Kinetics of amorphous silica dissolution and the paradox of the silica polymorphs
Kinetics of amorphous silica dissolution and the paradox of the silica polymorphs
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DOI：
10.1073/pnas.0803798105
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发表时间：
2008-07-22
影响因子：
11.1
通讯作者：
De Yoreo, James J.
中科院分区：
文献类型：
--
作者：
Dove, Patricia M.;Han, Nizhou;De Yoreo, James J.
非晶态二氧化硅的溶解机制已被证明是难以捉摸的，因为非晶态材料缺乏结构有序性，使得它们可以通过应用于晶体的经典的阶梯、壁架、扭结模型来研究。这似乎意味着非晶相的表面在原子尺度上是无序的，因此SiO4四面体向溶液的转移总是保持固体的表面自由能不变。因此，非晶相的溶解速度应该简单地随着驱动力(欠饱和)的增加而线性增加，因为分离二氧化硅四面体的可能性更高。相反，通过检查两个非晶态二氧化硅玻璃的速率测量，我们发现了一个悖论。在电解液溶液中，这些二氧化硅表现出与其晶体对应物石英相同的驱动力指数依赖关系。我们通过考虑无定形二氧化硅提供了两种主要的表面配位的二氧化硅四面体来分析这个谜团。电解质克服了高配位物种成核脱离的能量障碍，产生了活性较弱的配位基团的外围，从而增加了表面能量。结果是一个看似合理的基于机理的模型，与为晶体生长而开发的经典多核理论在形式上是一致的。该模型还解释了报道的天然、生物和合成胶体二氧化硅的脱矿率。原则上，当反应单元由其组成物种的能量定义时，这些见解应该适用于具有各种组成和结构顺序的材料。
The mechanisms by which amorphous silica dissolves have proven elusive because noncrystalline materials lack the structural order that allows them to be studied by the classical terrace, ledge, kink-based models applied to crystals. This would seem to imply amorphous phases have surfaces that are disordered at an atomic scale so that the transfer of SiO4 tetrahedra to solution always leaves the surface free energy of the solid unchanged. As a consequence, dissolution rates of amorphous phases should simply scale linearly with increasing driving force (undersaturation) through the higher probability of detaching silica tetrahedra. By examining rate measurements for two amorphous SiO2 glasses we find, instead, a paradox. In electrolyte solutions, these silicas show the same exponential dependence on driving force as their crystalline counterpart, quartz. We analyze this enigma by considering that amorphous silicas present two predominant types of surface-coordinated silica tetrahedra to solution. Electrolytes overcome the energy barrier to nucleated detachment of higher coordinated species to create a periphery of reactive, lesser coordinated groups that increase surface energy. The result is a plausible mechanism-based model that is formally identical with the classical polynuclear theory developed for crystal growth. The model also accounts for reported demineralization rates of natural biogenic and synthetic colloidal silicas. In principle, these insights should be applicable to materials with a wide variety of compositions and structural order when the reacting units are defined by the energies of their constituent species.