Determination of Surface Potential and Electrical Double-Layer Structure at the Aqueous Electrolyte-Nanoparticle Interface

PHYSICAL REVIEW X 6, 011007 (2016) Determination of Surface Potential and Electrical Double-Layer Structure at the Aqueous Electrolyte-Nanoparticle Interface Matthew A. Brown, 1,* Zareen Abbas, 2 Armin Kleibert, 3 Richard G. Green, 4 Alok Goel, 1 Sylvio May, 5 and Todd M. Squires 6 Laboratory for Surface Science and Technology, Department of Materials, ETH Zurich, Vladimir-Prelog-Weg 5, CH-8093 Zurich, Switzerland Department of Chemistry and Molecular Biology, University of Gothenburg, SE-41296 Gothenburg, Sweden Swiss Light Source, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland Measurement Science and Standards, National Research Council Canada, Ottawa, Ontario K1A 0R6, Canada Department of Physics, North Dakota State University, Fargo, North Dakota 58108-6050, USA Chemical Engineering Department, University of California Santa Barbara, Santa Barbara, California 93106-5080, USA (Received 31 July 2015; revised manuscript received 4 December 2015; published 28 January 2016) 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 (Li þ , Na þ , K þ , and Cs þ ) 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. DOI: 10.1103/PhysRevX.6.011007 Subject Areas: Materials Science, Physical Chemistry, Soft Matter I. INTRODUCTION The formation of excess positive or negative charge on solid surfaces upon contact with aqueous solutions gives rise to an electrical double layer (EDL) as ions in the adjacent electrolyte rearrange to screen the charge. The microscopic structure of this EDL is difficult to interrogate experimentally [1–5] and is extensively debated in the literature because of its fundamental and technological significance in controlling surface structure [6,7], regulat- ing interfacial reactivity [8] and colloid-colloid interactions [9], governing transport in microfluidics [10] and matthew.brown@mat.ethz.ch Published by the American Physical Society under the terms of the Creative Commons Attribution 3.0 License. Further distri- bution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. nanofluidics [11], and mitigating drug-carrier cell inter- actions [12]. The EDL is also at the heart of many processes of global interest [13], including capacitive deionization of ground water [14,15] and the harnessing of intermittent power sources (e.g., wind and solar) through large-scale electric double-layer capacitors [16]. An important property of the EDL that remains poorly understood is the surface potential (Φ 0 ). In dispersed nanoparticle (NP) systems, surface potentials are tradition- ally inferred from electrokinetic (zeta) potentials, which depend only on the charge in the diffuse part of the EDL [17], since the direct experimental determination of surface potentials is widely believed to be impossible [17–19]. In the absence of detailed experimental results, the burden of determining surface potentials for these systems falls to theory, which often depends on parameters that cannot be measured directly, but are instead chosen to fit measured data—a less than ideal situation. Given the importance of Published by the American Physical Society