Surfactant effects on colloidal interactions: Concentrated micellar solutions of nonionic surfactant

The effect of nonionic surfactants on interaction energies between polystyrene latex or silica spheres and a flat glass surface has been studied using total internal reflection microscopy (TIRM). The measurements reveal that both Triton X-100 and C12E5 contribute to the Debye screening length, an effect that is essentially removed on first exposing the surfactants to ion-exchange resin. Whereas particles can be embedded in Triton X-100 concentrations at least as high as 285 times the critical micelle concentration, the same particles become elastically tethered at intermediate separations from the glass surface in somewhat less concentrated micellar solutions of C12E5. The elastic energy is caused by a connecting structure between the particle and glass surface, which remains intact on dilution and on lowering the ionic strength of the solvent.