ENVR 60 |
| The self-assembled monolayer of octadecyltrichlorosilane (OTS) on fused silica has been used extensively as a representative hydrophobic surface in the study of water/hydrophobic interfaces. Recent sum-frequency vibrational spectroscopy (SFVS) found that the spectrum in the OH stretch region of the OTS/water interface increases significantly in strength as pH increases from 5.7 to 11, presumably due to surface charging effect, but how it changes the interfacial water structure is not clear. We have carried out a study on OTS/water interface using the newly developed phase-sensitive sum-frequency vibrational spectroscopy (PS-SFVS). It allows measurement of both real and imaginary parts of the surface spectral response with the latter playing a role equivalent to absorption and emission coefficients and provides information on net polar-orientations of various interfacial water species contributing to the different parts of the spectrum. The result shows that at low pH (pH~2), water molecules in both ice-like and liquid-like regions have weak net polar-orientations with H pointing towards the liquid. At high pH (pH~11), they are well aligned with H pointing to the solid substrate. It indicates that OTS/silica is negatively charged at high pH so that the surface field will reorient part of the interfacial water molecules. This seems to suggest that even though the silica surface is covered by the OTS monolayer, deprotonation of the silica surface could still be effective. Alternatively, one may consider that OH- ions could adsorb on the methyl terminals of OTS making the water/OTS interface negatively charged. This work was supported by a NSF grant through WaterCAMPWS. |
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Advances in Adsorption Processes for Drinking Water Treatment and Sourcewater Protection
8:30 AM-9:35 AM, Monday, April 7, 2008 Morial Convention Center -- Rm. 236, Oral
Division of Environmental Chemistry |