Dynamic Properties of Supercritical C₁₄TEMPO Monolayers at the Air/Water Interface

Abstract

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.

Critical Temperature and LE/G Phase Transitions in Monolayer Films of the Amphiphilic TEMPO Derivatives at the Air/Water Interface

“…These are separated by a bulky, lipophilic piperidine ring with four side methyl groups. These structural features are critical to the observed phase behavior reported recently and discussed in this report . Specifically, we postulate that in the high-pressure region, these compounds assume a vertical orientation of the TEMPO headgroup, minimizing their cross sectional area (see Figure ).…”

“…2D Electrochemistry. In our previous paper, we reported a discontinuity in the 2D voltammetry of tetradecane ferroceneketone (C 14 Fc) upon slow expansion of its monolayer on the water surface . Our observations indicated that the 2D voltammetric “line” microelectrode which resides by self-positioning exactly in the plane of the air/water interface functions as a nucleating site for the monolayer gas phase.…”

“…Once nucleated at the line microelectrode, the gas phase isolates this sensor from the LE phase, and the current signal abruptly vanishes, as the surface concentration of the amphiphile in the G phase is substantially lower and below detection limit of 2D voltammetry. We were able to reproducibly determine (within ±2 Å 2 /molecule) the position of the LE/G phase transition by 2D voltammetry both during C 14 Fc monolayer expansion and compression . In the latter case, the appearance of the current signal coincided with the final collapse of the coexistence region.…”

“…We have recently reported measurements suggesting an unusually low LE/G critical temperature of Langmuir monolayers of 4-tetradecaneamido-2,2,6,6-tetramethyl-1-piperidinyloxy (C 14 TEMPO) . In this report, we address the molecular basis of this finding and present results obtained with four additional members of this homologous series with longer alkyl chains (C 16 , C 18 , C 20 , and C 22 ).…”

“…We have recently reported measurements suggesting an unusually low LE/G critical temperature of Langmuir monolayers of 4-tetradecaneamido-2,2,6,6-tetramethyl-1-piperidinyloxy (C 14 TEMPO). 14 In this report, we address the molecular basis of this finding and present results obtained with four additional members of this homologous series with longer alkyl chains (C 16 , C 18 , C 20 , and C 22 ). Our measurements involving surface pressure and surface potential measurements as well as Brewster angle microscopy (BAM) and 2D electrochemistry 15 allowed us to determine that at room temperature only C 20 -TEMPO and C 22 TEMPO monolayers exhibit LE/G phase transitions and that the derivatives with shorter alkyl chains are supercritical.…”