A multimodal screening length analysis of concentrated electrolytes

The phenomenon of underscreening, where the screening of the electrostatic potential in the bulk electrolyte is weaker than it should be according to the canonical Debye-Hückel theory, has significant implications for colloidal stability in highly concentrated electrolytes. Current experimental and computational investigations of this phenomenon have been limited to single mode analyses, despite statistical mechanics predicting that many modes are present simultaneously. We hypothesise that using a multi-modal approach will provide insights not yet observed.

Here we apply Fourier analysis to radial charge densities, derived from polarisable molecular dynamic simulations of aqueous alkali chloride electrolytes, to determine if multiple modes are present. Prony's method is then applied to a multi-modal ansatz to estimate screening lengths associated with each mode.

Fourier analysis revealed that there are many modes present in the radial charge density. For all electrolytes considered at low concentrations the dominant mode was a non-oscillatory Yukawa decay mode, while at higher concentrations modes with non-zero spatial frequencies dominated. Resulting screening modes with oscillatory wavelengths ∼5-15 Å from Prony's method agree with the largest experimental screening lengths from surface force apparatus and fluorescence experiments. Concurrently, screening lengths with shorter oscillatory wavelengths, 3-5 Å, have smaller magnitudes and agree with other experiments such as atomic force microscopy and optical second harmonic scattering experiments.