Surface Acidity Distribution on Precipitated Silicas

Abstract

Precipitated silicas are carriers for a variety of formulations used in agricultural applications. In such formulations, interaction between the agricultural chemicals and the carriers must be minimized. Precipitated silicas are known to have reactive acid sites on their surfaces, which can cause catalyzed chemical decomposition. An investigation into factors that affect the acid site distribution of precipitated silicas is described in this report. Commercially available and laboratory-prepared precipitated silicas were suspended in isooctane treated with incremental amounts of n-butylamine solution. The distribution of acid sites of different strength was determined by using six Hammett indicators, corresponding to a range of +6.8 to −8.2 pKa, in conjunction with spectrophotometric analysis of the suspensions. Variations in blocking agents, impurities, and salt content are shown to account for alterations in the acid site distribution on the silica surface. Control of these factors will aid the agricultural formulator in minimizing chemical/carrier interaction.