The stationary phases for RP columns are surface modified silica gels or polymers with bounded alkyl chains which have hydrophobic/covalent properties. The longer the linked chain, the more hydrophobic the phase will be. For the surface treatment commonly silanols are used. The most typical and best known reversed stationary phase is the C18 modification.
Available KNAUER modifications of Eurospher II materials are C4, C8, C8 A, C18, C18 A, C18 H, C18 P and Phenyl. 