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

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.

Figure 1

Drawing of the experimental geometry in the SFB: mica sheets mounted in crossed-cylinder configuration are brought together in a mixture of ionic liquid and polar solvent. The distance between the sheets $D$, their curvature $R$, and their interaction force $F_N$ are all determined using the spectral fringes of equal chromatic order (FECO). The resulting $F_N$ vs $D$ profiles reveal the sequential squeeze-out of layers of molecules; cartoons on the right show proffered molecular arrangements at low and at high salt concentration, giving rise to solvation force wavelengths determined by solvent and by ion pairs, respectively.

Figure 2

(a) Chemical structures and (b)–(h) measured forces $F_N$ between two mica surfaces (normalized by radius of curvature $R$) as a function of surface separation $D$ across liquids with concentrations ranging from pure propylene carbonate to pure ionic liquid. Concentration values correspond to actual concentrations injected into the SFB rather than derived from a fit to data. Colored data indicate repulsive forces measured on approach of the surfaces, and triangles indicate points measured from the jump apart of the surfaces from adhesive minima. Lines are a guide to the eye to show the oscillatory nature of the forces, with solid lines through the measured regions and dashed lines through regions inaccessible to measurement.

Figure 3

(a) Wavelength of the measured oscillatory forces, corresponding to each of the concentrations shown in Figs. 2. Values were calculated using the measured distances between adjacent minima and between adjacent maxima in the force profiles. (b) Magnitude of the oscillatory force at each of the concentrations studied. Values were calculated from the preexponential factor associated with the decay of adhesive minima in the force oscillations for each concentration.

Figure 4

Two decay lengths are required to characterize the interaction between charged bodies in concentrated electrolytes. The decay length of the oscillatory structural force is determined from an exponential fit to the minima of oscillations shown in Figs. 2. The decay of the long-range electrostatic force is determined from an exponential fit to the long range tail. This was recently shown to demonstrate an anomalous increase at high concentrations [6]. The long-range screening length measured in pure PC, not shown, is 112 nm.