Evidence for Entropically Controlled Interfacial Hydration in Mesoporous Organosilicas
Evidence for Entropically Controlled Interfacial Hydration in Mesoporous Organosilicas
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DOI：
10.1021/jacs.1c11342
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发表时间：
2022-01-18
影响因子：
15
通讯作者：
Scott, Susannah L.
中科院分区：
文献类型：
--
作者：
Moon, Hyunjin;Collanton, Ryan P.;Scott, Susannah L.
在水界面上，吸附物的分布和动态受界面水行为的影响。疏水表面的水化可以通过界面水的有序化来存储熵，从而贡献了溶质结合的吉布斯能。然而，几乎没有实验证据证明这种熵储库的存在，而且几乎没有在包含扩展界面的系统中对其进行合理设计的先例。在本研究中，对两个系列的介孔二氧化硅进行了不同的改性：(1)通过表面硅醇的缩合，逐步加深了热脱羟基；(2)增加了非极性有机连接物在二氧化硅骨架中的掺入。这两种方法都会降低表面的平均极性，表现为吸附染料的荧光发生蓝移。对于无机二氧化硅，随着表面硅醇数目的减少，水中的氢键变得不那么广泛。Overhauser动态核极化(ODNP)弛豫测量表明，地表水的扩散率增强，反映了焓水合作用的丧失。相比之下，有机硅的地表水扩散率随着极性的降低而单调下降，反映出疏水水化作用的增强。分子动力学模拟预测有机硅界面水的四面体程度增加，这意味着纳米级有机结构域附近的有序性增加(相对于无机硅，无机硅必然没有这样的结构域)。这些发现证实了界面上的疏水水化受疏水区域微观长度尺度控制的预测。他们进一步表明，结构不均匀的二氧化硅表面的水合热力学可以被调节以促进吸附，这反过来又调节催化反应中的选择性。
At aqueous interfaces, the distribution and dynamics of adsorbates are modulated by the behavior of interfacial water. Hydration of a hydrophobic surface can store entropy via the ordering of interfacial water, which contributes to the Gibbs energy of solute binding. However, there is little experimental evidence for the existence of such entropic reservoirs, and virtually no precedent for their rational design in systems involving extended interfaces. In this study, two series of mesoporous silicas were modified in distinct ways: (1) progressively deeper thermal dehydroxylation, via condensation of surface silanols, and (2) increasing incorporation of nonpolar organic linkers into the silica framework. Both approaches result in decreasing average surface polarity, manifested in a blue-shift in the fluorescence of an adsorbed dye. For the inorganic silicas, hydrogen-bonding of water becomes less extensive as the number of surface silanols decreases. Overhauser dynamic nuclear polarization (ODNP) relaxometry indicates enhanced surface water diffusivity, reflecting a loss of enthalpic hydration. In contrast, organosilicas show a monotonic decrease in surface water diffusivity with decreasing polarity, reflecting enhanced hydrophobic hydration. Molecular dynamics simulations predict increased tetrahedrality of interfacial water for the organosilicas, implying increased ordering near the nm-size organic domains (relative to inorganic silicas, which necessarily lack such domains). These findings validate the prediction that hydrophobic hydration at interfaces is controlled by the microscopic length scale of the hydrophobic regions. They further suggest that the hydration thermodynamics of structurally heterogeneous silica surfaces can be tuned to promote adsorption, which in turn tunes the selectivity in catalytic reactions.