Cation Alkyl Side Chain Length and Symmetry Effects on the Surface Tension of Ionic Liquids

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%.

keywords

EXTENDED PRESSURE RANGE; MOLECULAR-FORCE FIELD; PHYSICAL-PROPERTIES; THERMODYNAMIC PROPERTIES; REFRACTIVE-INDEX; TEMPERATURE; DENSITY; ANION; WATER; SIMULATIONS

subject category

Chemistry; Materials Science

authors

Almeida, HFD; Freire, MG; Fernandes, AM; Lopes-da-Silva, JA; Morgado, P; Shimizu, K; Filipe, EJM; Lopes, JNC; Santos, LMNBF; Coutinho, JAP

Groups

G4 - Renewable Materials and Circular Economy
G5 - Biomimetic, Biological and Living Materials

Projects

BIIPP - Biorefinaria Integrada na Indústria da Pasta e Papel (BIIPP)

Centre for Research in Ceramics and Composite Materials - CICECO (LA0011: 2013-2014) (PEst-C/CTM/LA0011/2013)

Ionic Liquids as Promoters of Aqueous Biphasic Systems: The Role of van der Waals and Coulomb Interactions (PTDC/QUI-QUI/121520/2010)

acknowledgements

This work was financed by QREN SI-I&DT Project 11551 from UE/FEDER through COMPETE program. The authors also acknowledge FCT-Fundacao para a Ciencia e a Tecnologia for the projects Pest-C/CTM/LA0011/2013, PEst-OE/QUI/UI0100/2013, FCT-ANR/CTM-NAN/0135/2012 (including a postdoctoral grant of KS), PTDC/CTM-NAN/121274/2010 and PTDC/QUI-QUI/121520/2010. P.M. acknowledges the postdoctoral grant SFRH/BPD/81748/2011.