C8 vs C18 HPLC columns

 1- C8 :

Has only 8 carbon atoms chain bonded to silica atom, less dense than C18 (Decreases the surface area) so the elution will be faster (Decreases retention time).

2- C18 :

Has only 18 carbon atoms chain bonded to silica atom, more dense than C8 (Increases the surface area) so the elution will be slower (Increases retention time).