The Mechanism of Chromatographic Retention - Chromatographic Interactions > Polar Interactions > Dipole-Dipole Interactions. > Page 10

The low dipole moment of dioxane (i.e., 0.45 Debye) is an example of internal dipole compensation. Compared with diethyl ether, which has a dipole moment of 1.15 Debyes, the dipole moment would be expected to be about half that of dioxane.However, there is strong intramolecular compensation between the dipoles from each ether group within the molecule 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 substances that contain permanent dipoles and can exhibit polar interactions with other molecules are alcohols, esters, ethers, amines, amides, nitriles, etc.

The retentive characteristics of a polar stationary phase are displayed in the lower chromatogram in figure 2 and can be compared with the retentive characteristics of a dispersive phase shown in the chromatogram above. The polar stationary phase is a cyanopropyl polymer that exhibits relatively weak dispersive interactions but strong polar interactions. The aliphatic hydrocarbons, that are well retained by the dispersive stationary phase, are rapidly eluted on the polar phase but the aromatics are strongly retained and well resolved from one another. On the dispersive stationary phase, all the solutes are spread along the chromatogram, roughly in order of their increasing molecular weights.

Finally, it has been shown that the polarizability of a substance containing no dipoles will, among other factors, determine the strength of any dispersive interactions that might take place with another molecule. Unfortunately, in the case of polar materials, due to possible self-association and/or internal compensation, the dipole moment determined from bulk dielectric constant measurements will not always give a true indication of the strength of any polar interaction that might take place with another molecule.