Aromatic hydrocarbon-polar group interaction

Aromatic hydrocarbon-polar group interaction in microemulsions, effect of aromatic compounds, 41,4 /... 

Silica gel, per se, is not so frequently used in LC as the reversed phases or the bonded phases, because silica separates substances largely by polar interactions with the silanol groups on the silica surface. In contrast, the reversed and bonded phases separate material largely by interactions with the dispersive components of the solute. As the dispersive character of substances, in general, vary more subtly than does their polar character, the reversed and bonded phases are usually preferred. In addition, silica has a significant solubility in many solvents, particularly aqueous solvents and, thus, silica columns can be less stable than those packed with bonded phases. The analytical procedure can be a little more complex and costly with silica gel columns as, in general, a wider variety of more expensive solvents are required. Reversed and bonded phases utilize blended solvents such as hexane/ethanol, methanol/water or acetonitrile/water mixtures as the mobile phase and, consequently, are considerably more economical. Nevertheless, silica gel has certain areas of application for which it is particularly useful and is very effective for separating polarizable substances such as the polynuclear aromatic hydrocarbons and substances... 

Polar interactions between molecules arise from permanent or Induced dipoles existing in the molecules and do not result from permanent charges as in the case of Ionic interactions. Examples of polar substances having permanent dipoles would be alcohols, ketones, aldehydes etc. Examples of polarizable substances would be aromatic hydrocarbons such as benzene or toluene. It is considered that, when a molecule carrying a permanent dipole comes Into close proximity to a polarizable molecule, the field from the molecule with the permanent dipole induces a dipole in the polarizable molecule and thus electrical interaction can occur. It follows that to selectively retain a polar solute, then the stationary phase must also be polar and contain, perhaps, hydroxyl groups. If the solutes to be separated are strongly polar, then perhaps a polarizable substance such as an aromatic hydrocarbon could be employed as the stationary phase. However, to maintain strong polar interactions with the stationary phase (as opposed to the mobile phase) the mobile phase must be relatively non-polar or dispersive in nature. 

Any increase of polarity of the hydrocarbon, for example by substitution with chlorine or by replacing CH2 groups by ether oxygens, also increases incompatibility with perfluoroalkanes as polar interactions become more important [89, 96]. Polar groups are fundamental constituents of the mesogenic cores, especially as linking units and as substituents at aromatic moieties, and these groups increase the incompatibility between the aromatic cores and Rp-chains. 

This change Is caused by the interaction between the aromatic hydrocarbon and the polar group of the surfactant. This phenomenon has been clarified for normal micelles of ionic surfactants (24) for Inverse micelles material Is available only for nonlonlc surfactants. Christenson and collaborators (25-27) made an extensive study using NMR and calorimetry. The results of both studies agree showing a partition coefficient for benzene between the surfactant polar group and Its hydrocarbon chain of approximately 3. 

These results demonstrate two facts. The aromatic hydrocarbons have a strong Interaction with the polar groups of surfactants as evidenced by NMR and calorimetry Investigations. In addition, the light scattering and dipole moment determination show that this Interaction Influences the transition from monomeric aggregates to Inverse micelles. 

Isolated compounds include alkyl, aromatic, ahcyclic, or functional groups with significant hydrocarbon structure. All isolates have a potential for nonpolar interaction (except inorganic ions and compounds with polar groups, e.g., carbohydrates). Because of this fact, the nonpolar interactions are nonselective and allow the extraction of... 

As seen in Fig. 10, in aromatic hydrocarbons the Jt-electron-rich regions can interact with cations through so-called cation-7t interactions [40], In mixtures with ionic liquids, the aromatic molecules may be solvated in the nonpolar domains but they may have sufficient affinity for the cationic head-groups to be found among the polar domain as well. When looking at the structure of such mixtures, it is useful to study the local environments of the ions and of the aromatic compound. 

Factor (c) appears to depend on complex interacting effects and deserves detailed discussion. When the proton nmr spectrum of a polar substance, typically but not always a ketone (83), dissolved in an aromatic hydrocarbon is compared with that obtained in a saturated hydrocarbon, large shifts of up to 1.5 ppm are frequently observed. These shifts are either upfield or downfield, depending on the stereochemistry of each proton-bearing group (84). This general behavior, known as ASIS (aromatic solvent-induced shifts), has been reviewed by Foster and Lazio (85). 

Compounds, such as those containing the aromatic nucleus and thus tt electrons, are polarizable. When such molecules are in close proximity to a molecule with a permanent dipole, the electric field from the dipole induces a counterdipole in the polarizable molecule. This induced dipole acts in the same manner as a permanent dipole and, thus, polar interactions occur between the molecules. Induced-dipole interactions are, as with polar interactions, always accompanied by dispersive interactions. Aromatic hydrocarbons can be retained and separated in GC purely by dispersive interactions when using a hydrocarbon stationary phase or they can be retained and separated by combined induced-polar and dispersive interactions using a poly(ethylene glycol) stationary phase. Molecules can possess different types of polarity, phenyl ethanol, for example, will possess both a permanent dipole as a result of the hydroxyl group and also be polarizable due to the... 

---