The interactions between the adsorbed molecules on the oil-water interface at various salt concentrations

In this work we investigated the interactions between the adsorbed molecules of different structures on the oil water interfaces using zeta potential measurements and surface-sensitive second harmonic generation (SHG) spectroscopy. It was observed that with the increase of the salt concentration in the aqueous solution of the oil-water emulsions, the equilibrium on the oil-water interface was broken and further adsorption of surfactants (n alkane sulfate) or dye molecules (MG: Malachite Green; D289: 4 (4 diethylaminostyry) I methyl pyridinium iodide) led to a zeta potential higher than that in the original equilibrium state. This observation against the Le Chatelier's principle was induced by the chain-chain interactions between the surfactants or the pi-pi interactions between the dyes. For surfactants with short chains, such observation was absent. The increased adsorption of dye molecules at an increased ionic strength environment was confirmed by the SHG scattering measurements. Furthermore, we found that the type of oil (hexadecane or benzene) has no influence on these observations. (C) 2019 Elsevier B.V. All rights reserved.