Séminaire

Mardi 7 Juin 2022 à 11h00.

Aqueous Interfaces studied by Nonlinear Spectroscopies

''Water/air interface is one of the most significant platforms for many atmospheric and chemical processes in nature, and the reaction kinetics therein is unavoidably dictated by the surface properties such as surface pH or surface charge.
The interaction between water and silicon dioxide (SiO2) plays a major role in a large range of geochemical, environmental, pharmaceutical, biological and catalytical processes since silicon dioxide is the most abundant mineral on the continental Earth crust. Four kinds of silanols have been identified and studied by IR and NMR. The relative amounts of the different types of silanols have been estimated as the surface dehydroxylates with increasing temperature. We used vibrational sum frequency generation, an intrinsic surface specific technique, to study the interfacial water in close or direct proximity to the silica surface for two different pretreatments: plasma cleaning and heat treatment. In this study, we observed, identified and studied in details, for the first time, the isolated silanol at the silica/water interface1. The isolated silanol is contact with water is characterized by a vibrational, relatively sharp, band around 3680 cm-1.
In the past two decades, many researches indicated that hydronium ions exhibit manifest surface propensity2-3, which results in a surface pH value different from the bulk solution. In this presentation, I shall report our recent development of phase-sensitive sum-frequency spectroscopy (PS-SFS) technique4-5 and its application to quantitative analysis of hydronium adsorption to the water/air surface. The PS-SFS technique can now provide vibrational spectra of the Stern layer and the diffuse layer separately, so that structural and charge information of the water surface can be derived at the microscopic level. The results turn out quantitative Gibbs adsorption energy for the hydronium to the water surface for the first time6, offering a basis for comparison with nowadays theoretical studies7.
In a third step, I will explain the newly developed phase-sensitive optical second harmonic generation (PS-SHG) method8 that allows direct determination of the absolute surface charge density and surface potential without need of any interfacial information.

References:
[1] L. Dalstein, E. Potapova and E. Tyrode, PCCP, 2017
[2] Buch, V.; Milet, A.; Vácha, R.; Jungwirth, P.; Devlin, J. P., 2007, Proceedings of the National Academy of Sciences, 104, 7342-7347.
[3] Mucha, M.; Frigato, T.; Levering, L. M.; Allen, H. C.; Tobias, D. J.; Dang, L. X.; Jungwirth, P., 2005, The Journal of Physical Chemistry B, 109, 7617-7623.
[4] Wen, Y.-C.; Zha, S.; Tian, C.; Shen, Y. R., 2016. The Journal of Physical Chemistry C, 120, 15224-15229.
[5] Wen, Y.-C.; Zha, S.; Liu, X.; Yang, S.; Guo, P.; Shi, G.; Fang, H.; Shen, Y. R.; Tian, C., 2016,. Physical Review Letters, 116, 016101.
[6]Chiang, K.-Y.; Dalstein, L.; Wen, Y.-C., 2020, The Journal of Physical Chemistry Letters, 11,3,696-701.
[7] Pezzotti, S.; Gaigeot, M.-P.,2018,. Atmosphere, 9, 396.
[8] Dalstein, L.; Chiang, K.-Y.; Wen, Y.-C., 2019, The Journal of Physical Chemistry Letters, 10,17,5200-5205.
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