American Chemical Society, Langmuir, 30(31), p. 8371-8378, 2015
DOI: 10.1021/acs.langmuir.5b01977
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The behavior of ionic liquids trihexyl(tetradecyl)phosphonium bis(trifluoromethylsulfonyl)imide and trihexyl(tetradecyl)phosphonium dicyanamide, [P6 6 6 14][NTf2] and [P6 6 6 14][N(CN)2], at the water-air interface was investigated using the Langmuir trough technique. The obtained surface pressure versus mean molecular area (MMA) isotherms, π-A, and surface potential versus MMA isotherms, ΔV-A, show distinct interfacial behaviour between the two systems. The results were interpreted at a molecular level using Molecular Dynamics simulations: the different compression regimes along the [P6 6 6 14][NTf2] isotherm correspond to the self-organization of the ions at the water surface into compact and planar monolayers that coalesce at a MMA value of ca. 1.85 nm2/ion-pair to form an expanded liquid-like layer. Upon further compression the monolayer collapses at around 1.2 nm2/ion-pair to yield a progressively thicker and less organized layer. These transitions are much more subdued in the [P6 6 6 14][N(CN)2] system due to the more hydrophilic nature of the dicyanamide anion. The numerical density profiles obtained from the MD simulation trajectories are also able to emphasize the very unusual packing of the four long alkyl side chains of the cation above and below the ionic layer that forms at the water surface. Such distribution is also different for the two studied systems during the different compression regimes.