Stationary phases water-wettable

In RP-TLC, silica gel plates impregnated with a strong hydrophobic agent (paraffin oil or silicone oil, usually 5%) have been extensively used in the past as nonpolar stationary phases. Nowadays, plates covered with octa-decyl-silanized (ODS) silica gel are available. In this material, the silanol groups are etherified with alkyls containing 8 (Cg) or 18 (Cig) carbon atoms. The low wettability of HPTLC plates coated with highly etherified silica gel poses limitations in the water content of the mobile phase. This problem is circumvented by the use of RP-Cig plates with 50% etherification. However, the presence of free silanol groups may lead to undesirable silanophilic interactions, especially with low water content in the mobile phase. 

It should again be mentioned at this point that it is essential that the layer should be wettable by a solvent, as no development could otherwise occur. This is because the capillary forces within the layer would not be sufficient to overcome the repulsive forces between the stationary phase and the mobile phase. Thus, for example, a very hydrophobic RP-18 phase cannot be used with solvent systems that have a high water content. 

Alternatively samples of low molecular weight, particularly those of very low and very high polarity, can be separated by reversed-phase (RP) chromatography using partitioning processes. Compared to stationary phases for RP-HPLC, TLC material cannot tolerate a high water content of the mobile phase due to the limited wettability. To ensure migration of the mobile phase by capillary action the carbon load on the surface of the silica gel is therefore limited (water-wettable layers). Table 1 summarizes principal applications of some polar and nonpolar bonded phases. 

---