Physical Chemistry, University of Paderborn , Warburger Str. 100, 33098 Paderborn, Germany.
Langmuir. 2017 Jun 20;33(24):6071-6083. doi: 10.1021/acs.langmuir.7b00887. Epub 2017 Jun 6.
The early stages of silica polymerization in aqueous solution proceed according to a mechanism based on three steps: nucleation, particle growth, and agglomeration of the particles. Application of time-resolved static and dynamic light scattering as a powerful in situ technique in combination with spectrophotometric analysis of the monomer consumption based on the molybdenum blue method was carried out to further investigate this 3-step process. Experiments were carried out at four different initial silicic acid contents covering a range between 350 and 750 ppm in the presence of either 10 mM NaCl or 5 mM of a mixture of CaCl and MgCl. The process in all cases was initiated with a drop of pH to 7. Addition of the salts made possible an analysis of the impact of an electrolyte on the process. Independent of the presence or absence of salt, particle growth in step two proceeded as a monomer-addition process without being interfered significantly by Ostwald-ripening. The growing particles were compact with a homogeneous density. The size of the particles approached final values between 5 and 20 nm with the actual value increasing with decreasing initial silicic acid content. Above a certain concentration of initial silica content, which depends on the level of added salt, particle-particle interactions caused agglomeration. The presence of electrolyte shifted this level from ∼2000 ppm to a range between 500 and 750 ppm. The resulting agglomerates had a fractal dimension of 2. Independent of the conditions, particle growth could be described with a simple nucleation and growth model.
成核、颗粒生长和颗粒团聚。应用时间分辨静态和动态光散射作为一种强大的原位技术，结合基于钼蓝法的单体消耗分光光度分析，进一步研究了这三个步骤。实验在四个不同的初始硅酸含量下进行，范围在 350 到 750ppm 之间，存在 10mMNaCl 或 5mM 的 CaCl 和 MgCl 的混合物。在所有情况下，过程都是通过将 pH 降低到 7 来引发的。添加盐使得分析电解质对过程的影响成为可能。无论是否存在盐，第二步的颗粒生长都是单体添加过程，奥斯特瓦尔德成熟过程没有显著干扰。生长的颗粒是紧凑的，密度均匀。颗粒的大小接近最终值在 5 到 20nm 之间，实际值随着初始硅酸含量的降低而增加。在初始硅浓度的某个特定浓度以上，这取决于添加盐的水平，颗粒-颗粒相互作用会导致团聚。电解质的存在将这个水平从约 2000ppm 转移到 500 到 750ppm 之间。所得团聚物的分形维数为 2。无论条件如何，颗粒生长都可以用简单的成核和生长模型来描述。
