Understanding the Different Steps of Surfactant Adsorption at the Oil–Water Interface with Second Harmonic Generation

Probing the behavior of surfactants at oil–water interfaces is crucial to understand their functionality. In this work, we present detection of the adsorption of several common surfactants at the hexadecane–water interface with second harmonic generation (SHG) and zeta potential measurements. Water molecules were used as reliable indicators of the adsorption of ionic surfactants in SHG analysis. With the change of the interfacial potential monitored by both SHG and zeta potential measurements, unique information about the multiple steps involved in the adsorption of typical surfactants at the oil–water interface is provided. It was revealed that the adsorption of sodium dodecyl sulfate (SDS) and cetyltrimethyl­ammonium bromide (CTAB) at the hexadecane–water interface is initialled by a step dominated by the adsorption of the hydrophobic part of the surfactant, and a latter step involves comparable contributions from both the hydrophobic part and the counterion. The adsorption free energies involved in the initial step can be quantitatively analyzed. In addition, the adsorption of two oil-soluble amphiphiles at the hexadecane–water interface was also studied. Analysis of the ionic strength dependent SHG signal at the hexadecane–water interface also reveals that the origin of the SHG emission is mainly the water molecules at the interfacial layer. The preferential orientation of water molecules is with the hydrogen atoms pointing to the oil phase.