Effect of particle and surfactant acid–base properties on charging of colloids in apolar media

The present work examines role of particle and surfactant acid–base properties using model spherical silica particles, one type with acidic functionality and the other with basic functionality, dispersed in an apolar solvent containing either a basic or an acidic surfactant. The electrical properties of each of the four systems types are quantified by measuring the particle electrophoretic mobility using phase angle light scattering (PALS). Since the magnitude of the mobility in each case also depends strongly on the concentration of the surfactant, this dependence was investigated concurrently. 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. Both the polarity and magnitude of the surface charge are found to depend strongly on the acid–base properties of the surface and the surfactant. The basic surfactant produces negatively-charged particles, and the magnitude of the mobility increases with the particle surface acidity. The acidic surfactant, on the other hand, charges the particle positively and produces the highest mobility when used with basic particles. This finding is consistent with the acid–base charging mechanism initially proposed by Fowkes and coworkers. It is also found that further increases in micelle concentration can have the effect of lowering the particle mobility. The decline in electrophoretic mobility (or zeta potential) at high surfactant concentration is caused by the increasing concentration of electrically charged micelles, which may partially neutralize the surface charge or compress the electrical double layer.

Graphical abstractSurface charging in apolar media by reverse micelles via acid–base interaction. Here proton transfers from acidic surfaces to basic, neutrally charged micelles producing negative surfaces and positive counter-charges.Figure optionsDownload full-size imageDownload high-quality image (101 K)Download as PowerPoint slide