Volume 246, 2023

Ion adsorption and hydration forces: a comparison of crystalline mica vs. amorphous silica surfaces

Abstract

Hydration forces are ubiquitous in nature and technology. Yet, the characterization of interfacial hydration structures and their dependence on the nature of the substrate and the presence of ions have remained challenging and controversial. We present a systematic study using dynamic Atomic Force Microscopy of hydration forces on mica surfaces and amorphous silica surfaces in aqueous electrolytes containing chloride salts of various alkali and earth alkaline cations of variable concentrations at pH values between 3 and 9. Our measurements with ultra-sharp AFM tips demonstrate the presence of both oscillatory and monotonically decaying hydration forces of very similar strength on both atomically smooth mica and amorphous silica surfaces with a roughness comparable to the size of a water molecule. The characteristic range of the forces is approximately 1 nm, independent of the fluid composition. Force oscillations are consistent with the size of water molecules for all conditions investigated. Weakly hydrated Cs+ ions are the only exception: they disrupt the oscillatory hydration structure and induce attractive monotonic hydration forces. On silica, force oscillations are also smeared out if the size of the AFM tip exceeds the characteristic lateral scale of the surface roughness. The observation of attractive monotonic hydration forces for asymmetric systems suggests opportunities to probe water polarization.

Graphical abstract: Ion adsorption and hydration forces: a comparison of crystalline mica vs. amorphous silica surfaces

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Article information

Article type
Paper
Submitted
21 Feb 2023
Accepted
13 Mar 2023
First published
17 Mar 2023
This article is Open Access
Creative Commons BY license

Faraday Discuss., 2023,246, 274-295

Ion adsorption and hydration forces: a comparison of crystalline mica vs. amorphous silica surfaces

I. Siretanu, S. R. van Lin and F. Mugele, Faraday Discuss., 2023, 246, 274 DOI: 10.1039/D3FD00049D

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