A set-up for simultaneous measurement of second harmonic generation and streaming potential and some test applications

Abstract

We present a measurement cell that allows simultaneous measurement of second harmonic generation (SHG) and streaming potential (SP) at mineral-water interfaces with flat specimen that are suitable for non-linear optical (NLO) studies. The set-up directly yields SHG data for the interface of interest and can also be used to obtain information concerning the influence of flow on NLO signals from that interface. The streaming potential is at present measured against a reference substrate (PTFE). The properties of this inert reference can be independently determined for the same conditions. With the new cell, for the first time the SHG signal and the SP for flat surfaces have been simultaneously measured on the same surface. This can in turn be used to unambiguously relate the two observations for identical solution composition. The SHG test of the cell with a fluorite sample confirmed previously observed differences in NLO signal under flow vs. no flow conditions in sum frequency generation (SFG) investigations. As a second test surface, an inert ("hydrophobic") OTS covered sapphire-c electrolyte interface was studied to verify the zeta-potential measurements with the new cell. For this system we obtained combined zeta-potential/SHG data in the vicinity of the point of zero charge, which were found to be proportional to each other as expected. Furthermore, on the accessible time scales of the SHG measurements no effects of flow, flow velocity and stopped flow occurred on the interfacial water structure. This insensitivity to flow for the inert surface was corroborated by concomitant molecular dynamics simulations. Finally, the set-up was used for simultaneous measurements of the two properties as a function of pH in automated titrations with an oxidic surface. Different polarization combinations obtained in two separate titrations, yielded clearly different SHG data, while under identical conditions zeta-potentials were exactly reproduced. The polarization combination that is characteristic for dipoles perpendicular to the surface scaled with the zeta-potentials over the pH-range studied, while the other did not. The work provides an advanced approach for investigating liquid/surface interactions which play a major role in our environment. The set-up can be upgraded for SFG studies, which will allow more detailed studies on the chemistry and the water structure at a given interface, but also the combined study of specific adsorption including kinetics in combination with electrokinetics. Such investigations are crucial for the basic understanding of many environmental processes from aquatic to atmospheric systems.

Probing the Mineral–Water Interface with Nonlinear Optical Spectroscopy

“…These results were interpreted as a flow‐induced modification of the surface charge, which, compared to static conditions, requires a change in the pH value of up to 2 units. Similar flow‐dependent changes at low pH values are observed with SHG in p‐in/p‐out and p‐ and s‐in/s‐out polarization [58] …”

Liquid flow reversibly creates a macroscopic surface charge gradient

“…A flow-dependent surface charge not only paves the way for novel electrokinetic effects 14 , but it also has important consequences for the interpretation of zeta potential measurements. However, the mechanism by which the flow alters the surface charge is poorly understood as of yet 15 .…”

The Role of Ions on the Surface-Bound Water Structure at the Silica/Water Interface: Identifying the Spectral Signature of Stability

“…In summary, we determined the effect of ionic strength on the SFG spectra of the surfacebound waters at the silica/water interface through deconvolution of the spectra into surface-bound water, or (2) , and diffuse layer water, or (3) 3 , components. In lieu of a calculated surface potential for the analysis, we obtained the zeta potential from streaming current measurements, which has been related to both SFG [77][78] and SHG, [79][80][81] but not used to deconvolute the (2) and (3) contributions. With the reported phase-sensitive SFG 32 and SHG 31 measurements of the silica/water interface as reference, we applied the maximum entropy method to our measured SFG intensities to obtain the complex SFG spectra.…”