Relationship between Oil–Water Interfacial Tension and Oily Soil Removal in Mixed Surfactant Systems

Abstract

Detergency of oily soil is a complex kinetic process that includes contributions from the wash system physical properties, time and temperature of wash, and the hydrodynamic forces exerted during the wash process. Generally, surfactant mixtures that exhibit a low oil–water interfacial tension are considered to provide superior oily soil detergency. In our investigations, we measured the oil–water interfacial tension for C12EO3/NaLAS and C12EO7/NaLAS blends as a function of temperature (18–80°C) and time (0–60 min). The oil–water interfacial tension was found to decrease as a function of time for all blends containing nonionic surfactants, and this decline was most rapid for blends containing a higher fraction of ethoxylated alcohol with less than 3 mol of ethylene oxide. It is proposed that the diffusion of this hydrophobic fraction into the oil phase leads to the measured decline in the oil–water interfacial tension. Also, due to this limitation, no general correlation of oil–water interfacial tension with oily soil removal was observed. In view of these observations, it appears that the measurement of the oil–water interfacial tension (using a spinning drop tensiometer) for solutions containing hydrophobic surfactants may give misleading results.