  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.
