Fumed silicas possessing different morphology and hydrophilicity

Fumed silicas synthesized under varied conditions (stoichiometric or nonstoichiometric amounts of reactants SiCl4, O-2 (mixed with N-2), and H-2, different burner diameter, flow velocity and turbulence, flame temperature, etc.) were studied using adsorption of nitrogen, argon, and water, infrared (IR) 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 decrease in the amounts of hydrogen/oxygen in the flame and elevating synthesis temperature or flame turbulence (due to an increase in the burner diameter or changes in the reactant amounts) enhances the size of primary particles, which become slightly micropous on addition of hydrogen (though an annular nozzle) reacting at the flame periphery. Dividing of the flow in the burner to several smaller flows reducing the turbulence without changes in other synthesis conditions significantly enhances the specific surface area. The concentration of silanols increases with growing primary particle size, and the hydrophilicity (C-w,C-105 + C-w,C-900) decreases at oxygen deficiency on the synthesis. (C) 2001 Academic Press.