Ionic Liquid Films at the Water鈥揂ir Interface: Langmuir Isotherms of Tetra-alkylphosphonium-Based Ionic Liquids

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• 作者：Karina Shimizu ; Jos茅 N. Canongia Lopes ; Am茅lia M. P. S. Gon莽alves da Silva
• 刊名：Langmuir
• 出版年：2015
• 出版时间：August 4, 2015
• 年：2015
• 卷：31
• 期：30
• 页码：8371-8378
• 全文大小：475K
• ISSN：1520-5827

文摘

The behavior of ionic liquids trihexyl(tetradecyl)phosphonium bis(trifluoromethylsulfonyl)imide and trihexyl(tetradecyl)phosphonium dicyanamide, [P_{6鈥?鈥?鈥?4}][Ntf₂] and [P_{6聽6聽6聽14}][N(CN)₂], respectively, at the water鈥揳ir interface was investigated using the Langmuir trough technique. The obtained surface pressure versus mean molecular area (MMA) isotherms, 蟺鈥?i>A, and surface potential versus MMA isotherms, 螖V鈥?i>A, show distinct interfacial behavior between the two systems. The results were interpreted at a molecular level using molecular dynamics simulations: the different compression regimes along the [P_{6聽6聽6聽14}][Ntf₂] isotherm correspond to the self-organization of the ions at the water surface into compact and planar monolayers that coalesce at an MMA value of ca. 1.85 nm²/ion pair to form an expanded liquidlike 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 [P_{6聽6聽6聽14}][N(CN)₂] system because of 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 a distribution is also different for the two studied systems during the different compression regimes.