JOURNAL OF COLLOID AND INTERFACE SCIENCE 26, 62-69 (1968) Controlled Growth of Monodisperse Silica Spheres in the Micron Size Range 1 WERNER STOBER, ARTHUR FINK Department of Radiation Biology and Biophysics, Medical School, University of Rochester, New York 14620 AD ERNST BOHN, Max-Planck-Institut fir experimentelle Medizin, GOttingen, Germany Received August 3, 1967 A system of chemical reactions has been developed which permits the controlled growth of spherical silica particles of uniform size by means of hydrolysis of alkyl silicates and subsequent condensation of silicie acid in alcoholic solutions. Ammonia is used as a morphological catalyst. Particle sizes obtained in suspension range from less than 0.05 t, to 2 in diameter. In many experimental studies which in- volve the use of colloidal suspensions of matter in form of hydrosols and aerosols, it would be desirable to have the suspended phase consisting of homogeneous particles of uniform shape and size. Such monodisperse particulate suspensions offer many experi- mental and theoretical advantages. They not only facilitate easy calibration proce- dures for analytical equipment, but also simplify data reduction, evaluation and in- terpretation of experiments designed to elu- cidate physicoehemieal properties or physi- opathological effects of colloids and aerosols. The results would no longer be biased by parameters of size and shape distributions. Some monodisperse suspensions of parti- cles in the colloidal size range are available in form of spheres of organic high polymers (1). In the aerosol field, they are primarily used as model substances and for calibration purposes. Generators producing monodis- perse particle clouds from soluble or volatile materials are used in various aerosol studies I This paper is based in part on work performed under contract with the U.S. Atomic Energy Commission at the University of Rochester Atomic Energy Project and has been assigned Publication No. UR-49-815. and have been described in the literature (2). However, no successful attempt has been made to generate monodisperse suspen- sions of silica particles. A commercial form of highly disperse silica produced by com- bustion of silicon tetraehloride in a hydrogen torch (3) consists of primary silica spheres of sizes below 0.1 u, but they are aggregated to coarse and irregular clusters which cause a very poorly defined state of suspension. The following investigation was made to explore the possibilities of producing mono- disperse suspensions of silica spheres in the colloidal size range. Such material can be used in both hydrosol and aerosol studies. It will be of particular interest to investi- gators in the medical field because of its known eytotoxicity and inhalation hazard. The experiments were based on the fact that silica particles can be produced by chemical reaction of tetraesters of silicie acid (tetraalkyl silicates) with certain solu- tions. In 1956, Kolbe (4) described the formation of silica particles by reacting tetraethyl silicate in alcoholic solution with water in the presence of certain bases. With very pure reactants he observed in several eases a slowly proceeding reaction leading to the formation of uniform spherical silica 62 CONTROLLED GROWTH OF MONODISPERSE SILICA SPHERES 63 particles. In an attempt to duplicate these findings, many of our experiments resulted in gel formation and only in a few cases did the electron mierographs show particles of ellipsoidal shape and a size range near 0.08 t. Then, a systematic study of the reaction parameters was made and after some drastic changes of the experimental conditions, quasi-monodisperse suspensions of silica spheres of sizes up to 2 were finally obtained within less than an hour and the reaction no longer required ex- tremely pure reactants. EXPERIMENTAL Reagents Methanol, ethanol, n-propanol and n- butanol used as solvents were of analytic reagent quality. Tetraesters of silieic acid (tetraalkyl silicates) were either supplied in technical grade (methyl, ethyl) or prepared by react- ing silicon tetrachloride and alcohol (n- propyl, n-butyl, wpentyl). All esters were redistilled before used in the experiments. Ammonia (anhydrous, 99.99%) from a tank was passed through a drying column filled with sodium hydroxide pellets and bubbled through the alcohol at OC. Con- eentrations obtained were about 8 M in methanol, 5 M in ethanol, 3 M in n-pro- panol, n-butanol and n-pentanol. Ammonium hydroxide, 26P., U.S.P., was used as supplied. Titrations indicated an ammonia concentration of 14.2 M. Instruments Electron microscopes Zeiss EM 9 and Philips EM 100 B and partiee size analyzer Zeiss TGZ 3 were used. Procedures Throughout the investigation, ammonia was used as the catalyst causing the forma- tion of spherical particles. In many cases, it was applied by adding saturated alcoholic solutions of ammonia to the reaction vessel. In other cases, particularly when high ammonia concentratio