Surface and Aggregation Properties of N-(4-Nitrophenyl)perfluorononanamide

It was determined that N -(4-nitrophenyl)perfluorononanamide ( 1 ) forms a stable monolayer which suffers a liquid expanded (LE) → liquid condensed (LC) phase transition upon compression. The surface pressure of the transition was linearly dependent on the temperature indicating a diffuse first-order transition process. The transition was ascribed to rotation of the C−N bond of the amide. In the LE state, 1 has an “L” conformation and in the LC state the phenyl group is aligned with the perfluoroalkyl chain. The p K of 1 is about 2 units higher than the value expected in the water solution. The surface potential of 1 at the air−water interface is negative at all measured pH values, indicating that the molecule on the surface can be considered as a vector having its air end negative. In solution, 1 is in an aggregated form even at 2 × 10 - 6 M concentration and these aggregates are destroyed in solutions containing 0.01 M of sodium dodecyl sulfate (SDS) or 7 × 10 - 3 M of perfluorononanoic acid 2 . The kinetics of deaggregation occurs with two characteristic times with values of τ 1 74.6 and 21.7 s and τ 2 294 and 83 s for SDS and 2, respectively, at 25 °C.