Abstract
The structure of the electrical double layer has been debated for well over a century, since it mediates colloidal interactions, regulates surface structure, controls reactivity, sets capacitance, and represents the central element of electrochemical supercapacitors. The surface potential of such surfaces generally exceeds the electrokinetic potential, often substantially. Traditionally, a Stern layer of nonspecifically adsorbed ions has been invoked to rationalize the difference between these two potentials; however, the inability to directly measure the surface potential of dispersed systems has rendered quantitative measurements of the Stern layer potential, and other quantities associated with the outer Helmholtz plane, impossible. Here, we use x-ray photoelectron spectroscopy from a liquid microjet to measure the absolute surface potentials of silica nanoparticles dispersed in aqueous electrolytes. We quantitatively determine the impact of specific cations (, , , and ) in chloride electrolytes on the surface potential, the location of the shear plane, and the capacitance of the Stern layer. We find that the magnitude of the surface potential increases linearly with the hydrated-cation radius. Interpreting our data using the simplest assumptions and most straightforward understanding of Gouy-Chapman-Stern theory reveals a Stern layer whose thickness corresponds to a single layer of water molecules hydrating the silica surface, plus the radius of the hydrated cation. These results subject electrical double-layer theories to direct and falsifiable tests to reveal a physically intuitive and quantitatively verified picture of the Stern layer that is consistent across multiple electrolytes and solution conditions.
1 More- Received 31 July 2015
DOI:https://doi.org/10.1103/PhysRevX.6.011007
This article is available under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
Published by the American Physical Society
Popular Summary
A solid-liquid boundary is characterized by an electrical double layer in which charge builds up both on the solid surface and on the adjacent aqueous layer.
The structure of this electrical double layer mediates colloidal interactions, regulates surface structure, controls reactivity, sets capacitance, and represents the central element of electrochemical supercapacitors, but the local ion arrangement near the charged surface remains poorly understood. Traditionally, a layer of nonspecifically adsorbed ions has been invoked to rationalize the difference between the surface and electrokinetic potentials. However, the inability to directly measure the absolute surface potential of dispersed systems prohibits a quantitative determination of electrical double-layer structure. Here, we use x-ray photoelectron spectroscopy from a liquid microjet to measure, for the first time, the absolute surface potentials of silica nanoparticles dispersed in aqueous electrolytes.
We employ a liquid microjet that permits the passage of a colloidal solution. The sample is only exposed to x-ray radiation for approximately , so we anticipate that x-ray-induced damage is minimal. We quantitatively determine the impact of specific ion effects on the surface potential of silica nanoparticles. Using 420- and 840-eV synchrotron radiation, we ionize the sample, and we find that the magnitude of the surface potential increases linearly with the hydrated cation radius. We also determine that the thickness of the Stern layer—the charge-free region directly above the surface—corresponds to a single layer of water molecules hydrating the silica surface, plus the radius of the hydrated cation. Our findings are consistent with a simple hydration-modified Poisson-Boltzmann model.
We anticipate that our results will be applicable to a wide range of colloidal and quantum dot suspensions.