Munoz-Aguado MJ, Gregorkiewitz M
Instituto de Ciencia de Materiales, CSIC, Madrid-Cantoblanco, E-28049, Spain
J Colloid Interface Sci. 1997 Jan 15;185(2):459-65. doi: 10.1006/jcis.1996.4564.
Aqueous sodium silicate solutions and hydrochloric acid were used to prepare silica gels without any additives. In order to establish the optimum conditions for the preparation of microporous and monodisperse gels, the sol-gel reaction has been studied as a function of pH, concentration of silicate, time, and temperature. From 29Si NMR spectroscopy it was found that polycondensation sets on immediately after mixing the reagents, with a continuous increase of the fraction of 4-connected silicate tetrahedra (Q4) at the expense of Q1 and Q2. Polycondensation continues beyond the gelation point for 1 </= pH </= 7, but not for 7 </= pH </= 10. Two mechanisms were also observed for the gelation kinetics which is about second order with respect to [Si] for pH </= 7, whereas for pH > 7, the order increases rapidly with pH. Gels formed at pH > 7 had an electrostatic nature and redissolved in water, whereas for pH </= 7, gelation was irreversible as expected for a gel where individual colloid particles are linked by chemical ( identical withSi-O-Si identical with) bonds. From nitrogen adsorption, it is found that the pore size depends mainly upon pH, the finest pores being obtained for pH 2-3 (BET surface area 650-700 m2/g, porosity 35%, pore radius 5-11 A).
使用硅酸钠水溶液和盐酸制备不含任何添加剂的硅胶。为了确定制备微孔和单分散凝胶的最佳条件，研究了溶胶 - 凝胶反应与pH值、硅酸盐浓度、时间和温度的关系。通过29Si核磁共振光谱发现，试剂混合后立即发生缩聚反应，4连接的硅酸盐四面体（Q4）的比例持续增加，而Q1和Q2的比例相应减少。在凝胶点之后，对于1≤pH≤7，缩聚反应仍会继续，但对于7≤pH≤10则不会。还观察到两种凝胶化动力学机制，对于pH≤7，凝胶化动力学对[Si]约为二级反应，而对于pH>7，反应级数随pH迅速增加。在pH>7时形成的凝胶具有静电性质，可重新溶解于水中，而对于pH≤7，正如预期的那样，通过化学（Si - O - Si）键连接单个胶体颗粒形成的凝胶，凝胶化是不可逆的。通过氮气吸附发现，孔径主要取决于pH值，在pH为2 - 3时可获得最细的孔（BET表面积650 - 700 m2/g，孔隙率35%，孔半径5 - 11 Å）。
