The interaction between sodium dodecyl sulfate (SDS) and a phospholipid (1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC)) molecular layer was investigated experimentally to understand the mixing effect of SDS on biomembranes. Both the SDS concentration-dependencies of the contact angle between a pair of water droplets in oil and the surface pressure of a DOPC phospholipid monolayer changed at 1 similar to 2 mM SDS, which was lower than the critical micelle concentration. Attenuated total reflection Fourier transform infrared spectroscopy was used to evaluate the interaction between DOPC and SDS molecules. (C) 2016 Elsevier B.V. All rights reserved.