Directing Long-Range Molecular Ordering in Ionic Liquid Films: A Tale of Two Interfaces

Abstract

We report surface-dependent, long-range ordering behavior of two ionic liquids, 1-butyl-1-methylpyrrolidinium tetracyanoborate (P14 B­(CN)4) and 1-butyl-1-methylpyrrolidinium dicyanamide (P14 N­(CN)2). The ionic liquids are supported as liquid films and examined using vibrational spectroscopy and spectroscopic ellipsometry. Both systems show changes in the infrared peak profile characteristic of major dipole reorientations for both anions, but the changes observed are in different spectral regions. Specifically, tetracyanoborate shows changes in the B–C stretching modes, and dicyanamide displays changes in the CN and N–C modes. Surprisingly, B­(CN)4 induces a final film environment, which is identical to the solid–liquid interfacial orientation. However, the N­(CN)2 indicates propagation of the gas–liquid interfacial environment through the film toward the solid substrate. The extent of ordering within the films extends to 0.4 and 1.1 μm for P14 N­(CN)2 and P14 B­(CN)4, respectively. This observation is intriguing as it presents a possibility for controlling molecular (re)­orientations at 100 s of nanometer scales by tuning fundamental intermolecular interactions at surfaces. These results for the cyano-functionalized anions should be generally applied for other systems, contributing directly to a better understanding and design for applications in molecular electronics, lubrication, and energy storage devices.

Comparative Operando XPS and SEM Spatiotemporal Potential Mapping of Ionic Liquid Polarization in a Coplanar Electrochemical Device

“…In these efforts, in addition to the well-established electrochemical metrology, advanced optical, microscopy, and scattering techniques have been utilized. − These have revealed that the screening dynamics evolving on multiple time and length scales are due to various diffusion and relaxation processes. This has been also corroborated by simulations. , These studies have revealed that the polarization of ILs includes medium–slow (0.1 s–10 s) and ultraslow (10 2 –10 4 s) evolutions, in addition to the fast electrical double-layer (EDL) formation (1–10 ns), which also proceeds on short (1–10 nm), medium (<1 μm), and long (>100 μm) length scales. − Therefore, for extended IL devices, the information reflecting not only the solid/liquid interface but also the bulk liquid and electrodes must be accounted for comprehensive analyses.…”

The effect of dilution on induced free charge density gradients in room temperature ionic liquids

“…7,11–13 The Fayer group, in a series of elegant experiments using small molecules as probes, has identified organization with a persistence length on the order of tens to hundreds of nm in RTILs, 14–19 and the Shaw group has identified the evolution of structural order on the micrometer length scale in thin RTIL films. 20–23 Very recently, the Welton group has identified spatial variation in n , the refractive index, over distances of tens to hundreds of nm using Raman scattering. 24 It is clear from all these studies that RTILs exhibit structural order over length scales vastly in excess of what is typical for liquid phase solvents.…”

Observation of the Pockels Effect in Ionic Liquids and Insights into the Length Scale of Potential-Induced Ordering

“…Here, we attempt to understand the characteristic electric field effects on the Raman spectra of the ILs in the context of the Pockels effect because the Δ I signal has been found to be proportional to the applied potential E (see Figure S2b). The Pockels effect occurs only in non-centrosymmetric media like crystals, but if some ordering is induced in ILs near the interface with an electrode as revealed in recent studies, − it could break the inversion symmetry and allow the Pockels effect to occur in the interfacial region of the IL. The resulting change in the refractive index, Δ n (= a 1 F ), would then affect the power of the excitation light at the focal point ( I exc in eq ) through eqs and .…”

“…18,19 scale ordering can be induced in IL fluid films has been gained by IR reflection absorption spectroscopy and second harmonic generation. 20,21 Even greater organization on the order of 100 μm has been suggested by other laboratories for ILs at a charged surface. 22−24 The electric field effects on ILs are a crucial key to address the controversial question of the length scale of such an organization.…”

“…Studies have shown that when direct-current voltage is applied to an electrode in ILs, an interfacial structure is formed in which the cations and anions are stacked alternately within a few nanometers from the electrode’s charged surface and this ordering structure causes electrostatic screening of the externally applied electric field (short Debye lengths). − Thus, the IL that is more than 10 nm away from the electrode is regarded as behaving just as the bulk. Contrary to this traditional view, force–distance measurements with a surface force apparatus (SFA) have revealed the existence of long-range (10 nm or more) surface forces that operate between charged surfaces for many different ILs. , Furthermore, experimental evidence that micrometer-scale ordering can be induced in IL fluid films has been gained by IR reflection absorption spectroscopy and second harmonic generation. , Even greater organization on the order of 100 μm has been suggested by other laboratories for ILs at a charged surface. − The electric field effects on ILs are a crucial key to address the controversial question of the length scale of such an organization.…”