There has been much research and work done in this area and many books have been written about this topic.

However, to give a very short answer you could say that much of the retention of a column in Reversed Phase RP HPLC is due to what is commonly called hydrophobicity. This natural phenomena is due mostly to the size of the hydrophobic (water resistant) area of a molecule. Retention will increase with the amount of water in the mobile phase. In general, the more hydrophobic the molecule the longer it should be retained.

If a molecule is “solvo-phobic” the molecule will retain elsewhere, namely the column. This goes to the old adage in chromatography “like likes like”.

NOTE: This does not apply to Normal Phase HPLC, Aqueous Normal Phase ANP or Ion Exchange Chromatography.