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Optical imaging of surface chemistry and dynamics in confinement

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Science  25 Aug 2017:
Vol. 357, Issue 6353, pp. 784-788
DOI: 10.1126/science.aal4346

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Imaging surfaces with water

The surfaces of real materials are often highly chemically heterogeneous, and the reported values of even simple properties such as surface acidity can vary widely in many cases. Macias-Romero et al. developed a microscope that images surfaces on the basis of second-harmonic generation from the orientation of interfacial water (see the Perspective by Hunger and Parekh). They followed the deprotonation of silica along glass micropipettes by changing solution pH and found many regions where the surface acidity deviated strongly from the average for the entire micropipette.

Science, this issue p. 784; see also p. 755

Abstract

We imaged the interfacial structure and dynamics of water in a microscopically confined geometry, in three dimensions and on millisecond time scales, with a structurally illuminated wide-field second harmonic microscope. The second harmonic images reported on the orientational order of interfacial water, induced by charge-dipole interactions between water molecules and surface charges. The images were converted into surface potential maps. Spatially resolved surface acid dissociation constant (pKa,s) values were determined for the silica deprotonation reaction by following pH-induced chemical changes on the curved and confined surfaces of a glass microcapillary immersed in aqueous solutions. These values ranged from 2.3 to 10.7 along the wall of a single capillary because of surface heterogeneities. Water molecules that rotate along an oscillating external electric field were also imaged.

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