Transfer of oxyethylated alcohols through water/heptane interface: Transition from non-oscillatory to oscillatory behaviour

Spontaneous non-linear oscillations were observed during the transfer of commercial non-ionic surfactants, oxyethylated alcohols C8EO4 and C8EO5 through the water/heptane interface. The oscillations appear in a certain range of initial surfactant concentrations in the water phase. The experimental results are discussed using numerical simulations on the basis of the model proposed recently [N.M. Kovalchuk, D. Vollhardt, Oscillation of interfacial tension produced by transfer of nonionic surfactant through liquid/liquid interface, J. Phys. Chem. C 112 (2008) 9016-9022]. The explanation is proposed for the transition from non-oscillatory to oscillatory regime and again back to non-oscillatory regime observed by an increase in the initial surfactant concentration. The effect of the surfactant partition coefficient and the parameters of equilibrium isotherm on both upper and lower concentration limits is discussed.