The structure of the octadecyl (C18) chain layer attached to a silica surface in the presence of binary solvents (acetonitrile/water methanol/water) was investigated by electron paramagnetic resonance (EPR) and reverse-phase high-performance liquid chromatography (RP-HPLC) using 4-hydroxy-2 2 66 tetra-methylpiperidine-N-oxyl (Tempol) to mimic the behavior of pollutants with medium-low polarity The computer-aided analysis of the EPR spectra provided structural and dynamical information of the probe and its environments which clarified the modifications of the chain conformations that occur at different solvent compositions Capacity factors k' were calculated as a function of the percentage of water/organic solvent (mobile phase) and the retention behavior of the C18-functionalized silica surface (stationary phase) was compared with the results obtained with EPR analysis under static conditions In particular EPR analysis showed that at percentages of ACN equal or higher than 50% the chain layer assume a quite ordered structure whereas at percentages lower than 50% the chains tend to collapse and fold on the silica surface In this latter situation the hydrophobic net of the 08 chains strongly limits Tempol mobility In methanol/water mixtures both EPR and RP-HPLC analysis showed that the probe was adsorbed into a poorly ordered interphase If the residual silanols at the silica surface were bonded to a sililating agent (endcapping) both EPR and RP HPLC analysis showed a decreased adsorption of the probe with respect to the non endcapped silica at the same mobile phase composition (C) 2010 Elsevier Inc All rights reserved