Effect of Solid Substrates on the Molecular Structure of Ionic Liquid Nanofilms

Abstract

Fundamental understandings of the interfacial molecular structure of solid-confined ionic liquids (ILs) have significant impacts on the development of many cutting-edge applications. Among the extensive studies on the molecular structure at the IL/ solid interface, direct observation of a double-layering quantized growth of [C n mim][FAP] on mica was recently reported. In the current work, the atomic force microscopy (AFM) results directly show that the growths of [Bmim][FAP] nanofilms on silica and amorphous carbon are different from the double-layering growth on mica. The growth of [Bmim][FAP] nanofilms on silica is dominated by the aggregation of the IL molecules, which can be attributed to the inadequate negative charging of the silica surface resulting in a weak electrostatic interaction between silica and the IL cation. [Bmim][FAP] on amorphous carbon shows a fairly smooth film for the thinner nanofilms, which can be attributed to the π−π + parallel stacking between the cation imidazolium ring and the randomly distributed sp 2 carbon on the amorphous carbon surface. Our findings highlight the effect of different IL/solid interactions, among the several competing interactions at the interface, on the resulting molecular arrangements of various IL.

Atomic force microscopy probing interactions and microstructures of ionic liquids at solid surfaces

“…Except for the surface roughness, the chemical composition of the surface also influences the structure and dynamics of ILs at solid interfaces. 161 A solid surface functionalized by 3-hydroxytyramine hydrochloride was found to enable the catecholic anchor of imidazolium ILs to improve the nanofrictional performance of the surface. 162 With the fraction of hydroxylation in graphene oxide (GO) increasing from 0 to 15%, the friction coefficient of [BMIM][BF 4 ] confined in GO nanochannels can be enhanced by almost 60 times, and the slip length can be reduced by 3 orders of magnitude.…”

Immobilizing Ionic Liquids onto Functionalized Surfaces for Sensing Volatile Organic Compounds

“…There exist IL droplets on the functionalized surfaces revealed by the bright spots in elemental mapping images (Figure a,b), while the thickness of the structuring IL layers near the functionalized surfaces is measured to be less than 10 nm (Figure b). At this situation, the main contribution to the gas adsorption originates from the near-surface ion layers of the IL film. , The aggregation dominating growing mode of [BMIM]­[PF 6 ] IL is found on −N + - and −NH 2 -functionalized surfaces (left and middle panels in Figure ba,), probably due to the fact that the surface–IL interactions are still not strong enough to induce a strong templating effect on the IL ions . Unlike on the −N + - and −NH 2 -functionalized surfaces, the IL forms a film on the −COOH-functionalized surface without obvious aggregation (right panels in Figure a,b).…”

“…50,51 The aggregation dominating growing mode of [BMIM][PF 6 ] IL is found on −N + -and −NH 2 -functionalized surfaces (left and middle panels in Figure 44ba,), probably due to the fact that the surface−IL interactions are still not strong enough to induce a strong templating effect on the IL ions. 52 Unlike on the −N +and −NH 2 -functionalized surfaces, the IL forms a film on the −COOH-functionalized surface without obvious aggregation (right panels in Figure 4a,b). This is because of the π−π + stacking interaction that occurred between the delocalized π + The thickness of the functionalized surfaces before and after being immobilized by the IL layer has been further acquired through AFM (Figure 5).…”

“…Unlike on the −N + - and −NH 2 -functionalized surfaces, the IL forms a film on the −COOH-functionalized surface without obvious aggregation (right panels in Figure a,b). This is because of the π–π + stacking interaction that occurred between the delocalized π + electrons from the [BMIM] + imidazolium ring and the localized π electrons from the sp 2 carbon − on the −COOH group. The randomly distributed sp 2 carbon in exposed −COOH groups provides high conformational flexibility for the arrangement of [BMIM] + cations on the surface, with subsequent packed [PF 6 ] − anions as a result of the Coulombic force between the IL ions to maintain electroneutrality .…”

Effect of Electrode Surface Chemistry on Ion Structuring of Imidazolium Ionic Liquids

“…Electrostatic interaction plays a determinant role in the BP IL structuring on Au–NH 2 , although it is fairly weak for the [BMIM] + cation with the positively charged Au–NH 2 surface. Thus, the aggregating BP IL on the Au–NH 2 surface is due to the weaker IL–surface interactions, which is unable to induce a strong templating effect on the IL ions . In contrast, the negatively charged Au–COOH surface shows a stronger electrostatic interaction with the [BMIM] + cation, favorable for the IL layering.…”