Variation of Ionic Strength Reveals the Interfacial Water Structure at a Charged Mineral Surface

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• 作者：Kailash C. Jena ; Dennis K. Hore
• 刊名：Journal of Physical Chemistry C
• 出版年：2009
• 出版时间：August 27, 2009
• 年：2009
• 卷：113
• 期：34
• 页码：15364-15372
• 全文大小：485K
• 年卷期：v.113,no.34(August 27, 2009)
• ISSN：1932-7455

文摘

We have used visible-infrared sum-frequency generation spectroscopy to study the structure of water molecules that form the interface with fused silica. Spectra throughout the OH-stretching region were collected in two different polarization schemes so that the orientation of interfacial water molecules could be studied. A depth profiling experiment was performed by varying the ionic strength, thereby peeling away the outskirts of this interfacial region, leaving water molecules progressively closer to the charged interface. As the ionic strength increases, the spectral intensity drops since the interfacial region is becoming thinner. However, we notice a change in the relative contribution of tetrahedrally coordinated molecules compared to those that make less hydrogen bonds. We have the opportunity to study these two water species individually; we conclude that the higher coordination species exists some distance away from the interface and is oriented close to the plane of the surface with a mean tilt angle 70° and a narrow tilt and twist distribution. We have evidence that lower coordination water molecules are close to the interface and offer two possibilities for their orientation that are consistent with our data. One has a twist angle distribution centered in the plane of the interface, tilted 35° from the normal. The other possibility is that they are twisted out of the plane of the interface and tilted 30° from the normal. As the electrolyte concentration increases, we observe a reduction of the mean tilt angle in all cases. This is consistent with a picture of molecules nearer to the surface better aligned in the presence of a stronger interfacial field.