Effect of alkyl functionalization on charging of colloidal silica in apolar media

The present work examines the effect of alkyl-silane treatment on the charging of colloids in apolar solvent using two otherwise identical 250 nm diameter, spherical silica particles, one with untreated surface and the other treated with hexadecyltrimethoxysilane (C16), dispersed in an apolar isoparaffin solvent (Isopar-L) containing one of three oil-soluble surfactants: Aerosol-OT, OLOA 11,000, and zirconyl 2-ethyl hexanoate. The electrophoretic mobility of each dispersion was determined using phase angle light scattering (PALS). It was found that at sufficiently high surfactant concentration, i.e., where micelles begin to form in the bulk, the particle surfaces could be electrically charged. All three surfactants studied imparted a negative surface charge to the untreated silica particles. In all cases, the C16-treated particles were also found to be negatively charged but had a much higher magnitude of mobility than the untreated silica. Although the increase in magnitude of mobility as a result of the alkyl functionalization was surprising, it could be attributed to the increase in the number of surface hydroxyl groups arising from the hydrolysis of unbound methoxy groups of the silane molecules. The added hydroxyl groups provided additional potential acid–base interaction sites, resulting in higher particle mobility. It was also found that further increases in surfactant concentration lowered the particle mobility, attributed to the increasing concentration of electrically charged micelles, which may partially neutralize the surface charge or compress the electrical double layer.

Graphical abstractCharging of alkyl-functionalized surface in apolar media by reverse micelles. Surfactants interact with available hydroxy groups via acid–base process. Acidic surfaces become negatively charged as photons transfer from the surface to micellar core.Figure optionsDownload full-size imageDownload high-quality image (109 K)Download as PowerPoint slideResearch highlights► Untreated and alkyl functionalized silica particles were dispersed in an apolar solvent containing AOT, OLOA 11,000, or ZrO(Oct)2 reverse micelles. ► All systems had negatively charged surfaces. ► Treated particles always had higher magnitude of mobility than untreated silica. ► Increase in mobility is attributed to increase in surface hydroxyl groups on the treated particles.