Adsorption of N,N-Diethyldodecanamide at the Air–Water Interface

The adsorption of N,N-diethyldodecanamide (DEDA) at the air–water interface has been determined at 24°C by surface tension measurements. At monolayer saturation each molecule has a surface area, depending on the equation of state assumed, of about 41.2–45.6 Å2, comparable to that observed for sodium dodecyl sulfate and other somewhat similar surfactants. Micellization of DEDA appears not to occur below its aqueous saturation concentration of 1.4 × 10−5 M. At saturation the surface tension is lowered to 37 dyn cm−1, and the standard free energy of adsorption is about −46.5 kJ mol−1. The surface adsorption deviates slightly from the Szyszkowski–Langmuir surface equation of state at coverages below 50%, and a two-state adsorption mechanism proposed by Lunkenheimer and Hirte appears to best account for this deviation.