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

29 April 2022, Version 2
This content is a preprint and has not undergone peer review at the time of posting.

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 and heterodyned second harmonic generation techniques 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 cross-term potential contributions. We quantify how these contributions result in a considerable influence on the vibrational lineshapes. Our findings show that a purely Gouy-Chapman (Stern) view is insufficient to accurately describe the electrical double layer of aqueous interfaces.

Keywords

sum frequency generation
second harmonic generation
silica
water
ions
pH
nonlinear optics
ultrafast laser
zeta potential
streaming potential
surface potential
outer helmholtz plane
coulombic
dipolar
dipole potential
maximum entropy method
MEM
Stern
stern layer
diffuse layer
electrical double layer
EDL
gouy chapman
second order nonlinear susceptibility
third order nonlinear susceptibility
surface charge density
electrokinetic
surface
interface
heterodyned
homodyned
SFG
SHG

Supplementary materials

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Title
Supporting Information: Water Structure in the Electrical Double Layer and the Contributions to the Total Interfacial Potential at Different Surface Charge Densities
Description
The supporting information contains: experimental details, calculations for local field effects in SFG spectra, in-depth details of error phase prediction using the MEM, and electrokinetic charge densities calculated from zeta potentials.
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