Effect of the mobile phase composition on the retention behavior of polydisperse ethoxylated nonylphenols in reversed-phase high performance liquid chromatography

The paper presents the effect of organic modifiers on the reversed-phase retention behavior of the polydisperse ethoxylated nonylphenols (NPEO 7-15 ). The retention plots (log k' vs. organic solvent concentration) obtained with methanol-water as eluent are linear, while those with acetonitrile-water are not and the deviation from linearity increases with the ethoxylation degree of the surfactant. For both elution systems the retention plots reverse at about 20% (vol.) water in the eluent. The physical properties of the hydro-organic eluents such as surface tension and viscosity were investigated as a function of the binary mixture composition and related to the retention data of the surfactants. The surface tension values decreased linearly with the methanol content in the eluent, but remained constant in the case of acetonitrile-water mixtures. The viscosity data indicated that acetonitrile interacts with water more feebly than methanol does. This suggests that acetonitrile molecules unassociated with water in the mobile phase adsorb on the hydrophobic stationary phase. Between the ethylene oxide (EO) groups close to the alkyl chains of the surfactants and the acetonitrile layer adsorbed onto the stationary phase specific interactions appear, which account for the increase of the surfactant retention with the length of the poly(ethylene oxide) (PEO) chain. Considerations about the retention mechanism of these compounds in reversed-phase systems are put forward.