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Probing the Surface Tension of Ionic Liquids Using the Langmuir Principle.

Authors :: Shimizu K
Heller BSJ
Maier F
Steinrück HP
Canongia Lopes JN
Source :: Langmuir : the ACS journal of surfaces and colloids [Langmuir] 2018 Apr 10; Vol. 34 (14), pp. 4408-4416. Date of Electronic Publication: 2018 Mar 29.
Publication Year :: 2018
Abstract: At 298 K, the surface tension of ionic liquids (ILs) of the 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide series, [C <subscript>n</subscript> C <subscript>1</subscript> Im][NTf <subscript>2</subscript> ], ranges from around 35 mN·m <superscript>-1</superscript> for [C <subscript>2</subscript> C <subscript>1</subscript> Im][NTf <subscript>2</subscript> ] to just below 30 mN·m <superscript>-1</superscript> for [C <subscript>12</subscript> C <subscript>1</subscript> Im][NTf <subscript>2</subscript> ]. However, the decrease rate along the series is not constant: a large decrease from [C <subscript>2</subscript> C <subscript>1</subscript> Im][NTf <subscript>2</subscript> ] to [C <subscript>8</subscript> C <subscript>1</subscript> Im][NTf <subscript>2</subscript> ] is followed by almost constant values from [C <subscript>8</subscript> C <subscript>1</subscript> Im][NTf <subscript>2</subscript> ] to [C <subscript>12</subscript> C <subscript>1</subscript> Im][NTf <subscript>2</subscript> ]. Such behavior is hard to interpret from a molecular point of view without suitable information about the free-surface structure of the different ILs. In this work, we have successfully used the Langmuir principle in combination with structural data obtained from angle-resolved X-ray photoelectron spectroscopy experiments and molecular dynamics simulations, to predict the correct surface tension trend along the IL series. The concepts unveiled for this particular homologous IL family can be easily extended to other systems.