In many experimental studies which

In many experimental studies which involvethe use of colloidal suspensions ofmatter in form of hydrosols and aerosols, itwould be desirable to have the suspendedphase consisting of homogeneous particles ofuniform shape and size. Such monodisperseparticulate suspensions offer many experimentaland theoretical advantages. Theynot only facilitate easy calibration proceduresfor analytical equipment, but alsosimplify data reduction, evaluation and interpretationof experiments designed to elucidatephysicoehemieal properties or physiopathologicaleffects of colloids and aerosols.The results would no longer be biased byparameters of size and shape distributions.Some monodisperse suspensions of particlesin the colloidal size range are availablein form of spheres of organic high polymers(1). In the aerosol field, they are primarilyused as model substances and for calibrationpurposes. Generators producing monodisperseparticle clouds from soluble or volatilematerials are used in various aerosol studiesI This paper is based in part on work performedunder contract with the U.S. Atomic EnergyCommission at the University of RochesterAtomic Energy Project and has been assignedPublication No. UR-49-815.and have been described in the literature(2). However, no successful attempt hasbeen made to generate monodisperse suspensionsof silica particles. A commercial formof highly disperse silica produced by combustionof silicon tetraehloride in a hydrogentorch (3) consists of primary silica spheres ofsizes below 0.1 u, but they are aggregated tocoarse and irregular clusters which cause avery poorly defined state of suspension.The following investigation was made toexplore the possibilities of producing monodispersesuspensions of silica spheres in thecolloidal size range. Such material can beused in both hydrosol and aerosol studies.It will be of particular interest to investigatorsin the medical field because of itsknown eytotoxicity and inhalation hazard.The experiments were based on the factthat silica particles can be produced bychemical reaction of tetraesters of silicieacid (tetraalkyl silicates) with certain solutions.In 1956, Kolbe (4) described theformation of silica particles by reactingtetraethyl silicate in alcoholic solution withwater in the presence of certain bases. Withvery pure reactants he observed in severaleases a slowly proceeding reaction leadingto the formation of uniform spherical silica particles. In an attempt to duplicate thesefindings, many of our experiments resultedin gel formation and only in a few cases didthe electron mierographs show particles ofellipsoidal shape and a size range near0.08 t~. Then, a systematic study of thereaction parameters was made and aftersome drastic changes of the experimentalconditions, quasi-monodisperse suspensionsof silica spheres of sizes up to 2 ~ werefinally obtained within less than an hourand the reaction no longer required extremelypure reactants.