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

Abstract

The second-order nonlinear susceptibility, c (2) , in the Stern layer, and the total interfacial potential drop, F(0)tot, across the oxide:water interface are estimated from SHG amplitude and phase measurements for divalent cations (Mg 2+ , Ca 2+ , Sr 2+ , Ba 2+ ) 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 χ (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 2for 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.

The Local Vibrational Mode Theory and Its Place in the Vibrational Spectroscopy Arena

“…Using linear spectroscopy, such as FTIR and Raman, can reveal essential information about hydrogen bond strengths and the local environment of water through the OH stretching mode frequency; however, these spectra often suffer from spectral congestion and band broadening. , Beyond purely vibrational spectroscopy, vibrational SFG has recently been used in visualizing the extent of interfacial water and probing water–surface interactions. ,, This technique improves upon pure IR measurements by distinct electronic excitations that suggest molecular locality and directionality in addition to characteristic vibrational modes. A special case of SFG is Second Harmonic Generation (SHG) spectra where the visible and IR excitation frequencies are identical; recent studies emphasizing second order nonlinear susceptibility measured in SHG have diminished some issues researchers have experienced in modeling more distinct features in charged interfaces and water–silica interfaces, − including acceptor–donor, acceptor–donor–donor (ADD), acceptor–acceptor–donor (AAD), as the main examples . The AAD class is characterized by a free OH group while the ADD class is composed of exclusively bound OH groups.…”

Revealing Silica's pH-Dependent Second Harmonic Generation Response with Overcharging

“…[11][12][13][14] Among the nonlinear optical techniques used to characterize the interface, second harmonic generation (SHG) has been influential in providing molecular-level information and electrical properties at the silica/water interface. 2,4,6,[12][13][14][15][16][17][18][19] The underlying theory of the sensitivity of nonresonant SHG to charged solid/aqueous interfaces comes from the seminal model proposed by Eisenthal and coworkers 20 which is commonly referred to as the 𝜒 (3) method. 13 This approach emerged from early SHG studies at the silica/water interface and postulated that the signal measured in these experiments was dominated by the orientation or polarization of water molecules responding to the presence of a static electric field originating from the charged silica surface.…”

“…Divalent ions such as Ca 2+ have been known to reduce the aqueous interfacial potential and cause overcharging in two main cases: (i) as the pH is increased with constant Ca 2+ concentration 48 or (ii) at neutral pH and sufficiently high Ca 2+ concentrations (>100 mM). 15,49,50 In our previous work, we utilized vSFG and zeta potential measurements to examine the former, i.e., the mechanism behind overcharging of the silica/water interface in the presence of 100 mM CaCl2 with increasing pH. 48 In Figure 2, we compare our previously reported zeta potentials and the square root of the integrated vSFG area to the corresponding SHG measurements of 100 mM CaCl2 over a similar pH range.…”

Sixty Years of Surface-Specific Spectroscopy

“…SHG has also been used to study ion adsorption, and how this process affects the electrical double layer at charged interfaces. For example, Ma and Geiger employed the SHG phase and amplitude measurements through heterodyne-detected SHG to examine how ionic charge affects the structure and surface potential at SiO 2 interfaces (Figure ), and how dynamics differ in different regions of the double layer . The way the surface potential can be extracted from SHG experiments was also discussed by Hore and co-workers .…”

“…(a) Heterodyne-detected SHG experiments to measure the surface potential of SiO 2 interfaces. Reprinted with permission from ref . Copyright 2021 American Chemical Society.…”