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Abstract: The effect of the time of addition of organosilanes to silica particle formation reaction mixtures on the resulting surface availability of the added functional groups was investigated. Base catalyzed particle formation was initiated by the addition of tetraethyl orthosilicate (TEOS) to a water-in-oil microemulsion. Subsequently, amine, mono-carboxylate, ethylenediaminetriacetic acid, or dihydroimidazoleterminated organosilanes were added to the microemulsion. Continuous growth in size of monodispersed spherical particles over time was monitored by transmission electron microscopy and light- scattering measurements. Surface primary amine and carboxylate groups on the resulting particles were labeled with fluorescamine and 1-pyrenyldiazomethane (PDAM), respectively, and the effect of varying the time of organosilane addition to the microemulsion was determined by fluorescence spectroscopy. The effect of the time of organosilane addition on surface expression of dihydroimidazole groups was determined using a direct titration method. The results demonstrate that the degree of surface functionalization of the silica particles varied with the time of organosilane addition. For each type of amine terminated organosilane used, the highest surface availability of organo-functional groups was obtained when the organosilane was added 30 min after particle growth was initialized. A progressive decrease in the surface availability of primary amine groups was observed when 3- aminopropyltrimethoxysilane (APTMS) was added later than 30 min after particle formation was initiated. For each type of carboxylate-terminated organosilane used, the highest surface availability of organo-functional groups was obtained when the organosilane was added 5.5 h after particle growth was initialized. (C) 2000 Elsevier Science B.V. All rights reserved