目的:重金属离子如Pb(II)吸附到带负电荷的矿物如二氧化硅上有望改变二氧化硅/水界面的结构和相互作用。除了溶液的pH值,Pb(II)的内球吸附有望调节表面电荷/电势,假设控制一价阴离子在水环境中的作用。这些复杂的图片可以直接使用表面敏感的和频生成(SFG)光谱探测。
方法:使用SFG检查了在不同离子的影响下二氧化硅/水界面处双层内的pH依赖性水结构。使用最大熵方法结合双电层理论,将记录的SFG光谱解卷积为Stern层(SL)和扩散层(DL)。
结果:独立的一价钠盐对二氧化硅/水界面不表现出离子特异性作用。然而,Pb(II)物种和每种盐的混合物对二氧化硅/水界面的结构表现出深远的离子特异性影响,表明Pb(II)作为所研究的单价阴离子的离子特异性的推动者的作用。有趣的效果来自物理过程之间复杂的相互作用(即,静电相互作用,筛选效果,等。)和化学过程,如Pb(II)离子的水解,离子络合,表面硅烷醇基团的质子化和去质子化。
OBJECTIVE: The adsorption of heavy metal ions such as Pb(II) onto negatively charged minerals such as silica is expected to alter the structure and the interactions at the silica/aqueous interfaces. Besides the solution pH, the inner-sphere sorption of Pb(II) is expected to regulate the surface charge/potential, hypothesized to control the actions of monovalent anions in the aqueous environment. These complex pictures can be probed directly using surface-sensitive sum-frequency generation (SFG) spectroscopy.
METHODS: The pH-dependent water structure within the double layer at silica/aqueous interfaces under the influence of different ions was examined using SFG. The recorded SFG spectra were deconvoluted into the Stern layer (SL) and diffuse layer (DL) using the maximum entropy method in conjunction with the electrical double-layer theory.
RESULTS: Standalone monovalent sodium salts do not exhibit ion-specific effects on the silica/aqueous interfaces. However, the mixture of Pb(II) species and each of these salts display profound ion-specific effects on the structure of silica/aqueous interfaces, indicating the role of Pb(II) as an enabler of the ion-specificity of the investigated monovalent anions. The interesting effect arises from a complex interplay between the physical processes (i.e., electrostatic interactions, screening effects, etc.) and chemical processes such as the hydrolysis of Pb(II) ions, ion complexation, protonation and deprotonation of the surface silanol group.