通过二次谐波产生研究羧酸和酯基对铬（VI）与功能化二氧化硅/水界面结合的控制。

Al-Abadleh Hind A, Mifflin Amanda L, Bertin Paul A, Nguyen Sonbinh T, Geiger Franz M

Department of Chemistry and Center for Nanofabrication and Molecular Self-Assembly, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, USA.

J Phys Chem B. 2005 May 19;109(19):9691-702. doi: 10.1021/jp050782o.

Resonantly enhanced surface second harmonic generation (SHG) measurements carried out at pH 7 and room temperature were performed to study how surface-bound carboxylic acid and methyl ester functional groups control the interaction of chromate ions with fused silica/water interfaces. These functional groups were chosen because of their high abundance in humic and fulvic acids and related biopolymers commonly found in soils. They were anchored to the silica surface using organosilane chemistry to avoid competing complexation processes in the aqueous solution as well as competitive adsorption of the organic compounds and chromate. The SHG experiments were carried out at room temperature and pH 7 while using environmentally representative chromate concentrations ranging from 1 x10(-6) to 2 x 10(-4) M. Chromate is found to bind to the acid- and ester-functionalized silica/water interfaces in a reversible fashion. In contrast to the plain silica/water interface, chromate binding studies performed on the functionalized silica/water interfaces show S-shaped adsorption isotherms that can be modeled using the Frumkin-Fowler-Guggenheim (FFG) model. This model predicts a coverage-dependent binding constant of K(ads) x exp(gtheta). Values for g are found to be 3.2(2), 2.1(2), and 1.3(2) for the carboxylic acid-, the ester-, and the nonfunctionalized silica/water interfaces, respectively, and are consistent with stabilizing lateral adsorbate-adsorbate interactions among the Cr(VI) species adsorbed to the functionalized surfaces. The FFG model allows for the parametrization of the solid-liquid partition coefficient and chromate retardation factors in silica-rich soil particles whose surfaces contain organic adlayers rich in carboxylic acid and methyl ester groups. The straightforward model presented here predicts that chromate retardation increases by up to 200% when carboxylic acid functional groups are present at the silica/water interface. Increases up to 50% are predicted for methyl ester-containing organic adlayers, and the retardation factor remains effectively near unity for the plain silica/water interface (no siloxanes present).

在pH值为7和室温条件下进行了共振增强表面二次谐波产生（SHG）测量，以研究表面结合的羧酸和甲酯官能团如何控制铬酸根离子与熔融石英/水界面的相互作用。选择这些官能团是因为它们在腐殖酸、富里酸以及土壤中常见的相关生物聚合物中含量很高。利用有机硅烷化学方法将它们锚定在二氧化硅表面，以避免水溶液中竞争性络合过程以及有机化合物和铬酸盐的竞争性吸附。SHG实验在室温及pH值为7的条件下进行，使用的铬酸盐浓度范围为1×10⁻⁶至2×10⁻⁴M，具有环境代表性。发现铬酸根以可逆方式结合到酸官能化和酯官能化的二氧化硅/水界面。与普通二氧化硅/水界面相比，在官能化二氧化硅/水界面上进行的铬酸根结合研究显示出S形吸附等温线，可用弗鲁姆金-福勒-古根海姆（FFG）模型进行模拟。该模型预测吸附常数为K(ads)×exp(gθ)，其值取决于覆盖度。对于羧酸官能化、酯官能化和未官能化的二氧化硅/水界面，g值分别为3.2(2)、2.1(2)和1.3(2)，这与吸附在官能化表面的Cr(VI)物种之间稳定的横向吸附质-吸附质相互作用一致。FFG模型允许对富含二氧化硅的土壤颗粒中的固液分配系数和铬酸根阻滞因子进行参数化，这些土壤颗粒表面含有富含羧酸和甲酯基团的有机吸附层。这里提出的简单模型预测，当二氧化硅/水界面存在羧酸官能团时，铬酸根阻滞作用最多可增加200%。对于含甲酯的有机吸附层，预测增加量高达50%，而对于普通二氧化硅/水界面（不存在硅氧烷），阻滞因子实际上仍接近1。