Interaction Forces between a Bare Silica Surface and an α-Alumina Surface Bearing Adsorbed Polyelectrolyte and Surfactant

Abstract

The interactions between silica and α-alumina surfaces with and without polyelectrolyte and combinations of both polyelectrolyte (sodium poly(styrene sulfonate) or PSS) and surfactant (cetyltrimethylammonium bromide or CTAB) have been measured using the atomic force microscope (AFM) colloid probe technique. In this case, silica particles were glued to the AFM cantilever to give a surface of known geometry. The forces between silica and α-alumina in electrolyte only were well described by the Derjaguin−Landau−Verwey−Overbeek theory at all separation distances. However, introduction of a solution of negatively charged PSS and mixtures of PSS and a positively charged surfactant, CTAB, at the point of zero charge of α-alumina, modified the interaction forces, introducing short-range steric interactions superimposed onto electrostatic interactions. Thus, the adsorption of strongly charged polyelectrolytes onto net neutral surfaces resulted in a flat conformation with few loops and tails. When PSS and CTAB were added sequentially, the interaction forces were further modified, resulting in weak electrostatic interactions and the presence of attractive van der Waals or bridging forces between the surfaces. The sign of the surfaces was altered from negative to positive in this case. Increasing the concentration of the added CTAB resulted in swelling of the adsorbed layer and an increase in the effective surface potential fitted to the data.