Baute Debbie, Frydman Veronica, Zimmermann Herbert, Kababya Shifi, Goldfarb Daniella
Department of Chemical Physics, Weizmann Institute of Science, Rehovot, Israel.
J Phys Chem B. 2005 Apr 28;109(16):7807-16. doi: 10.1021/jp044538t.
The properties of the silica layer during the formation of the mesoporous material MCM-41 were investigated by electron paramagnetic resonance (EPR) experiments carried out on a specifically designed, organo(trialkoxy)silane spin probe, SL1SiEt. Minute amounts of the spin probe were co-condensed with the silica source, tetraethyl orthosilicate (TEOS), in the synthesis of MCM-41 with cetyltrimethylammonium bromide (CTAB) under basic conditions. The mobility and location of the spin probe were followed in the CTAB micellar solution before the reaction, in the reaction mixture and in the final ordered material. It was found that the EPR spectra of hydrolyzed SL1SiEt throughout the room temperature part of the reaction are characteristic of a fast tumbling species, indicating that the silica is highly fluid prior to drying. After filtering, a slow motion type spectrum was observed, showing that the spin-label experiences considerable motional hindrance. The liquidlike behavior could be restored upon stirring the material in water. When the reaction is performed with a hydrothermal stage, the spectrum of SL1SiEt in the final product is the same as that of the room temperature synthesized material, but the addition of water did not restore the high mobility, due to a higher degree of silica cross-linking. The location of SL1SiEt throughout the formation process was obtained from electron spin-echo envelope modulation (ESEEM) measurements on MCM-41 prepared with CTAB deuterated either at the N-methyl or the alpha position and in a reaction carried out in D2O. Comparing the deuterium modulation depth, k(2H), induced by CTAB-alpha-d2, CTAB-d9, or D2O in CTAB micellar solutions of a number of reference spin probes with those of SL1SiEt revealed that the hydrolyzed SL1SiEt is located near the polar heads of the surfactant in the absence of base and TEOS. This supports the postulation of charge matching at the interface as a driving force for the formation of the mesostructure. Similar experiments carried out on reaction mixtures containing SL1SiEt showed a decrease of k(2H) from CTAB-alpha-d2 and CTAB-d9 compared to the micellar solution, exhibiting practically no time dependence. This indicates that the spin probe is pulled away from the micelle-water interface into the loosely linked, forming silica network. After drying, the modulation depth induced by CTAB-alpha-d2 and CTAB-d9 increases, showing that, once the water is removed, the silica walls contract around the micelles, pushing the silica-linked spin probe into the organic phase within the mesopores.
通过对一种专门设计的有机（三烷氧基）硅烷自旋探针SL1SiEt进行电子顺磁共振（EPR）实验，研究了介孔材料MCM - 41形成过程中二氧化硅层的性质。在碱性条件下，将微量的自旋探针与二氧化硅源正硅酸四乙酯（TEOS）以及十六烷基三甲基溴化铵（CTAB）共缩合，用于合成MCM - 41。在反应前的CTAB胶束溶液、反应混合物以及最终的有序材料中跟踪自旋探针的迁移率和位置。结果发现，在反应的室温阶段，水解后的SL1SiEt的EPR谱具有快速翻转物种的特征，表明在干燥前二氧化硅具有高度流动性。过滤后，观察到慢运动型谱，表明自旋标记经历了相当大的运动阻碍。将材料在水中搅拌可恢复其类似液体的行为。当反应进行水热阶段时，最终产物中SL1SiEt的谱与室温合成材料的谱相同，但由于二氧化硅交联程度较高，加水并不能恢复高迁移率。通过对用在N - 甲基或α位氘代的CTAB以及在D2O中进行的反应制备的MCM - 41进行电子自旋回波包络调制（ESEEM）测量，获得了SL1SiEt在整个形成过程中的位置。将CTAB - α - d2、CTAB - d9或D2O在多种参考自旋探针的CTAB胶束溶液中诱导的氘调制深度k(2H)与SL1SiEt的进行比较，结果表明，在没有碱和TEOS的情况下，水解后的SL1SiEt位于表面活性剂的极性头部附近。这支持了界面电荷匹配作为介观结构形成驱动力的假设。对含有SL1SiEt的反应混合物进行的类似实验表明，与胶束溶液相比，CTAB - α - d2和CTAB - d9诱导的k(2H)降低，且几乎没有时间依赖性。这表明自旋探针从胶束 - 水界面被拉离，进入松散连接的、正在形成的二氧化硅网络。干燥后，CTAB - α - d2和CTAB - d9诱导的调制深度增加，表明一旦水被去除，二氧化硅壁在胶束周围收缩，将与二氧化硅连接的自旋探针推入介孔内的有机相中。
