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Physical properties
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During the last 30 years a large number of studies have been performed concerning the properties of the reversed phase surface. There are two main reasons for conducting such studies:

- to gain a better understanding of the retention mechanism of reversed phase chromatography.

- to facilitate the manufacturing of reproducible RP-materials.

Most studies concern the properties of the RP-C18 material, which therefore is the main topic here.

 

A variety of techniques have been used to study the physico-chemical properties of the C18 layer. These include NMR-, IR- fluorescence spectroscopy and adsorption studies. It is obvious that the properties depend on the type of phase, i.e. a monomeric or polymeric phase, and the ligand density. Furthermore, the properties also depend on the type and composition of the fluid phase, which may be a gas or a mixture of organic solvent and water. It is found that the properties vary with the composition of the fluid phase.

 

The nonpolar layer of a C18 stationary phase cannot be considered to be a sharp and distinct solid or liquid. The reason is that the carbon chains move relative to each other at the same time as they are anchored to the solid support at one end. Furthermore, in contact with the mobile phase, the layer selectively adsorbs the organic modifier and swells. For certain organic modifiers, e.g. acetonitrile, the situation is even more complex because they tend to aggregate and form microenvironments at the interface between the mobile and stationary phase.

 

A number of studies have shown that the chemical composition of the stationary phase is inhomogenous. The alkyl chains are not homogenously distributed over the surface. They form islands of high ligand concentrations and areas with low density.  For a monomeric phase about 50 - 60% of the original silanlo groups are attached to an alkyl ligand. Because of their size, these ligands shield about 20-25% of the original silanol groups. This means that 20-25% of the original silanol groups are accessible for interaction with solvent or solute molecules. After end-capping the shielded silanol groups are intact and the relative amount of free silanol groups is approximately 5%.

 

In the original silica there are a number of impurities, Na, Fe, Al and B, which tend to concentrate at the surfaceduring the manufacturing process, These impurities form sites with high internal energy which further amplifies the inhomogenous nature of the surface.

 

In conclusion, the C18 phase is rather inhomogenous. Patches on the surface have a high ligand density and other patches have low density. Silanol groups and impurities are present on the surface and have a strong influence of its properties.

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