![]() ![]() Silica gels can be used directly as a chromatographic material or after the surface of the particles are chemically modified. Most of these involve the preparation of powders, such as Stöber spheres. ![]() Silica gels have long been used for chromatographic applications including flash chromatography in organic synthesis, thin-layer chromatography, and packings for columns used in liquid and gas chromatography. ![]() Loy, in Encyclopedia of Physical Science and Technology (Third Edition), 2003 VII.B.1 Sol–Gel Preparation of Bulk Phases for Chromatography The hydrophobicity of herbicides can be modified by the addition of cyclodextrins.ĭouglas A. Hexane–dioxane mixtures are used with alumina layers. Methanol or acetonitrile with water is recommended when reversed-phase stationary phases are employed. Mobile phases frequently used in the TLC of herbicides in combination with silica gel include dichloromethane, hexane–acetone and hexane–butyl acetate mixtures. TLC layers covered with a transparent polymer film have been recommended to prevent evaporation of the mobile phase and volatile herbicide samples and to suppress the adsorption of environmental impurities. Good separation of herbicides can also be obtained on alumina. Silica gel impregnated with diethylene glycol is suitable for triazine herbicides. Silica gel is the most common stationary phase in TLC and HPTLC of herbicides but reversed-phases (silica gel modified with C 8, C 18, e.g., RP-18 W, Nano-Sil C 18-100, silica gel impregnated with paraffin oil) can also be used. Pacáková, in Encyclopedia of Separation Science, 2000 Chromatographic Systems at high temperature and water vapour pressure) in the production of wide-pore material with a relatively small specific surface area and high hydroxyl group concentration. Hydrothermal treatment has been found useful ( i.e. Depending on the preparation method, silica gel can be obtained with a specific surface in a range from units to hundreds of m 2 g −1. To ensure wide application of silica gel, methods have been developed for the preparation of a chemically pure material with a narrow distribution of wide pores. Most commercial silica gels contain a mixture of Al 2O 3 and Fe 2O 3, contributing to the irreversible sorption, which can also have a catalytic effect. As the elementary structure of silica gel consists of spherical species with quite regular dimensions, the pore size is also quite uniform. For example, silica gel with small pores is obtained in acidic media (pH ≈ 4), while wide-pore silica gel is formed in weakly alkaline media. the pH and the ratio between the rates of polymerization and condensation, silica gel can be obtained with various porosity characteristics. Depending on the conditions of preparation, i.e. Silica gel is partly hydrated SiO 2 and is a typical adsorbent on which specific interactions occur. In Comprehensive Analytical Chemistry, 1991 Silica gel Table 2.12 presents a comparison between zeolite molecular sieves and silica gels. Unlike the zeolites, silica gels possess pores with large diameters. It has a network of interconnected pores and channels. Due to its unique internal structure, silica gel is radically different to other SiO 2-based materials. Silica gel is a porous, amorphous form and is composed of SiO 2. Due to these properties, silica gel is used in environments where a low temperature, moderate water vapor pressure, and high adsorption capacity are required. In contrast, silica gel is easier to reduction can be heated to as low as 150☌, while zeolite must be heated to about 350☌. The adsorption capacity of silica gel at low temperatures is higher than that of alumina and zeolite however, if the moisture content is low, it is less adsorbent than zeolite. Silica gel is a suitable adsorbent for water, alcohol, phenols, amines, and so on. Silica gel is a three-dimensional polymer composed of four-dimensional silicon dioxide units. Silica gel is composed of SiO 2 units with high porosity. ![]()
0 Comments
Leave a Reply. |
Details
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |