Hexagonal mesoporous silica materials were synthesized in an acidic medium using different chain length of the quaternary ammonium surfactants as a template. The effects of chain length on the physical property, morphology of mesoporous materials, and the temperature on synthesis of materials in C n TMBr-TEOS-HNO 3 -H 2 O (n 5 12,14,16,18) system were systematically studied. The synthesized products were characterized by X-ray diffraction patterns, scanning electron microscopy, transmission electron microscopy, and nitrogen sorption analysis. It was found that degree of ordering, the d spacing values, and pore size all increase with an increase in the chain length of the template. The optimum synthesis temperature for mesoporous silica using C 18 TMBr-TEOS-HNO 3 -H 2 O system is slightly higher than the Krafft point. The temperature and pH can all affect the expandability of micelles, and so an increase in temperature and decrease in pH all lead to an increase in the pore size. It is also found that the shear flow and chain length are two key factors inducing the formation of millimeter-scaled silica ropes and micrometer-scaled rope fibers.
The effects of variant counterions with ionic strength of 0.05, 0.10, 0.20 and 0.25 mol•kg -1 on the stability and particle size of silica sols have been studied using the traditional methods of Ubbelohde viscosity measurement, TEM and titration respectively, finding that the stability and particle size of the silica sols are all concerned with the acidic, positively electric properties and the sizes of the counterions, as well as the attraction between the counterions and surface silicon hydroxyl groups of the silica sols. The small positively charged counterions lead to the decrease in particle sizes, making the silica sol the most stable. But the larger weakly acidic counterions can restrict the particle sizes of the silica sols and easily make the sols coagulate. It was also found that there existed a linear relationship between log r and log η, which has not ever been reported. The effect of temperature on the stability and particle sizes was also discussed.Keywords silica sol, counterion, stability, particle size, temperature IntroductionSilica sol is called "colloidal silica", referring to stable dispersions or sols of discrete silica particles. The reactive silica particles are able to become miscible with organic polymers by a surface modification, so they are widely applied to adhesion of organic and inorganic materials, dip-coated particulate thin films, coatings, precision casting, catalytic carriers and so on. The silica particles in silica sols are so small in size that they have a large specific surface, and there are huge interfaces between dispersed phases of silica sols, which leads to the silica sols having huge surface Gibbs free energy. 1According to the principle of minimum Gibbs free energy, the silica sols have been considered to be thermodynamically unstable, the colloidal silica particles are spontaneously congregated into larger particles, and finally form gel. Though stable concentrated silica sols that do not gel or settle out for at least several years became available in 1940s, at present, there exists a big disparity between the local products and the same kind of products abroad. Many effects can cause this disparity, but the most important one is how to control aggregation of discrete silica particles, and increase the stability of the silica sols to prolong their storage. As proposed by Iler, the silica sols can be stabilized by limiting counterions, for example, alkali-stabilized sols not only are stable against gelling, but also hinder the spontaneous growth of particles and their thermodynamical aggregation. Iler pointed out that the stabilizing energy came from the adsorption of OH -and the counterions on the colloid surface.1 During the period of year 2000 to 2006, several studies on the stability of an aqueous silica sol were published. Peukert 2 ever studied the stability of an aqueous silica sol by changing the electrolyte concentrations and pH-values in an investigation of dip-coated particulate thin films, and found both the concentration and the valency of the cations...
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