Interaction forces between two hard surfaces in particle-containing aqueous systems

The interaction forces between two hard surfaces in the presence of particle-containing aqueous systems were measured with an atomic force microscope. Bare silica and sodium dodecyl sulfate (SDS)-modified paraffin were used for the surfaces. SDS micelles and colloidal silica were used for the particles. Increasing the number of micelles between two silica surfaces increased the number and magnitude of the oscillations. The effect of the compression speed was seen only with fast speeds greater than or equal to 3420 nm s(-1), probably because of a hydrodynamic force. The decay length of the observed electrostatic repulsion decreased with a SDS concentration increase, as a result of an increase in the number of free ions above the critical micelle concentration resulting from a finite dissociation of ions from the surfactant. The effect of using a silica particle instead of a micelle did not show any measurable differences in the oscillations of the force curves. The effect of using an asymmetric system (silica-solution-SDS-modified paraffin) instead of a symmetric system (silica- solution-silica) showed differences only in the Derjaguin-Landau-Verway-Overbeek forces. In all cases, the forces could be explained well by a summation of an electrostatic, van der Waals, and a structural force.