The behavior of hydroxide and hydronium ions at the hexadecane–water interface studied with second harmonic generation and zeta potential measurements

Gan, Wei; Wu, Wei; Yang, Fangyuan; Da, Hu; Fang, Hui; Lan, Zhenggang; Yuan, Qunhui

doi:10.1039/c7sm00813a

“…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 𝜒 𝑆 (2) , and diffuse layer water, or 𝜒 (3) 𝑔 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 𝜒 𝑆 (2) and 𝜒 (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.…”

Section: Discussionmentioning

confidence: 99%

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

Rehl¹,

Gibbs

²

2020

Preprint

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We explore the influence of salt addition on the structure of water interacting closely with a charged silica surface. Isolating these surface effects is challenging, even with surface-specific techniques like sum frequency generation (SFG), because of the presence of aligned water nanometers to microns away from the charged silica. Here we combine zeta potential and SFG intensity measurements with the maximum entropy method and reported heterodyne second harmonic and sum frequency generation results to deconvolute from the total signal intensity the SFG spectral contributions of the waters adjacent to the surface. Deconvolution reveals that at very low ionic strength the surface water structure is similar to that of a neutral silica surface near the point of zero charge with waters oriented in opposite directions. This result suggests the known metastability of silica near the PZC and the stability of silica in low ionic strength solutions may originate from the same source, these oppositely oriented surface-bound waters. Orientation changes are induced upon adding salt, which lead to a decrease in the total amount of aligned water at the surface.

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“…Tian and Shen using phase-sensitive sum frequency vibrational spectroscopy showed that the OH – has the highest adsorption energy compared to those of H 3 O + and Cl – at a water–silane interface . A study with second harmonic generation spectroscopy showed that hydroxide ions exhibit affinity for the water–oil interface . Wu et al used second harmonic generation spectroscopy to report that the water–oil interface is negatively charged and described the role of the interfacial electric field in particle adsorption .…”

mentioning

confidence: 99%

Strong Electric Field Observed at the Interface of Aqueous Microdroplets

Xiong

¹

,

Lee

²

,

Zare

³

et al. 2020

J. Phys. Chem. Lett.

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Chemical reactions in aqueous microdroplets often exhibit unusual kinetic and thermodynamic properties not observed in bulk solution. While an electric field has been implicated at the water interface, there has been no direct measurement in aqueous microdroplets, largely due to the lack of proper measurement tools. Herein, we employ newly developed stimulated Raman excited fluorescence microscopy to measure the electric field at the water−oil interface of microdroplets. As determined by the vibrational Stark effect of a nitrile-bearing fluorescent probe, the strength of the electric field is found to be on the order of 10 7 V/cm. This strong electric field aligns probe dipoles with respect to the interface. The formation of the electric field likely arises from charge separation caused by the adsorption of negative ions at the water−oil interface of microdroplets. We suggest that this strong electric field might account in part for the unique properties of chemical reactions reported in microdroplets.

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“…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.…”

Section: Discussionmentioning

confidence: 99%

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

Rehl¹,

Gibbs²

2020

Preprint

1

0

View full text Add to dashboard Cite

We explore the influence of salt addition on the structure of water interacting closely with a charged silica surface. Isolating these surface effects is challenging, even with surface-specific techniques like sum frequency generation (SFG), because of the presence of aligned water nanometers to microns away from the charged silica. Here we combine zeta potential and SFG intensity measurements with the maximum entropy method and reported heterodyne second harmonic and sum frequency generation results to deconvolute from the total signal intensity the SFG spectral contributions of the waters adjacent to the surface. Deconvolution reveals that at very low ionic strength the surface water structure is similar to that of a neutral silica surface near the point of zero charge with waters oriented in opposite directions. This result suggests the known metastability of silica near the PZC and the stability of silica in low ionic strength solutions may originate from the same source, these oppositely oriented surface-bound waters. Orientation changes are induced upon adding salt, which lead to a decrease in the total amount of aligned water at the surface.

show abstract

The behavior of hydroxide and hydronium ions at the hexadecane–water interface studied with second harmonic generation and zeta potential measurements

Cited by 30 publications

References 64 publications

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

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

Strong Electric Field Observed at the Interface of Aqueous Microdroplets

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

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