Charging behavior and colloidal stability of amidine latex particles are studied in the presence of poly(sodium styrene sulfonate) (PSS) and KCl. Detailed measurements of electrophoretic mobility, adsorbed layer thickness, and aggregation (or coagulation) rate constant on varying the polymer dose, molecular mass of the polymer, and ionic strength are reported. Polyelectrolyte adsorption leads to the characteristic charge reversal (or overcharging) of the colloidal particles at the isoelectric point (IEP). In accordance with classical Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, uncharged particles tend to aggregate because of van der Waals attraction, whereas charged particles are stabilized by electrical double layer repulsion. Attractive patch-charge interactions originating from the laterally inhomogeneous structure of the adsorbed polymer substantially decrease the suspension stability or even accelerate the aggregation rate beyond diffusion control. These electrostatic non-DLVO forces become progressively important with increasing molecular mass of the polymer and the ionic strength of the solution. At higher polymer dose of typically 10 times the IEP, one observes the formation of a saturated layer of the adsorbed polymer with a thickness of several nanometers. Its thickness increases with increasing molecular mass, whereby the layer becomes increasingly porous. This layer does not seem to be involved in the suspension stabilization, since at such high polymer doses the double layer repulsion has attained sufficient strength to stabilize the suspension.

The interaction forces between individual positively charged amidine functionalized latex particles with adsorbed negatively charged sodium poly(styrene sulfonate) were studied with the colloidal probe technique based on atomic force microscopy (AFM). When the polymer dose is progressively increased, the strength of the repulsive force between the particles decreases as the charge neutralization point is approached, then increases again due to overcharging, and finally reaches a plateau. Surface potentials obtained from fits of the force profiles to Poisson-Boltzmann theory agree well with potentials measured with electrophoresis. Close to the charge neutralization point, attractive forces exceeding van der Waals interactions are found. These attractive forces increase in strength with increasing molecular mass of the polymer and decreasing ionic strength. These attractive interactions are of electrostatic origin and result from lateral patch-charge heterogeneities within the adsorbed polyelectrolyte layer. The measured forces are shown to be in semiquantitative agreement with model calculations based on charge distributions with square lattice symmetry.

Polyelectrolytes adsorb strongly at oppositely charged surfaces, thereby dramatically influencing the corresponding interaction forces. In this letter, we report on direct force measurements with the atomic force microscope (AFM) between two individual particles in an aqueous colloidal suspension in the presence of polyelectrolytes near the isoelectric point. From systematic variations of the molecular mass, the ionic strength, and analysis of adhesion events, we conclude that the observed attractive forces are mainly due to electrostatic patch-charge interactions. The same type of attractive forces is equally influencing interactions between proteins as well as hydrophobic or mineral surfaces.

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Interaction forces between a spherical silica probe and a cross-linked poly(dimethylsiloxane) (PDMS) colloidal droplet have been determined by atomic force microscopy (AFM). On approach of the surfaces, as a result of deformation of the PDMS, the repulsive force increases much less rapidly than expected for electrical double layer interaction of rigid particles. The amount of deformation is dependent on the loading force and the drive velocity and reflects the nanorheological response of the PDMS. On retraction of the surfaces, force curve hysteresis is observed and is dependent on the rate of approach/retraction. Hysteresis is due to the viscoelastic response of the PDMS droplet and can be described by theory (Attard, P. Phys. Rev. E 2001, 63, 061604). The moduli and the characteristic relaxation time of the PDMS colloid have been determined from the force measurements and offer a novel description of the nanorheological properties.