Effect of Hydrogen Bonding between Ions of Like Charge on the Boundary Layer Friction of Hydroxy-Functionalized Ionic Liquids

Abstract

Atomic force microscopy has been used to measure the lubricity of a series of ionic liquids (ILs) at mica surfaces in the boundary friction regime. A previously unreported cation bilayer structure is detected at the IL−mica interface due to the formation of H-bonds between the hydroxy-functionalized cations [(cc) H-bonds], which enhances the ordering of the ions in the boundary layer and improves the lubrication. The strength of the cation bilayer structure is controlled by altering the strength of (c-c) H-bonding via changes in the hydroxyalkyl chain length, the cation charge polarizability, and the coordination strength of the anions. This reveals a new means of controlling IL boundary nanostructure via H-bonding between ions of the same charge, which can impact diverse applications, including surface catalysis, particle stability, electrochemistry, etc.

Understanding the Photoexcitation of Room Temperature Ionic Liquids

“…Interactions due to hydrogen bonds between ions of the same charge can control the boundary laver friction at an RTIL‐mica interface. Therefore, it can have an impact on various applications, electrochemistry [7,38,39] and physical properties like enthalpies. Especially the enthalpy of vaporization is influenced by hydrogen bonds in hydroxyl‐functionalized RTILs [40] …”

Thermodynamically Stable Cationic Dimers in Carboxyl-Functionalized Ionic Liquids: The Paradoxical Case of “Anti-Electrostatic” Hydrogen Bonding

“…Until then, like-charge attraction in "real systems" had been reported mainly for large-scale structures, assemblies, or stabilizing frameworks [13][14][15][16][17][18][19][20][21]. Overall attractive interaction between the "likes" in liquids and solutions required screening effects caused by neighboring counterions, polar molecules, or generally favorable dielectric environments [22][23][24][25][26][27][28][29][30][31][32][33][34][35]. Our attempts aimed not only to detect but to control like-charge attraction by varying molecular parameters.…”

“…Obviously, hydrogen bonds between ions of like charge can be stronger than those between ions of opposite charge despite the repulsion in the first interaction and the attractive Coulomb interaction in the latter case. Recently, we showed by means of molecular dynamics (MD) simulations that the hydrogen bonding lifetimes of the (c + -c + ) H-bonds can be longer than those of the (c + -a − ) H-bonds for alkyl chain length n = 4-5, reflecting the different interaction strengths in both types of hydrogen bonds, as indicated by IR and NMR spectroscopy [32][33][34][35]. However, the populations derived from neutron diffraction data showed that only less than 20% of all hydrogen bonds refer to (c + -c + ) bound clusters at room temperature.…”

“…The cationic dimers demonstrate that the short-range donor-acceptor covalency forces successfully compete with the powerful long-range electrostatic repulsions [22][23][24][25][26][27][28][29][30][31][32][33][34][35].…”

Anti-electrostatic hydrogen bonding between anions of ionic liquids: a density functional theory study

“…increasing their melting point and viscosity, reducing their conductivity, inhibiting crystallization under supercooled conditions and changing the interfacial nanostructures of ILs. 25,[33][34][35] However, to the best of our knowledge, no research on the HBs between the anions of ILs has been reported so far.…”