Water Structure in the Electrical Double Layer and the Contributions to the Total Interfacial Potential at Different Surface Charge Densities

Abstract

The electric double layer governs the processes of all charged surfaces in aqueous solutions; however, elucidating the structure of the water molecules is challenging for even the most advanced spectroscopic techniques. Here, we present the individual Stern layer and diffuse layer OH stretching spectra at the silica/water interface in the presence of NaCl over a wide pH range using a combination of vibrational sum frequency generation spectroscopy, heterodyned second harmonic generation, and streaming potential measurements. We find that the Stern layer water molecules and diffuse layer water molecules respond differently to pH changes: unlike the diffuse layer, whose water molecules remain net-oriented in one direction, water molecules in the Stern layer flip their net orientation as the solution pH is reduced from basic to acidic. We obtain an experimental estimate of the non-Gouy−Chapman (Stern) potential contribution to the total potential drop across the insulator/electrolyte interface and discuss it in the context of dipolar, quadrupolar, and higher order potential contributions that vary with the observed changes in the net orientation of water in the Stern layer. Our findings show that a purely Gouy−Chapman (Stern) view is insufficient to accurately describe the electrical double layer of aqueous interfaces.

A molecular dynamics study of the nonlinear spectra and structure of charged (101) quartz/water interfaces

“…When the paper was submitted for publication, a new article reporting on an experimental VSFG study of the amorphous This journal is © the Owner Societies 2022 silica/water interface has been published. 80 Phase-sensitive nonlinear spectra of the Stern and diffuse layers obtained in that study favorably compare with the spectra computed in the present work.…”

Influence of the Hydrogen-Bonding Environment on Vibrational Coupling in the Electrical Double Layer at the Silica/Aqueous Interface

“…Representative SSP vSFG intensity spectra corrected for local field effects from 2850 to 3750 cm –1 at the (a) silica/HOD and (b) silica/H 2 O interfaces as the pH was varied from 10 to 2 with under a 50 mM NaCl background electrolyte. Representative PPP vSFG intensity spectra without local field corrections at the (c) silica/HOD and (d) silica/H 2 O interfaces.…”

“…They found a remarkable similarity between the Stern layer spectra of HOD and H 2 O at pH 12 and high ionic strength, indicating that H 2 O molecules at the silica interface were vibrationally decoupled under these conditions. However, owing to the nature of these experiments, the influence of the ions and pH on the EDL could not be deconvoluted although both are known to strongly influence the structure of H 2 O in the EDL. ,,− Furthermore, as we have shown previously, the structure of H 2 O in the Stern layer changes significantly as the pH is lowered below 12 …”

“…By uncovering the diffuse layer complex spectrum, the imaginary component of the Stern layer complex spectrum could be predicted, which is primarily of interest when constructing a molecular description of the interface. In our previous work, we modified this approach by utilizing zeta (ζ) potentials from either streaming current or potential measurements in combination with the maximum entropy method to deconvolute the Stern and diffuse layer spectra as the ionic strength and pH were varied at the silica/H 2 O interface. , …”

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

“…A common theme in determining the vibrational spectroscopy of water is unraveling site-specific interactions in water clusters, namely in characterizing the free and bound O–H stretches. 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 .…”