Thermodynamic properties of polyethoxylated nonylphenols by reversed-phase high-performance liquid chromatography

The reversed-phase liquid chromatography (RPLC) was used to investigate the thermodynamic properties of homogeneous ethoxylated nonylphenols (NPE7–15) and their retention mechanism from methanol–water eluents. The enthalpies (ΔH°) and entropies (ΔS°) of transfer from the mobile phase to the octyl- and octadecylsilica stationary phase were determined using the change of the capacity factor (k′) with temperature. The ΔH° and ΔS° values were negative and decreased with the water content of the eluent and with the length of ligand bonded to the stationary phase. The ethylene oxide chain length of the surfactants does not significantly influence the thermodynamic functions of retention. The enthalpy–entropy compensation (EEC) behavior was employed to investigate the mechanism of surfactant retention in the reversed-phase systems. The obtained compensation temperatures were within the 430–590 K range. They were very closed to those of ethoxylated fatty alcohols (455–586 K) and fall within the range of hydrophobic organic compounds. The results reveal the similarity of interactions involved in the RPLC with nonpolar stationary phases and aqueous eluents.