Orientation and lateral mobility of insoluble Tempo amphiphiles at the air/water interface

The lateral diffusion constants of two water insoluble redox surfactants, 4-alkaneamide derivatives of 2,2,6,6-tetramethyl-1-piperidynyloxy radical (C_nTempo where n = 14 or 18) were measured using 2D voltammetry with 500 μm long line electrodes positioned in the plane of the air/water interface. In order to extend these measurements into a region of low surface concentrations, we first examined the line micro-band electrodes as well as the stability of the Tempo surfactants on the water surface. Only C₁₈Tempo proved to be sufficiently insoluble in water to form stable monolayer films over sufficiently long periods of time to assure reproducible measurements in the range of mean molecular areas (MMA) of 50-750 Å²/molecule. In this range, its diffusion constant increases linearly with MMA. At MMA of ca. 450 Å²/molecule, this dependence becomes significantly weaker suggesting proximity of a plateau region with a D of ca. 1.5 × 10⁻⁵ cm²/s. The orientation of C₁₈Tempo on the water surface in a compressed monolayer at 52 Å²/molecule was determined by X-ray reflectivity using a synchrotron source. Only the amide group of C₁₈Tempo appears to be immersed in the aqueous subphase while the alkane chain and the piperidine ring are located above the water surface pointing upwards.