Surface forces across colloidal dispersions

Abstract

Colloid based products are ubiquitous in our daily life and precise knowledge of the interactions in these systems is of great interest in basic as well as in applied sciences. This thesis studies forces between macroscopic surfaces interacting across concentrated colloidal dispersions, focussing on the mutual effect between the dispersions' nanostructure and surface forces. Besides the well-known DLVO-type surface forces, specific structuring of colloidal particles may induce the so-called oscillatory structural forces. In particular, this study utilises two model colloidal dispersions: suspensions of silica nanoparticles and dispersions of self-assembled surfactant micelles. Throughout this work, surface forces across these colloidal dispersions are measured using colloidal-probe atomic force microscopy. In dispersions of charged colloidal particles, electrostatic interactions cause a pronounced interparticle structuring. For spherical particles, the mean interparticle distance scales with the particles' volume fraction according to an inverse cubic root scaling law. Oscillatory structural forces across these dispersions typically show a wavelength which is directly related to the respective mean interparticle distance. Consequently, the measurement of oscillatory structural forces can be utilised as a tool for characterisation of colloidal particles, knowing the particles' volume fraction. This is demonstrated by measuring surface forces across concentrated silica nanoparticle suspensions to determine the respective nanoparticles' diameters. The validity of this inverse cubic root scaling law is checked upon variation of the particles' surface charges. Mixing nonionic and anionic surfactants form particles with tunable surface charge as demonstrated by small-angle neutron scattering. It is shown that the inverse cubic root scaling law is only valid for highly charged colloidal particles. If the particles carry little or no charges, their interparticle structuring decreases. By this means, the oscillatory structural forces across dispersions are precisely tuned not only by the particles' volume fraction but also by the amount of surface charges per particle. Complete description of the surface forces requires further contributions, other than the oscillatory structural forces. Here, forces between charged surfaces across concentrated silica nanoparticle suspensions are modelled as a superposition of two individual contributions - the electrostatic double layer and the oscillatory structural force. The electrostatic screening length of colloidal dispersions, an important parameter for the description of the electrostatic double layer force, is independently determined by conductivity measurements. In that way, both force contributions are untangled. This enables a uniform description of the surface forces from a few hundred nanometres down to the surfaces being almost in contact.