Revisionist look at solvophobic driving forces in reversed-phase liquid chromatography. IV. Partitioning vs. adsorption mechanism on various types of polymeric bonded phases

The partition and adsorption mechanisms of retention in reversed-phase liquid chromatography have been examined based on a comparison of the free energy of transfer of methylene groups from aqueous-organic mixtures to bulk hexadecane with those to a variety of polymeric bonded phases. The stationary phases studied include: conventional silica-based polymeric phases of various alkyl chain lengths, a so-called 'horizontally polymerized' octadecyl phase on silica and a series of polybutadiene-coated zirconia phases. The data indicate that for methylene groups a partition-like mechanism is dominant on all phases. On the polybutadiene-coated zirconia and 'horizontally polymerized' octadecyl phases the partition mechanism holds at all mobile phase compositions. In contrast on conventional polymeric silica phases the retention mechanism seems to become more adsorption-like at methanol compositions greater than about 70% (v/v).