Directly Probing the Effects of Ions on Hydration Forces at Interfaces

Abstract

Understanding the influence of water layers adjacent to interfaces is fundamental in order to fully comprehend the interactions of both biological and nonbiological materials in aqueous environments. In this study, we have investigated hydration forces at the mica-electrolyte interface as a function of ion valency and concentration using subnanometer oscillation amplitude frequency modulation atomic force microscopy (FM-AFM). Our results reveal new insights into the nature of hydration forces at interfaces due to our ability to measure high force gradients without instability and in the absence of lateral confinement due to the use of an atomically sharp tip. We demonstrate the influence of electrolytes on the properties of both primary and structural hydration forces and reveal new insights into the interplay between these phenomena in determining the interaction forces experienced by a nanoscale object approaching an interface. We also highlight the difficulty in directly comparing hydration force data from different measurement techniques where the nature of the perturbation induced by differing interaction geometries is likely to dramatically affect the results.

Trends in mica–mica adhesion reflect the influence of molecular details on long-range dispersion forces underlying aggregation and coalignment

“…Our rationale for estimating an effective separation (h eff ) is that the AFM cantilever cannot exert a sufficient pressure to desolvate the ions (i.e., penetrate nearby hydration layers), and thus leaves the adsorbed hydrated ions between the tip and substrate intact (see SI Appendix for details). This estimate is consistent with the dependence of pressure on mica-mica separation determined in previous SFA experiments (16). The hydrated ions lead to h eff of ∼2× the hydrated ion diameter (see SI Appendix, Fig.…”

“…Near-surface solvent structure and interaction forces between surfaces (such as those of mica plates) have been investigated via atomic force microscopy (AFM) (16), surface force apparatus (SFA) (13,17,18), and theoretical modeling (19,20). Those studies showed there are at least three minima in force vs. distance between two mica surfaces that exhibit a periodicity of about 3 Å corresponding to the approximate thickness of a water…”

Sub-nanometer Resolution Imaging with Amplitude-modulation Atomic Force Microscopy in Liquid

“…These may include poorly reproducible curves and amplitude vs distance curves that significantly deviate from a typical sigmoidal shape 42 . If contaminants, ionic or otherwise, are dispersed homogeneously throughout the fluid, they may not show up in topographic imaging but could disrupt the hydration structure of the sample 69 , which is crucial for maintaining a regular tip-sample interaction 29 and obtaining high resolution 70 . There may also be direct effects of the contaminants on the sample, especially in soft, biological experiments.…”

Methods of photoelectrode characterization with high spatial and temporal resolution

“…118 Other exciting opportunities to apply in situ AFM techniques towards photoelectrochemistry include in situ patterning and modification of electrode surfaces 117,119 and elucidation of the electrolyte double layer structure and its associated charge at electrode surfaces. [120][121][122][123][124][125] Of particular interest are recent AFM studies that have demonstrating the ability to obtain three-dimensional (3D) force maps at a solid-liquid interface that reveal hydration layer structure and dynamics with Angstrom-and sub-minute resolutions, respectively. 122,125 When applied to studying a photoelectrode surface, this application of AFM could be invaluable, not only for better understanding electrochemical charge transfer kinetics, but also the affect that charged species in the electrolyte may have on the space charge layer of photoelectrodes.…”