Cation Alkyl Side Chain Length and Symmetry Effects on the Surface Tension of Ionic Liquids
authors Almeida, HFD; Freire, MG; Fernandes, AM; Lopes-da-Silva, JA; Morgado, P; Shimizu, K; Filipe, EJM; Lopes, JNC; Santos, LMNBF; Coutinho, JAP
nationality International
journal LANGMUIR
keywords EXTENDED PRESSURE RANGE; MOLECULAR-FORCE FIELD; PHYSICAL-PROPERTIES; THERMODYNAMIC PROPERTIES; REFRACTIVE-INDEX; TEMPERATURE; DENSITY; ANION; WATER; SIMULATIONS
abstract Aiming at providing a comprehensive study of the influence of the cation symmetry and alkyl side chain length on the surface tension and surface organization of ionic liquids (ILs), this work addresses the experimental measurements of the surface tension of two extended series of ILs, namely R,R'-dialkylimidazolium bis-Rtrifluoromethyl)sulfonyllimide ([C(n)C(n)im][NTf2]) and R-alkyl-3-methylimidazolium bis[ (trifluoromethyl)sulfonyl] imide ([C-n C(1)im]-[NTf2D, and their dependence with temperature (from 298 to 343 K). For both series of ILs the surface tension decreases with an increase in the cation side alkyl chain length up to aliphatic chains no longer than hexyl, here labeled as critical alkyl chain length (CACL). For ILs with aliphatic moieties longer than CACL the surface tension displays an almost constant value up to [C(12)C(1)im][NTf2] or [C(16)C(1)im][NTf2]. These constant values further converge to the surface tension of long chain n-alkanes, indicating that, for sufficiently long alkyl side chains, the surface ordering is strongly dominated by the aliphatic tails present in the IL. The enthalpies and entropies of surface were also derived and the critical temperatures were estimated from the experimental data. The trend of the derived thermodynamic Properties highlights the effect of the structural organization of the IL at the surface with visible trend shifts occurring at a well-defined CACL in both symmetric and asymmetric series of ILs. Finally, the structure of a long-alkyl side chain IL at the vacuum-liquid interface was also explored using Molecular Dynamics simulations. In general, it was found that for the symmetric series of ILs, at the outermost polar layers, more cations point one of their aliphatic tails outward and the other inward, relative to the surface, than cations pointing both tails outward. The number of the former, while being the preferred conformation, exceeds the latter by around 75%.
publisher AMER CHEMICAL SOC
issn 0743-7463
year published 2014
volume 30
issue 22
beginning page 6408
ending page 6418
digital object identifier (doi) 10.1021/la501308q
web of science category Chemistry, Multidisciplinary; Chemistry, Physical; Materials Science, Multidisciplinary
subject category Chemistry; Materials Science
unique article identifier WOS:000337198900013
link 24834955

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