Monolayers of 4-tetradecaneamido-2,2,6,6-tetramethyl-1-piperidinyloxy (C 14 TEMPO) were investigated at the air/water interface. Pressure-area diagrams and Brewster angle microscopy provided evidence of supercritical state of this monolayer at temperatures as low as 2°C. The supercritical character of C 14 TEMPO Langmuir monolayers allowed the 2-D voltammetric measurements of its lateral diffusion constant, D, to be extended into a previously inaccessible region of low surface densities with mean molecular areas A ∼ 600 Å 2 /molecule. At A > 250 Å 2 /molecule, D becomes independent of the C 14 TEMPO surface concentration and depends solely on the hydrodynamic coupling of the polar headgroup to the aqueous subphase. These measurements and the supercritical state of C 14 TEMPO monolayer open a possibility to probe the viscoelastic properties of the water liquid-vapor interfacial region.

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