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Colloidal Probe Atomic Force Microscopy Reveals Anomalous Underscreening: A Matter of Experimental Conditions

Thomas Tilger, Esther Ohnesorge, Michalis Tsintsaris, Kazue Kurihara, Hayden Robertson, Regine von Klitzing

Abstract

There is considerable debate about anomalous underscreening in highly concentrated electrolytes: While surface force apparatus (SFA) measurements have confirmed anomalously long screening lengths, so far they have not yet been detected in experiments using colloidal probe atomic force microscopy (CP-AFM). CP-AFM measurements across aqueous LaCl$_3$ solutions demonstrate that by adapting the experimental conditions to those of SFA studies, similarly large screening lengths can be achieved at high salt concentrations. This represents the first observation of anomalous underscreening with CP-AFM. These findings leave room for speculations about the ordering of the confined electrolyte.

Colloidal Probe Atomic Force Microscopy Reveals Anomalous Underscreening: A Matter of Experimental Conditions

Abstract

There is considerable debate about anomalous underscreening in highly concentrated electrolytes: While surface force apparatus (SFA) measurements have confirmed anomalously long screening lengths, so far they have not yet been detected in experiments using colloidal probe atomic force microscopy (CP-AFM). CP-AFM measurements across aqueous LaCl solutions demonstrate that by adapting the experimental conditions to those of SFA studies, similarly large screening lengths can be achieved at high salt concentrations. This represents the first observation of anomalous underscreening with CP-AFM. These findings leave room for speculations about the ordering of the confined electrolyte.
Paper Structure (4 sections, 4 equations, 2 figures)

This paper contains 4 sections, 4 equations, 2 figures.

Table of Contents

  1. Introduction
  2. Experimental Section
  3. Results and Discussion
  4. Conclusion

Figures (2)

  • Figure 1: Normalized interaction force $F / R$ between a silica colloidal probe and a planar quartz substrate at separation $h$ immersed in aqueous LaCl3 electrolytes of various concentrations: 0.001; 0.030; 0.177; 0.285; 0.500; 0.700; 1.000. Top row: Data captured after an equilibration time of 30min with an approach speed of 200nm/s using a (a) 2.1µm and (b) 8.5µm colloidal probe. Bottom row: Captured with the larger 8.5µm colloidal probe after an equilibration time of 48h at an approach speed of (a) 200nm/s and (b) 30nm/s. The force profiles present an emerging repulsion in the highly concentrated regime with (a) $\rightarrow$ (b) increasing probe size, (b) $\rightarrow$ (c) longer equilibration time, and (c) $\rightarrow$ (d) decreasing approach speed. A linear representation, also highlighting attractive interactions, is provided as Fig. S1 in the Supplemental Material SuppMat.
  • Figure 2: The experimental screening length $\lambda_{\mathrm{S}}$ in aqueous LaCl3 electrolytes as extracted from the data presented in \ref{['fig:Fig1']} (d). (a) Screening length as a function of concentration $c$ and ionic strength $I = 6 c$. The dashed black line presents the predictions of the Debye-Hückel theory. (b) Screening length $\lambda_{\mathrm{S}}$ normalized by the Debye length $\lambda_{\mathrm{D}}$ as a function of the normalized mean ion size $a / \lambda_{\mathrm{D}}$. The black triangle denotes the slope predicted by the scaling law from \ref{['eqn:Scaling_law']} for $p = 3$, indicative of anomalous underscreening. A linear representation of these data and a superimposed model fit of the scaling law are presented in Fig. S4 in the Supplemental Material SuppMat.