Interaction forces between two silica surfaces in an apolar solvent containing an anionic surfactant

The forces between two silica surfaces in a surfactant-containing apolar solvent were studied by atomic force microscopy, as a function of the silica surface type, surfactant concentration, compression speed, and effect of water presence. In the apolar system, no electrostatic repulsive forces were measurable between two hydrophobized silica surfaces in dodecane. However, the introduction of the anionic surfactant of sodium bis(2-ethylhexyl) sulfosuccinate (NaAOT) into the dodecane caused a measurable surface potential because of the adsorption of the hydrophobic bis(2-ethylhexyl) sulfosuccinate (ACT) anions to silica surfaces. Increasing the NaAOT concentration from the critical micelle concentration of 7.6 to 500 mM caused a maximum in the surface potential at 100 mM because of the subsequent adsorption of the sodium cations onto the AOT anions that have already adsorbed. Oscillations in the force curves were only observed for the NaAOT concentration of 500 mM; a smaller concentration did not give enough reverse micelles to cause measurable structuring effects. The experimental results in this study are well-explained by an extended Derjaguin-Landau-Verway-Overbeek theory, which includes electrostatic, van der Waals, and structural forces.