Effect of thermal treatment and moisture content on the charge of silica particles in non-polar media

Surfactants, such as Aerosol-OT (AOT), are known to impart charge on particles in non-polar media and make them colloidally stable. Previous studies on the charge of silica particles, which are one of the most widely used and studied colloids in non-polar media, have shown significant inconsistencies in the observed charging behaviour. While some studies have shown that silica becomes positively charged with AOT, others have shown that silica gains a net negative charge with the same surfactant. In addition, we have noticed differences in the charge state of colloidal silica from different suppliers. To determine the possible origin of these discrepancies, the effect of surface chemistry and moisture on the charge of silica in AOT–toluene is examined. The surface chemistry of silica is altered by thermal treatment at temperatures up to 1000 °C, while the amount of water in silica dispersions is controlled by keeping them at three different relative humidities. We use Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA) and quartz crystal microbalance (QCM) to gain insight into the effect of thermal treatment and moisture on the charging of silica in non-polar media. Changes in the surface chemistry of silica particles are found to have a significant effect on their charge in AOT–toluene solution. In addition, an increase in the moisture content of silica dispersions is shown to adversely affect the charge in AOT–toluene and, in some cases, induce charge inversion. Our results show that the surface chemistry of silica as well as moisture content are crucial parameters that influence the charging of silica in non-polar media and that these parameters should be carefully controlled to enhance the reproducibility of surface charge measurements in non-polar media. Furthermore, both high temperature thermal treatment and moisture content could serve as control parameters to change the charge of silica in non-polar media.