), are termed polarizable. On close proximity of such compounds with a molecule having a permanent dipole, the electric field from the permanent dipole induces a counter dipole in the polarizable molecule.   Induced counter-dipole will behave in a similar manner to a permanent dipole and the electric forces between the two dipoles (permanent and induced) can result in strong polar interactions. Typical examples of polarizable compounds are the aromatic hydrocarbons. However, just as dipole-dipole interactions occur coincidentally with dispersive interactions, so are dipole-induced dipole interactions accompanied by dispersive interactions. It follows that using an n-alkane stationary phase, aromatic hydrocarbons can be retained and separated by purely dispersive interactions as in GC. This again is demonstrated in the upper chromatogram in figure 1. Alternatively, a polyethylene glycol stationary phase will separate aromatic hydrocarbons largely by dipole-induced dipole

Dipole-Dipole Interactions. The strength of the intermolecular force involved in polar interaction is determined by the strength of the dipoles and can vary widely. In the extreme (e.g., the association of water with methanol) the dipole-dipole interaction energy can approach that of a chemical bond; such energies are involved in hydrogen bonding. To a first approximation, the energy (UP) that arises during the interaction between two dipolar molecules is given by,  

forces between localized charges resulting from permanent or induced dipoles. They cannot occur in isolation, but must be accompanied by dispersive interactions and under some circumstances may also be combined with ionic interactions. Polar interactions can be very strong and result in molecular associations that approach, in energy, that of a weak chemical bond. Examples of such instances are 'hydrogen bonding' and in particular the association of water with itself. Dipole-Dipole Interactions The interaction energy (UP) between two dipolar molecules is given, to a first approximation, by where (a) is the polarizability of the molecule, (m) is the dipole moment of the molecule, and (r) is the distance between the molecules

of dioxane. This reduces the field, as measured by external techniques, and gives a value for dioxane of only about one-third of that of a diethyl ether molecule alone. However, another molecule approaching one ether group of the dioxane molecule will be subject to the uncompensated field of a single dipole and interact accordingly. Consequently, although dioxane has a very low dipole moment of 0.45, it is still a very polar substance that is completely miscible with water. An impression of a dipole-dipole interaction is depicted in figure 3. The dipoles are shown interacting directly, but, it must be emphasized that behind the dipole-dipole interactions will be dispersive interactions from the random charge fluctuations that continuously take place on both molecules. In the example given above, the net molecular interaction will be a combination of both dispersive interactions from the fluctuating random charges and polar interactions from forces between the two dipoles. Examples of

has also a significant number of different aromatic hydrocarbons present which, as already has been discussed (book 1), can be polarized and consequently interact with a polar stationary phase. Thus if a sample of gasoline is chromatographed on a strongly dispersive stationary phase the components would be separated roughly on a basis of molar volume. This is shown in the top chromatogram in figure 2. Polar Interactions Polar interactions can take two forms; those that result from direct dipole-dipole interaction (involving only molecules that have permanent dipoles) and those that result from dipole-induced dipoleinteraction Dipole-induced dipole interaction results from the interaction of a molecule with a permanent dipole and one that is polarizable. In general, dipole-dipole interactions are very strong polar interactions (e.g. the interaction of methanol with water) and dipole-induced dipole interactions relatively weak (e.g., the interaction of an ester with an aromatic