Abstract
The colloidal probe technique was used to measure the inner layer capacitance of an electrical double layer. In particular, the forces were measured between silica surfaces and sulfate latex surfaces in solutions of monovalent salts of different alkali metals. The force profiles were interpreted with Poisson-Boltzmann theory with charge regulation, whereby the diffuse layer potential and the regulation properties of the interface were obtained. While the diffuse layer potential was measured in this fashion in the past, we are able to extract the regulation properties of the inner layer, in particular, its capacitance. We find systematic trends with the type of alkali metal ion and the salt concentration. The observed trends could be caused by difference in ion hydration, variation of the binding capacitance, and changes of the effective dielectric constant within the Stern layer. Our results are in agreement with recent experiments involving the water-silica interface based on a completely independent method using X-ray photoelectron spectroscopy in a liquid microjet. This agreement confirms the validity of our approach, which further provides a means to probe other types of interfaces than silica.
Highlights
A solid surface immersed in an aqueous solution usually acquires charge at the solid-liquid interface [1,2]
This article presents measurements of the Stern capacitance in monovalent electrolytes by direct force measurements based on the atomic force microscope (AFM)
Since Brown et al [14,19] exclusively focused on pH 10, we focus on these CcoollnoiddsitIinotenrsfacoesn2ly01. 8I,n2d, xeFeOdR, oPuEErRmReEaVsIuEWrements of the inner capacitance compare reasonably wel1l3wofit1h7 the ones obtained by X-ray photoelectron spectroscopy (XPS)
Summary
A solid surface immersed in an aqueous solution usually acquires charge at the solid-liquid interface [1,2]. This process is governed by adsorption of ions and/or ionization of surface groups within a thin and compact layer, which is situated close to the interface This charge is neutralized by an accumulation of counterions from the electrolyte solution in a so-called diffuse layer, which may gradually extend far away from the interface. One possible approach is to study ion adsorption and dissociation of ionizable surface groups in particle suspensions [10,11,12,13] and to extract the Stern capacitance by fitting the measured adsorption isotherms with suitable adsorption models [10,14] Another approaches are second harmonic generation and X-ray reflectivity techniques, which provide additional information about the water structure near the water-solid interface, and permit to extract the inner capacitance in some situations [15,16,17,18]. Given the complexity of this approach, alternative techniques capable to measure the capacitance of the inner layer in such systems more directly would certainly be welcome
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