Surface Structure of Quaternary Ammonium-Based Ionic Liquids Studied Using Molecular Dynamics Simulation: Effect of Switching the Length of Alkyl Chains

The surface structure of four quaternary ammonium-based ionic liquids (QaILs) at the QaIL|vacuum interface has been analyzed using molecular dynamics simulation to investigate the effect of switching the length of alkyl chains (k) of the quaternary ammonium cations on the surface structure. These four QaILs are composed of a common anion, bis­(trifluoromethanesulfonyl)­amide (TFSA–), and different cations: butyltrimethylammonium (N1114 +, k = 1), dibutyldimethylammonium (N1144 +, k = 2), tributylmethylammonium (N1444 +, k = 3), and tetrabutylammonium (N4444 +, k = 4), where k represents the number of butyl chains. All the QaILs show the same features as well-studied imidazolium-based ionic liquids (ILs): the formation of the interfacial ionic layers and the orientational preference that nonpolar parts of ions point to the vacuum phase. The thickness of the first ionic layer decreases with increasing k. This results from two-dimensional nanosegregation between polar and nonpolar parts of ions, where the state of the polar parts changes from the continuous phase for small k to dispersed one for large k because of the enlargement of the nonpolar domain with increasing k. Orientational distributions of the butyl chains of the Qa cations indicate that the orientational preference of the butyl chains pointing to the vacuum phase is weakened with increasing k, especially significantly from k = 1 to 2. Even for k = 4, N4444 + still shows the orientational preference in spite of its symmetric structure. A linear relation is found between the interfacial potential differences and the surface densities of the Qa cations, suggesting a possibility to control surface absorptivity of dipolar gas molecules in ILs by changing the cation size.