Aggregation Kinetics and Fractal Structure of γ-Alumina Assemblages
Aggregation Kinetics and Fractal Structure of γ-Alumina Assemblages
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DOI：
10.1006/jcis.2001.7694
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发表时间：
2001-09
期刊：
影响因子：
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通讯作者：
T. Waite;J. Cleaver;J. Beattie
中科院分区：
文献类型：
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作者：
T. Waite;J. Cleaver;J. Beattie
各种不同氧化铝的悬浮液先前已显示需要非常高浓度的氯离子和硝酸根阴离子（> 0.5M）来诱导快速聚集。这种高稳定性被认为是由于表面力的存在，表面力被认为是由于在微酸性pH下形成高度带电的Al 13聚合物物质和在碱性条件下形成羟基氧化铝凝胶。这种稳定性对所得聚集体结构的影响在这里使用成熟的静态光散射技术进行研究。在所有情况下，散射光强度作为散射波矢量的函数的幂律行为被观察到，并且暗示分形结构。得到的分形维数落在预期范围内的1.8至2.3观察到的胶体聚集体，但似乎不遵循典型的观察胶体不稳定的中性电解质，其中较低的分形维数与快速（扩散限制）聚集和较高的分形维数与较慢（反应限制）聚集。事实上，相对恒定的分形维数（2.10至2.25）观察到的盐浓度范围内发生的缓慢到快速的聚集速率转换，如果有的话，一个稍微更高的分形维数观察到更高的聚集速率。特异性结合硫酸根阴离子的存在似乎否定了强近距离排斥力，导致在低（1至2 mM）硫酸根浓度下快速聚集。显着较低的分形维数（1.85至1.91），观察到使用硫酸根离子不稳定形成的聚集体比获得时，使用氯化物或硝酸盐，再次，在分形维数增加聚集率时，明显略有增加。
Suspensions of a variety of different aluminum oxides have previously been shown to require very high concentrations of chloride and nitrate anions (>0.5 M) to induce rapid aggregation. This high stability has been accredited to the presence of surface forces considered to be due to the formation of highly charged Al13 polymeric species at slightly acidic pH's and aluminum oxyhydroxide gel formation under alkaline conditions. The effect of this stability on the structure of the resulting aggregates is investigated here using well-established static light-scattering techniques. Power law behavior of scattered light intensity as a function of scattering wave vector is observed in all cases and is suggestive of fractal structure. The fractal dimensions obtained fall within the expected range of 1.8 to 2.3 observed for colloidal aggregates but do not appear to follow the typical observations for colloids destabilized by indifferent electrolytes where lower fractal dimensions are associated with rapid (diffusion-limited) aggregation and higher fractal dimensions with slower (reaction-limited) aggregation. Indeed, relatively constant fractal dimensions (2.10 to 2.25) are observed over the range of salt concentrations at which the slow to rapid aggregation rate transformation occurs with, if anything, a slightly higher fractal dimension observed for higher aggregation rates. The presence of specifically binding sulfate anions appears to negate the strong near-distance repulsive forces leading to rapid aggregation at low (1 to 2 mM) sulfate concentrations. Significantly lower fractal dimensions (1.85 to 1.91) are observed for aggregates formed by destabilization using sulfate ions than obtained when chloride or nitrate are used with, again, an apparent slight increase in fractal dimension upon increasing aggregation rate.