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

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 (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.

Quantifying Specific Ion Effects on the Surface Potential and Charge Density at Silica Nanoparticle–Aqueous Electrolyte Interfaces

“…63 It would be interesting to ascribe the differences in the interpretations of SHG experiments with those of the present potentiometric titrations, ATR-FTIR, XPS and the zeta-potential 18 and AFM 62 measurements of others to the radius of curvature of the NP, however, this remains to be confirmed. 17 Our XPS measurements reveal pronounced specific cation effects on the nanoparticles relative surface potentials, in agreement with our earlier results 60 and consistent with zeta-potential measurements available in the literature. 17 These effects are rationalized by the increasing hydrated-ion size going from ‫ݏܥ‬ ௬ௗ ା to ‫݅ܮ‬ ௬ௗ ା .…”

“…Cesium has been shown to approach the surface more closely than lithium, which results in a higher capacitance of the Stern layer and lower magnitude negative surface potential (Figure 7b). 60 In addition, the closer approach of Cs + offers enhanced screening of the charged site and a decrease in the Coulomb potential between deprotonated surface sites, 12, 51 which allows for more deprotonation for the same energy penalty (Figure 7a). Change in surface potential of 5 wt.% colloidal silica in different chloride electrolytes relative to that in NaCl as determined by the Si 2p binding energy (see Equation (2)).…”

“…Here, simply for comparison, we report the surface charge density and the relative surface potential in 50 mM LiCl, NaCl, KCl and CsCl. 60 The point of this comparison is to show that the analytical techniques employed herein are, in fact, sensitive to changes in surface charge density and relative surface potential, if they exist.…”

Field-effect sensors – from pH sensing to biosensing: sensitivity enhancement using streptavidin–biotin as a model system

“…30,35,70,119,180 Assumption (b) has been shown to be approximately the case for pH sensing and assuming a consistent oxide material between devices. 181,182 As a result of this, pH sensing FET Sensitivity can be predicted via eqn (9) prior to pH sensing experiments simply by measuring the subthreshold slope and using knowledge of the oxide material. In this work we perform a similar analysis incorporating streptavidin-sensing data in order to understand whether a similar trend can be identified, and as a method of graphically rationalising differences in Sensitivity between biosensing experiments.…”

Surface-specific vibrational spectroscopy of the water/silica interface: screening and interference

“…While eqn (4) discriminates between ions species with different valence, the effect of different ionic radii is ignored. However, ion size not only determines the Stern-layer thickness, 5 but might also be relevant for the screening ability since the ion mobility changes with the hydrated radius. For a negatively charged surface, the properties of the cations are especially important.…”