Polar retention forces

Partition retention forces intermolecular forces that result in attraction of solute molecules to the stationary phase. Polar retention forces include dipole-dipole attractions, van der Waal s forces and hydrogen bonding. Non-polar retention forces consist of London s dispersion forces arising from induced polarity in non-polar molecules, remember that like attracts like . 

Polar retention forces forces of attraction between a solute and the stationary phase due to dipole-dipole interactions, van der Waal s forces and hydrogen bonding. 

Hydrophobic Effect. The primary retention force in normal phase adsorption is the attraction of solute polar moieties to the polar stationary phase. In contrast, the retention force in RPLC is repulsion from the mobile phase. The stationary phase is a relatively passive surface, the solute attraction for the hydrocarbon stationary phase being weak and non-selective. How does this come about, and what are the resultant selectivity characteristics ... 

Polar van der Waal s retention forces are a consequence of dipole-dipole interactions and hydrogen bonding between molecules. Only components with dipoles similar to the solvent (stationary phase) will disperse, producing solute-solvent pairs. Dipole induced dipole interactions arise from the charge on one molecule (component or stationary phase) disturbing the electrons in a second associated molecule, producing a shift in charge which then forms the induced dipole. 

Gas chromatography GC, employs a gaseous mobile phase, e.g. nitrogen or helium, and usually a liquid stationary phase, e.g. non-polar Apiezon (alkane grease), OVIOI (polymethyl siloxane) or polar PEG20M (polyethylene glycol). Separation is effected by competition between attraction of the components for the stationary phase and volatility at the column temperature being used, that is, retention forces versus partial vapour pressure in the mobile phase. 

Non-polar dispersion forces used to describe the retention of non-polar solutes on a non-polar liquid stationary phase. London s dispersion forces postulate an intermolecular induced dipole mechanism to account for attraction of a component onto a non-polar stationary phase. 

Retention forces see polar, non polar and adsorption retention forces. 

It follows that the remaining dominant retentive forces will be polar or ionic in nature. In the second case, the mobile phase is predominantly water and thus provides very strong polar interactions with the solute but very weak dispersive interactions. It also follows, that the retention forces of the stationary phase, in this case, will be dominantly dispersive... 

Hammett s constant, proton-donor capacity, and steric effects of substituents) and is quite different firom that observed with other reversed-phase supports. Thus it was concluded that charge-induced interactions between the graphite surface as well as steric effects force the polar groups close to the graphite surface. This type of interaction is called polar retention effect on graphite, (PREG), and proved to be more important than hydro-phobic interactions in the retention mechanism of polar compounds. 

Electroultrafiltration (EUF) combines forced-flow electrophoresis (see Electroseparations,electrophoresis) with ultrafiltration to control or eliminate the gel-polarization layer (45—47). Suspended colloidal particles have electrophoretic mobilities measured by a zeta potential (see Colloids Elotation). Most naturally occurring suspensoids (eg, clay, PVC latex, and biological systems), emulsions, and protein solutes are negatively charged. Placing an electric field across an ultrafiltration membrane faciUtates transport of retained species away from the membrane surface. Thus, the retention of partially rejected solutes can be dramatically improved (see Electrodialysis). 

Molecular interactions are the result of intermolecular forces which are all electrical in nature. It is possible that other forces may be present, such as gravitational and magnetic forces, but these are many orders of magnitude weaker than the electrical forces and play little or no part in solute retention. It must be emphasized that there are three, and only three, different basic types of intermolecular forces, dispersion forces, polar forces and ionic forces. All molecular interactions must be composites of these three basic molecular forces although, individually, they can vary widely in strength. In some instances, different terms have been introduced to describe one particular force which is based not on the type of force but on the strength of the force. Fundamentally, however, there are only three basic types of molecular force. 

The induced counter-dipole can act in a similar manner to a permanent dipole and the electric forces between the two dipoles (permanent and induced) result in strong polar interactions. Typically, polarizable compounds are the aromatic hydrocarbons examples of their separation using induced dipole interactions to affect retention and selectivity will be given later. Dipole-induced dipole interaction is depicted in Figure 12. 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... 

Returning to the molecular force concept, in any particular distribution system it is rare that only one type of interaction is present and if this occurs, it will certainly be dispersive in nature. Polar interactions are always accompanied by dispersive interactions and ionic interactions will, in all probability, be accompanied by both polar and dispersive interactions. However, as shown by equation (11), it is not merely the magnitude of the interacting forces between the solute and the stationary phase that will control the extent of retention, but also the amount of stationary phase present in the system and its accessibility to the solutes. This leads to the next method of retention control, and that is the volume of stationary phase available to the solute. 

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 theoretical treatment outlined above does not involve any attractive interaction biptween stationary phase and eluite besides that caused by van der Waals>force. Consequently, the above obscrvniion is not predicted by the ther y. Nevertheless, surface silanols can have profound effect on the selectivity of the stationary phase in contact with eluents rich in organic solvent. The phenomenon and its practical importance in RPC of polar substances, particularly those carrying positive charge, have long been appreciated, Yet, only recently has retention behavior analyzed... 

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