Chromatography molecular interaction forces

Allenmark, S. Bomgren, B. Boren, H. Direct liquid chromatographic separation of enantiomers on immobilized protein stationary phases. IV. Molecular interaction forces and retention behaviour in chromatography on bovine serum albumin as a stationary phase. J. Chromatogr. 1984, 316 (12), 617-624. 

The selection of the solvent is based on the retention mechanism. The retention of analytes on stationary phase material is based on the physicochemical interactions. The molecular interactions in thin-layer chromatography have been extensively discussed, and are related to the solubility of solutes in the solvent. The solubility is explained as the sum of the London dispersion (van der Waals force for non-polar molecules), repulsion, Coulombic forces (compounds form a complex by ion-ion interaction, e.g. ionic crystals dissolve in solvents with a strong conductivity), dipole-dipole interactions, inductive effects, charge-transfer interactions, covalent bonding, hydrogen bonding, and ion-dipole interactions. The steric effect should be included in the above interactions in liquid chromatographic separation. 

The retention of a solute is directly proportional to the magnitude of its distribution coefficient (K) between the mobile phase (gas) and the stationary phase. The magnitude of K depends on the relative affinity of the solute for the two phases thus, the stronger the forces between the solute molecule and the molecules of the stationary phase, the larger the distribution coefficient and the more the solute is retained. It follows that the stationary phase must interact strongly with the solutes to be retained and to achieve a separation. Molecular interaction results from intermolecular forces, of which there are only two types effective in gas chromatography (GC). 

Molecular mechanics calculations are useful for studying retention order in chromatography. Although the MOPAC calculations did not clearly indicate the contributing force to the molecular interaction, they identified the molecular interaction center on an electron density map constructed by the tabulator of the CAChe program, and the change of atomic partial charge before and after the optimization of the complex form. The hydrocarbons... 

The results indicate that increasing the number of double bonds decreases the molecular interaction energy values, especially those of c/s-form compounds. The predominant interaction force is the van der Waals force. The electrostatic energy did not change after formation of the complex. No 71-71 interactions influenced the direct interaction. The calculated results supported the chromatographic behavior of fatty acids in reversed-phase liquid chromatography. 

According to Dalgliesh [2], three active positions on the selector must interact simultaneously with the active positions of the enantiomer to reveal differences between optical antipodes. This is a sufficient condition for resolution to occur, but it is not necessary. Chiral discrimination may happen as a result of hydrogen-bonding and steric interactions, making only one attractive force necessary in this type of chromatography. Moreover, the creation of specific chiral cavities in a polymer network (as in molecular imprinting techniques) could make it possible to base enantiomeric separations entirely on steric fit. 

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