole would feel its electrons attracted to the positive end of the dipole, developing a dipole with the opposite orientation. In this way, temporary dipoles are propagated through a liquid or solid. The motion of the molecules in the liquid or solid soon disrupts the pattern, but similar events take place continually. The larger the size of atoms and the more electrons they possess, the greater the probability of forming substantial transient dipole interactions. Molecules that are non-polar and non-polar functional groups of molecules only experience Van der Waals interactions with other molecules or functional groups.

To understand the differences in properties of larger molecules, the additivity of intermolecular interactions becomes important. In effect, the interaction of each group of atoms of a molecule with a group of atoms of a neighboring molecule can be considered to be independent of the interactions of other groups of atoms of the molecules. The total energy required to move two molecules apart is the sum of all the energies of the individual interactions. The more groups there are present and the stronger each individual interaction is in size, the greater the sum of energy of interactions. Among the non-polar linear alkanes, the boiling point for a molecule with many -CH₂ groups, such as liquid octane—CH₃(CH₂)₆CH₃—is higher than that of gaseous propane—CH₃CH₂CH₃— because of the greater number of Van der Waals interactions between the octane molecules. For large molecules such as the higher alkanes (heavy oils and waxes) and polymers such as polyethylene, the total attractive energy due to the Van der Waals force can be greater than the polar interactions or hydrogen bonding interactions of other, smaller molecules. Hence, molecules that have only Van der Waals interactions may still melt or boil at high temperatures.

See also Gases, properties of.