Interaction Forces between Two Silica Surfaces in an Apolar Solvent Containing an Anionic Surfactant

Abstract

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 (AOT) 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.

Force measurements with the atomic force microscope: Technique, interpretation and applications

“…The surfactants form reverse micelles and dissolve free ions in the liquid and at the same time charge the surfaces. This was demonstrated by McNamee et al [469]. They measured the force between two silica surfaces in n-dodecane with dissolved anionic surfactant AOT (bis(2-ethylhexyl)-sulfosuccinate).…”

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

“…While a change in the slip plane location could contribute to the decrease in zeta potential, such a decline has also been observed even after the surfactant adsorption reached the maximum [27]. Additionally, the surface potential of a hydrophobic silica surface in dodecane extracted from force measurements using atomic force microscopy (AFM) also exhibited a maximum as AOT concentration was increased [48]. In such experiments, the decrease in surface potential could not be explained by the change in slip plane location.…”

Measured electrical charge of SiO2 in polar and nonpolar media

“…Thus, while matter can be expected to be strongly charged in water (ε s ∼ 80), an ionizable surface should carry much less charge in a nonpolar medium with ε s ∼ 5. Although electrostatic interactions in apolar media are indeed weak relative to the aqueous phase, [2][3][4][5][6][7] the repulsion is often regarded as negligible 8,9 mainly on the basis of solvation energy arguments. Generally efforts are invested in enhancing repulsions in apolar media by the addition of micelle-forming surfactants that stabilize potentialdetermining ions [2][3][4][5][6][7]9 or by chemical modification of the surface.…”

“…Although electrostatic interactions in apolar media are indeed weak relative to the aqueous phase, [2][3][4][5][6][7] the repulsion is often regarded as negligible 8,9 mainly on the basis of solvation energy arguments. Generally efforts are invested in enhancing repulsions in apolar media by the addition of micelle-forming surfactants that stabilize potentialdetermining ions [2][3][4][5][6][7]9 or by chemical modification of the surface. 3,4,9 Electrostatic interactions in nonpolar media have been exploited in model systems for atomic crystals 10,11 in a) G. Kokot and M. I. Bespalova contributed equally to this work.…”