Fumed silicas possessing different morphology and hydrophilicity

Fumed silicas synthesized under varied conditions (stoichiometric or nonsto ichiometric amounts of reactants SiCl4, O-2 (mixed with N-2), and H-2, diff erent burner diameter, flow velocity and turbulence, flame temperature, etc .) were studied using adsorption of nitrogen, argon, and water, infrared (I R) and photon correlation spectroscopy, I H NMR, and electrokinetic methods . Prepared silicas posses different specific surface area (S-Ar = 85-512 m( 2)/g), structures of primary particles and their swarms, concentrations of silanols (C-OH = 1.9-5.2 mu mol/m(2)), and weakly (C-w,C-105 = 0.4-2.4 wt%) and strongly (C-w,C-900 = 0.4-2.2 wt%) bound waters. There is correlation between the specific surface area (S) of fumed silica and the flow velocity upsilon (f)(S similar to 1n upsilon (f) at upsilon (f) < 25-30 m/s). A dec rease in the amounts of hydrogen/oxygen in the flame and elevating synthesi s temperature or flame turbulence (due to an increase in the burner diamete r or changes in the reactant amounts) enhances the size of primary particle s, which become slightly micropous on addition of hydrogen (though an annul ar nozzle) reacting at the flame periphery. Dividing of the flow in the bur ner to several smaller flows reducing the turbulence without changes in oth er synthesis conditions significantly enhances the specific surface area. T he concentration of silanols increases with growing primary particle size, and the hydrophilicity (C-w,C-105 + C-w,C-900) decreases at oxygen deficien cy on the synthesis. (C) 2001 Academic Press.