Switching the Structural Force in Ionic Liquid-Solvent Mixtures by Varying Composition

Abstract

The structure and interactions in electrolytes at high concentration have implications from energy storage to biomolecular interactions. However, many experimental observations are yet to be explained in these mixtures, which are far beyond the regime of validity of mean-field models. Here, we study the structural forces in a mixture of ionic liquid and solvent that is miscible in all proportions at room temperature. Using the surface force balance to measure the force between macroscopic smooth surfaces across the liquid mixtures, we uncover an abrupt increase in the wavelength above a threshold ion concentration. Below the threshold concentration, the wavelength is determined by the size of the solvent molecule, whereas above the threshold, it is the diameter of a cation-anion pair that determines the wavelength.

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

“…Accordingly, increase in the concentration leads to the phenomenon of under-screening. 15,36 As introduced by Lee et al, 55 in a regime of concentration beyond the Kirkwood point, we observe that the screening length roughly scales as c 3/2 with respect to the Debye screening length.…”

“…19,30,45,49,59,60 To emphasize that, we chose to focus on the screening length behavior at a full range of concentrations, which is beyond the available theories as it captures the effect of secondary transverse instabilities that explain the experimentally observed second decay at high concentrations. 15,36 Notably, the advantage of spatiotemporal framework is in cases where understanding of temporal and/or morphological bulk effects is required, such as self-assembly. A detailed analysis of secondary instabilities is beyond the scope of this letter and will be presented elsewhere.…”

“…40 Indeed, the second decay in screening length have been reported at high concentrations, see cf. 15,36 and Figure 2.…”

“…The model is qualitatively tested and appear to capture and relate, in broad parameter regimes, the above mentioned phenomena to effects of polarization 3 and self-assembly 23 to explain the origins of non-monotonic electrical screening length variation. 15,36 In particular, at high concentrations, the system is expected to indeed form different morphologies in both the EDL and bulk region that are strongly dictated by spatiotemporal instabilities that drive the self-assembly process in the bulk, as in models for pure ionic liquids 37,40 and charged polymer diblock copolymers. 41 …”

Structural interactions in ionic liquids linked to higher-order Poisson-Boltzmann equations

“…This phenomenon has been observed in numerous experiments and simulations, see e.g. [3][4][5][6][7]; for a review, see [8].…”