Rehl Benjamin, Gibbs Julianne M
Department of Chemistry, University of Alberta, Edmonton, Alberta T6G 2G2, Canada.
J Phys Chem Lett. 2021 Mar 25;12(11):2854-2864. doi: 10.1021/acs.jpclett.0c03565. Epub 2021 Mar 15.
Isolating the hydrogen-bonding structure of water immediately at the surface is challenging, even with surface-specific techniques like sum-frequency generation (SFG), because of the presence of aligned water further away in the diffuse layer. Here, we combine zeta potential and SFG intensity measurements with the maximum entropy method referenced to reported phase-sensitive SFG and second-harmonic generation results to deconvolute the SFG spectral contributions of the surface waters from those in the diffuse layer. 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 in different hydrogen-bonding environments oriented in opposite directions. This similarity suggests that the known metastability of silica colloids against aggregation under both conditions could arise from this distinct surface water structure. Upon the addition of salt, significant restructuring of water is observed, leading to a net decrease in order at the surface.
即便采用像和频产生光谱（SFG）这样的表面特异性技术，要直接分离出水面处水的氢键结构仍具有挑战性，这是因为在扩散层中存在排列整齐的更远距离的水。在此，我们将zeta电位和SFG强度测量与最大熵方法相结合，并参考已报道的相敏SFG和二次谐波产生结果，以反卷积出扩散层中水体对表面水体SFG光谱贡献的影响。反卷积结果表明，在极低离子强度下，表面水结构类似于零电荷点附近的中性二氧化硅表面，不同氢键环境中的水呈相反方向排列。这种相似性表明，在这两种条件下二氧化硅胶体已知的抗聚集亚稳性可能源于这种独特的表面水结构。加入盐后，观察到水发生了显著的结构重组，导致表面有序度净下降。
