Ion Chromatography - Dipole-Dipole Interactions
The interaction energy between two dipoles is proportional to the square of the dipole moment, the magnitude of which can vary over a wide range of values. Unfortunately, the magnitude of the dipole moment, obtained from the usual bulk measurements of dielectric constant that are taken over a range of temperatures, will not always provide a true indication of the dipole's interactive strength. For example, the dipole moment of water, which is an extremely polar molecule, is only about 1.76 debyes. However, another very polar substance, methanol, has a dipole moment of 2.9 debyes which is greater than water that is, in fact, less polar than water. These low dipole moments values for strongly polar materials may result from electric field compensation caused by molecular association and/or when more than one dipole is present in the molecule, from internal electric field compensation.
Figure 2. Possible Self-Associates of Water and Methanol
Water, for example, associates very strongly with itself by so called 'hydrogen bonding'. This self-association could reduce the net dipole character of the associated molecules calculated from bulk dielectric constant measurements of the pure substance. In a similar manner, methanol also associates strongly with itself. Examples of possible associates of water and methanol are shown in figure 2. It is seen that if such associates exist, the electric field produced by each individual dipole would oppose the field produced by the other. It is clear that this would result in a reduction in the net electric field as measured externally. Consequently, bulk property measurements can provide false values for the moments of the individual dipoles. Another molecule, however, approaching a dipole of the water or methanol molecule would experience the uncompensated field of the single dipole and interact accordingly.
Another interesting example of internal electric field compensation is the unexpected small dipole moment of dioxane, (cf 0.45 debyes). Compared with the measured value of 1.15 debyes for diethyl ether (which theoretically would be expected to be about half that of dioxane) it is seen that there is strong internal compensation between the dipoles of each of the ether groups and, as a result, the dipole moment of dioxane is reduced to a fraction of what would be the true effective value. Again, however, a molecule, approaching one ether group of the dioxane molecule would experience the uncompensated field of that single dipole and, as a consequence, interact accordingly.
Figure 3. Polar Interactions: Dipole-Dipole Interactions
It is seen that although, the strength of the dispersive interactions of a substance containing no dipoles is related to its polarizability, unfortunately, due to possible self-association or internal compensation, the dipole moment of a substance, determined from bulk dielectric constant measurements, will not always give an indication of the strength of any polar interaction with another molecule. A diagrammatic impression of a dipole-dipole interaction is shown in figure 3.
As would be expected, the dipoles interact directly, but it is important to understand that behind the dipole-dipole interaction there will still be dispersive interactions arising from the charge fluctuations on both molecules. The net molecular interaction will, therefore, be the sum of both. Examples of some substances that have permanent dipoles and exhibit polar interaction with other molecules are alcohols, esters, ethers, amines, amides, nitriles, etc.