The Interfacial Tension of Nanoscopic Oil Droplets in Water Is Hardly Affected by SDS Surfactant

Surfactants such as sodium dodecylsulfate (SDS) can reduce the interfacial tension between bulk water and bulk n-hexadecane by 42 mN/m. Although reduction of interfacial tension should also take place on the interface of nanoscopic oil droplets in water, vibrational sum frequency scattering experiments indicate otherwise. In these measurements we have directly measured the adsorption of SDS onto hexadecane oil droplets with an average radius of 83 nm. We find that the interfacial density of adsorbed SDS is at least 1 order of magnitude lower than that at a corresponding planar interface. The derived maximum decrease in interfacial tension is only 5 mN/m.