In a recent study on mixtures of the ionic surfactant, sodium dodecyl sulfate (SDS), with the polar (but charge-neutral) surfactant, C 12 E 6 , surface-sensitive via) Electronic mail: colinegan@protonmail.com b) Electronic mail: ahassana@ictp.it brational sum frequency generation spectroscopic measurements demonstrated that the presence of a C 12 E 6 monolayer at the AWI enhances the interfacial binding affinity of SDS compared to that of SDS binding to the bare AWI. Umbrella sampling (US) molecular dynamics (MD) simulations corroborated this interpretation, showing a twofold enhancement of the free energy of DS -(the surfactant part of SDS) binding to the air-C 12 E 6 -water interface (ACWI) compared to the free energy of binding to the AWI. This result demonstrates the inadequacy of the Langmuir adsorption model in describing mixed surfactant monolayer self-assembly due to a combination of adsorbate-adsorbate interactions as well as collective effects between DS -, C 12 E 6 , and the interfacial water.