C18 silica gel is one kind of widely used chromatography packing materials.

We bond octadecyl on silica gel and get the C18 by ourselves.

It is also called reversed phase C18 silica, 

There are End-capped and Non End-capped.

It can react hydrophobic interactions with a wide variety of organic compounds. 

It is widely used for separation of nonpolar to moderately polar compounds, for example: Prostaglandins, Fatty Acids, Fat-Soluble Vitamins, Glycerides and so on.