Solid-liquid interfaces of ionic liquid solutions—Interfacial layering and bulk correlations

Abstract

The influence of the polar, aprotic solvent propylene carbonate on the interfacial structure of the ionic liquid (IL) 1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate on sapphire was investigated by high-energy x-ray reflectivity. Experiments at solvent concentrations between 17 mol. % and 83 mol. % bridge the gap between diluted electrolytes described by the classical Gouy-Chapman theory and pure ionic liquids. Analysis of our experimental data revealed interfacial profiles comprised of alternating anion and cation enriched regions decaying gradually into the bulk liquid. With increasing solvent concentration, we observed a decrease in correlation length of the interfacial layering structure. At high ion concentrations, solvent molecules were found to accumulate laterally within the layers. By separating like-charged ions, they reduce their Coulomb repulsion. The results are compared with the bulk structure of IL/solvent blends probed by x-ray scattering and predictions from fundamental fluid theory.

From Solvent-Free to Dilute Electrolytes: Essential Components for a Continuum Theory

“…This region is characterized by low enough concentrations so that steric effects are perturbative, and the model reduces to the PNP theory of dilute electrolytes (or Poisson-Boltzmann (PB) theory if steady states are of interest). The latter also demonstrates that sufficiently diluted IL, either by solvent 47 or possibly by formation of ion-pairs, 19,50 can behave as dilute electrolytes.…”

“…The insets A-C in Figure 2 present steady-state solutions of (4) with parameters corresponding to the ionic liquid, [C 4 C 1 Pyrr][NTf 2 ], diluted with propylene carbonate (C 4 H 6 O 3 ). Indeed, a transition from layered to monotonic EDL has been observed using high-energy x-ray reflectivity 47 and in situ AFM 48 within the range of parameters considered here. The molecular origin for the transition from monotonic to spatially oscillatory EDL, a.k.a.…”

“…4 gives rise to a richer picture of bulk and interfacial behavior, that is consistent with the picture arising in recent experimental and numerical studies. 21,23,33,35,[47][48][49] Indeed, using spatial linearization methods, see SI for details, we map the qualitatively distinct bulk and interfacial behaviors, and the corresponding regions in the parameter space spanned by ion size asymmetry γ and normalized concentration c, see Region A, describes the emergent self-assembly of bulk nano-structure, i.e., a spatially oscillatory profile throughout the whole domain, see top inset in Figure 1. Here, the region is obtained from the onset of the finite wavenumber instability, 38 see SI for details.…”

Mean-Field Theory of Electrical Double Layer In Ionic Liquids with Account of Short-Range Correlations

“…Introduction of overscreening into the theory [8] leads to a complicated set of equations, but its effect on capacitance is tricky: although it broadens somewhat the capacitance curve, alone, without the introduction of the compact layer, it does not substantially reduce the 1 Their existence has also been detected directly by X-ray [6,[45][46][47][48][49] and indirectly by scanning nano-probe techniques [50][51][52][53][54][55][56]. Generally, oscillating profiles and overscreening in dense ionic solutions have a long history.…”

“…The authors of Refs. 72-74 studied the effects of short-range correla-2 Experimental verifications of oscillating structures in the double layer of ionic liquids [6,[45][46][47][48][49][50][51][52][53][54][55][56] have all been detected near ideally atomically flat surfaces, and we are not aware of any systematic investigations of how they may be perturbed or eliminated by the atomic roughness of the surface [56]. However, computer simulations have specially studied step-edge defects in carbon electrodes [65,66] and surface roughness on a similar scale to molecular dimensions [5,67], which have demonstrated that the resonance effect could be damped.…”