Dipole-Dipole
Polar interactions between molecules can result from either dipole-dipole interactions of dipole–induced dipole interactions. Dipole-dipole interactions arise from localized permanent dipoles. The two opposite charges are on the same molecules and, thus, neutralize each other so there is no net charge on the molecule as there is with an ion. However, the individual charges can interact with the individual charges of opposite sign on another molecule, and the interaction can be extremely strong. The energy of dipole-dipole interactions can approach the energy of a weak chemical bond. Due to the high energy of the dipole-dipole interactions, molecules can associate with each other (e.g., methanol-water association); this type of association is often called hydrogen bonding. The energy of interaction is directly proportional to the product of the polarizability of the molecule and the square of the dipole moment. The interaction energy is also inversely proportional to the sixth power of the distance between the interacting charges.
Author: RPW Scott
Book:The Mechanism of Chromatographic Retention
Section:Retention Chromatographic-Interactions Polar Dipole-Induced-Dipole
), 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
Retention Chromatographic-Interactions Polar Dipole-Induced-Dipole
Author: RPW Scott
Book:Principles and Practice of Chromatography
Section:Principles Distribution-Coefficient Molecular Dipole-Induced-Dipole
Principles Distribution-Coefficient Molecular Dipole-Induced-Dipole
Author: RPW Scott
Book:The Mechanism of Chromatographic Retention
Section:Retention Chromatographic-Interactions Polar Dipole-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,
 
Retention Chromatographic-Interactions Polar Dipole-Dipole
Author: RPW Scott
Book:The Mechanism of Chromatographic Retention
Section:Retention Chromatographic-Interactions Polar
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
Retention Chromatographic-Interactions Polar
Author: RPW Scott
Book:The Mechanism of Chromatographic Retention
Section:Retention Chromatographic-Interactions Polar Dipole-Dipole
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
Retention Chromatographic-Interactions Polar Dipole-Dipole
Author: RPW Scott
Book:Principles and Practice of Chromatography
Section:Principles Distribution-Coefficient Molecular Dipole-Dipole
Principles Distribution-Coefficient Molecular Dipole-Dipole