Determination of thermodynamic parameters of ethoxylated nonionic surfactants by means of reversed-phase high-performance liquid chromatography

An approach to evaluate the thermodynamic properties of ethoxylated nonionic surfactants by using the reversed-phase high-performance liquid chromatography (RP-HPLC) is described. The enthalpies (ΔH°) and entropies (ΔS°) of transfer from the mobile to the stationary phase were determined using the change of capacity factor with temperature. The surfactants taken into consideration were homogeneous (C10–16E8) and polydisperse (C12–18EO10) ethoxylated fatty alcohols. The stationary phase was the alkyl-silica (i.e. octyl- and octadecyl-silica). The eluents were various methanol–water mixtures and the detection was done at 280 nm. The obtained ΔH° and ΔS° values were negative and decreased with the alkyl chain length of the surfactant, the water content of the eluent and the length of the ligand bonded to the stationary phase. The methylenic increments of enthalpy, entropy and Gibbs free energy of transfer from pure water as eluent to the nonpolar stationary phase were also determined. Their values were of −8.60 kJ mol−1, −18.71 J mol−1 K−1 and −2.74 kJ mol−1 on the octyl-silica column and, respectively of −17.28 kJ mol−1, −46.40 J mol−1 K−1 and −2.76 kJ mol−1 on the octadecyl-silica column. The enthalpy–entropy compensation behavior of the surfactants was evaluated and the compensation temperatures obtained were within the 455–586 K range. They agreed with those for alkylbenzenes and polycyclic aromatic hydrocarbons in the RPLC systems. The results reveal the likeness of the interactions involved in the retention of hydrophobic compounds on alkyl-bonded stationary phases and hydro-organic eluents.