Divalent Ion-Specific Outcomes on Stern Layer Structure and Total Surface Potential at the Silica:Water Interface

The second-order nonlinear susceptibility, chi(2), in the Stern layer, and the total interfacial potential drop, Phi(0)tot, across the oxide:water interface are estimated from SHG amplitude and phase measurements for divalent cations (Mg2+, Ca2+, Sr2+, Ba2+) at the silica:water interface at pH 5.8 and various ionic strengths. We find that interfacial structure and total potential depend strongly on ion valency. We observe statistically significant differences between the experimentally determined chi(2) value for NaCl and that of the alkali earth series, but smaller differences between ions of the same valency in that series. These differences are particularly pronounced at intermediate salt concentrations, which we attribute to the influence of hydration structure in the Stern layer. Furthermore, we corroborate the differences by examining the effects of anion substitution (SO4 2- for Cl-). Finally, we identify that hysteresis in measuring the reversibility of ion adsorption and desorption at fused silica in forward and reverse titrations manifests itself both in Stern layer structure and in total interfacial potential for some of the salts, most notable CaCl2 and MgSO4, but less so for BaCl2 and NaCl.

Solvation-Induced Onsager Reaction Field Rather than Double Layer Field Controls CO2 Reduction on Gold

<div><p>The selectivity and activity of the carbon dioxide reduction reaction (CO2R) are sensitive functions of the electrolyte cation. By measuring the vibrational Stark shift of in-situ generated CO on Au in the presence of alkali cations, we quantify the total electric field present during turnover and deconvolute this field into contributions from 1) the electrochemical Stern layer and 2) the Onsager, or solvation-induced, reaction field. The magnitude of the Onsager field is shown to be on the same order as the Stern layer field (∼10 MV/cm) but follows an opposite trend with cation, increasing from Li⁺< Na⁺< K⁺< Rb⁺≈Cs⁺. Contrary to theoretical reports,CO₂R kinetics are not correlated with the Stern field but instead are controlled by the strength of the Onsager reaction field with Cs⁺ as an exception. Spectra of interfacial water as a function of cation show that Cs⁺ induces a change in the interfacial water structure correlated with a dramatic drop in CO₂R activity, highlighting the importance of cation-dependent interfacial water structure on reaction kinetics. These findings show that both the Onsager reaction field and interfacial solvation structure must be explicitly considered for accurate modeling of CO₂R reaction kinetics.</p><br></div>

Solvation-Induced Onsager Reaction Field Rather than Double Layer Field Controls CO2 Reduction on Gold

<div><p>The selectivity and activity of the carbon dioxide reduction reaction (CO2R) are sensitive functions of the electrolyte cation. By measuring the vibrational Stark shift of in-situ generated CO on Au in the presence of alkali cations, we quantify the total electric field present during turnover and deconvolute this field into contributions from 1) the electrochemical Stern layer and 2) the Onsager, or solvation-induced, reaction field. The magnitude of the Onsager field is shown to be on the same order as the Stern layer field (∼10 MV/cm) but follows an opposite trend with cation, increasing from Li⁺< Na⁺< K⁺< Rb⁺≈Cs⁺. Contrary to theoretical reports,CO₂R kinetics are not correlated with the Stern field but instead are controlled by the strength of the Onsager reaction field with Cs⁺ as an exception. Spectra of interfacial water as a function of cation show that Cs⁺ induces a change in the interfacial water structure correlated with a dramatic drop in CO₂R activity, highlighting the importance of cation-dependent interfacial water structure on reaction kinetics. These findings show that both the Onsager reaction field and interfacial solvation structure must be explicitly considered for accurate modeling of CO₂R reaction kinetics.</p><br></div>

Solvation-Induced Onsager Reaction Field Rather than Double Layer Field Controls CO2 Reduction on Gold

<div><p>The selectivity and activity of the carbon dioxide reduction reaction (CO2R) are sensitive functions of the electrolyte cation. By measuring the vibrational Stark shift of in-situ generated CO on Au in the presence of alkali cations, we quantify the total electric field present during turnover and deconvolute this field into contributions from 1) the electrochemical Stern layer and 2) the Onsager, or solvation-induced, reaction field. The magnitude of the Onsager field is shown to be on the same order as the Stern layer field (∼10 MV/cm) but follows an opposite trend with cation, increasing from Li⁺< Na⁺< K⁺< Rb⁺≈Cs⁺. Contrary to theoretical reports,CO₂R kinetics are not correlated with the Stern field but instead are controlled by the strength of the Onsager reaction field with Cs⁺ as an exception. Spectra of interfacial water as a function of cation show that Cs⁺ induces a change in the interfacial water structure correlated with a dramatic drop in CO₂R activity, highlighting the importance of cation-dependent interfacial water structure on reaction kinetics. These findings show that both the Onsager reaction field and interfacial solvation structure must be explicitly considered for accurate modeling of CO₂R reaction kinetics.</p><br></div>

A dynamic Stern layer model to explain high salinity zeta potential measurements on sandstones

&lt;p&gt;Exploring the electrical properties of the mineral-water interface for interpreting geophysical electrical measurements is a very challenging work because of the low specific surface area of minerals such as quartz or calcite. Only few methods exist to probe the properties of the electrical double layer (EDL) compensating the surface charge of minerals. Among them, there is the streaming potential (SP) method where the applied water pressure difference generates a pore water flow displacing the mobile counter-ions in excess in the EDL, hence creating a measurable electrical potential difference, the streaming potential. During SP measurements, the exact position of the shear plane from the mineral surface is not known and it is widely accepted that the shear plane is located between the compact Stern layer and the diffuse layer. In our study, we show that the assumption that there is no water flow in the Stern layer has no physical basis for sandstones in contact with a NaCl electrolyte because water molecules around counter-ions in the Stern layer may have bulk-like properties. Using a basic Stern model to simulate surface complexation reactions and considering water flow in the Stern layer, we reproduced the zeta potential measurements on sandstones over a large salinity range from about 10&lt;sup&gt;-2&lt;/sup&gt; to 5.5 M NaCl. The &amp;#8220;anomalous&amp;#8221; high salinity zeta potential data can not be reproduced by a surface complexation model considering water flow only in the diffuse layer. Our ability to explain these measurements suggests that the shear plane may be located between the mineral surface and the Stern layer, i.e. closer to the surface than previously thought, which may have strong implications for the modelling of the surface electrical properties of the minerals.&lt;/p&gt;