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.

Phase behavior of five members of a homologous series of 4-alkaneamido-2,2,6,6-tetramethyl-1-piperidinyloxy (C n TEMPO with n = 14, 16, 18, 20, 22) was investigated with classical Langmuir monolayer techniques and with Brewster angle microscopy and 2D voltammetry. Surface potential data, BAM images, and 2D voltammetry showed that only C20- and C22TEMPO monolayers have a 2D LE/G critical point above room temperature and that the remaining three compounds with shorter alkane chains form supercritical monolayers. The unusually low values of the critical temperature for these amphiphiles was explained in terms of the molecular structure of their headgroup featuring two polar groups (the nitroxy and the amide moieties) located in the opposite positions of a bulky piperidine ring. The presence of the two polar groups causes a change of orientation of the headgroup during monolayer expansion and results in a substantial increase (ca. 50−100 Å2/molecule) of the cross sectional area of these amphiphiles. As a result, in the expanded monolayers, the van der Waals attraction between the alkane chains is weakened as they can only partially align. The decreased cohesion within the monolayers leads to the observed decrease of their 2D critical temperature. Analysis of the plots of BAM reflected light intensity as a function of amphiphile's surface concentration obtained at different temperatures yielded the C20TEMPO critical temperature of 28 °C. BAM and 2D voltammetry can both be used to precisely determine the position of the C22TEMPO LE/G phase transition (98.5 Å2/molecule at 23.5 °C). The proximity of the C20TEMPO critical point to the room-temperature results in a less precise value of this phase transition (127−130 Å2/molecule at 23.5 °C). A remarkably consistent ability of fresh line microelectrodes used in 2D voltammetry to nucleate the 2D gas phase makes this method less ambiguous and thus superior to BAM in determining the position of the LE/G phase transitions.