Small-angle X-ray scattering study of nanopore evolution of macroporous silica gel by solvent exchange
Small-angle X-ray scattering study of nanopore evolution of macroporous silica gel by solvent exchange
复制标题
小角X射线散射研究溶剂交换大孔硅胶纳米孔演化
DOI：
10.1039/fd9950100249
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发表时间：
1995
影响因子：
3.4
通讯作者：
N. Soga
中科院分区：
文献类型：
--
作者：
Ryōji Takahashi;K. Nakanishi;N. Soga
小角X射线散射（SAXS）与氮吸附，已被用来分析孔结构的演变行为，在纳米尺度上，通过不同的溶剂交换处理的硅胶具有双连续互连的大孔。对于在含有聚丙烯酸的体系中制备的凝胶，浸泡在乙醇和1 mol dm-3硝酸中的凝胶在湿态下的SAXS谱差异很小，但在干燥时显着增大。浸泡在乙醇中的干凝胶呈现出质量分形特征，而浸泡在1 mol dm-3硝酸中的干凝胶在几个纳米内是非分形的。在热处理过程中的演变行为也不同，导致较小的尺寸和较低的体积的孔在前凝胶。孔结构的这些变化的原因是在乙醇中发生溶胀，并在1摩尔dm-3硝酸溶液中的微观相分离。然而，对于在含有聚（环氧乙烷）的系统中制备的凝胶，类似处理的干凝胶的轮廓的差异不是那么明显，可能是因为微观相分离的进展受到限制的有机聚合物上的硅醇的凝胶基质。另一方面，在碱性条件下通过溶剂交换观察到平均孔径以及孔体积的显著增加，而与用于凝胶制备的聚合物或催化剂的种类无关。这种效应被解释为通过溶解和再沉淀机制的粗化过程。Porod斜率的时间演变，以及其对溶液的pH值在湿状态下的依赖性，进行了监测，通过在原位SAXS测量。Porod斜率的急剧增加，伴随着在湿状态下的幂律区域的扩展，表示重组内的硅胶结构。
Small-angle X-ray scattering (SAXS) together with nitrogen adsorption, have been employed to analyse the pore-structure evolution behaviour, on the nanometre scale, by varying solvent-exchange treatment for silica gels having bicontinuous interconnected macropores. For the gels prepared in the system containing poly(acrylic acid), the difference in SAXS profile between gels immersed in ethanol and 1 mol dm–3 nitric acid was small in the wet state, but was considerably enlarged on drying. The dried gel immersed in ethanol exhibited a mass fractal feature, while that immersed in 1 mol dm–3 nitric acid was non-fractal up to several nanometres. The evolution behaviour during heat-treatment also differed, leading to a smaller size and lower volumes of the pores in the former gel. These variations in pore structures are explained by the occurrence of swelling in ethanol, and microscopic phase separation in 1 mol dm–3 nitric acid solution. However, for gels prepared in the system containing poly(ethylene oxide), the difference in profiles of similarly treated dried gels was not so evident, possibly because the progress of microscopic phase separation was restricted by the adsorbed organic polymer on silanols of the gel matrix. On the other hand, a significant increase in the average pore size, as well as the pore volume, was observed by solvent exchange in basic conditions, irrespective of the kind of polymer or catalyst used for gel preparation. This effect is interpreted as a coarsening process through a dissolution and reprecipitation mechanism. The time evolution of the Porod slope, as well as its dependence on the solution pH in the wet state, were monitored by in situ SAXS measurements. The steep increase of the Porod slope accompanied by the extension of the power-law region in the wet state indicated reorganization within the silica gel structures.