Charge-Induced χ<sup>(3)</sup> Susceptibility in Interfacial Nonlinear Optical Spectroscopy Beyond the Bulk Aqueous Contributions: The Case for Silica/Water Interface

Ma²,

et al. 2022

Preprint

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 higher order 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.

Section: 𝑑𝑧mentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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

Ma²,

et al. 2022

Preprint

“…Moreover, the nonresonant SHG signal contains contributions from all polarizable species including interfacial silanol groups, water, the ions, and protonated and deprotonated surface sites, whereas our vibrational SFG measurements only probe OH oscillators. 61,74,91 Nevertheless, the HD-SHG measurements allow us to predict how the error phase changes between our two reference points (i.e. pH 6 and pH 2) (Fig.…”

Section: 𝑑𝑧mentioning

confidence: 99%

“…Specifically, the wave vector mismatch for the reflection geometries commonly used in most SFG setups is on the order of 10 2 nm -1 , leading to third-order (bulk allowed) contributions that add to the second-order (interface-specific) SFG response depending on the interfacial potential. [55][56][57][58][59][60][61] While it has been known for a while how these second-and third-order contributions are encoded in the total detected SFG signal, [62][63] the proper lineshape analysis requires knowledge of the phase relationship between these two terms and the total interfacial potential, which has remained elusive. To overcome this problem, we now combine vibrational sum frequency, nonresonant heterodyne-detected second harmonic generation (HD-SHG), and streaming potential measurements to obtain the interface-specific response for the fused silica/water interface as a function of pH and a total ionic strength of 50 mM.…”

Section: Introductionmentioning

confidence: 99%

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

Ma²,

et al. 2022

Preprint

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 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.

“…Specifically, the wave vector mismatch for the reflection geometries commonly used in most SFG setups is on the order of 10 2 nm -1 , leading to third-order (bulk allowed) contributions that add to the second-order (interface-specific) SFG response depending on the interfacial potential. [49][50][51][52][53][54][55] While it has been known for a while how these second-and third-order contributions are encoded in the total detected SFG signal, [56][57] the proper lineshape analysis requires knowledge of the phase relationship between these two terms and the total interfacial potential, which has remained elusive. To overcome this problem, we now combine vibrational sum frequency, nonresonant heterodyne-detected second harmonic generation (HD-SHG), and streaming potential measurements to obtain the interface-specific response for the fused silica/water interface as a function of pH and a total ionic strength of 50 mM.…”

Section: Introductionmentioning

confidence: 99%

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

Ma²,

et al. 2022

Preprint

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.