- The Surface Charge of Soft and Hard Sphere Colloidal Particles - Experimental Investigation and Comparison to Theory (2011)
- The focus of this thesis was aimed at the investigation of colloidal particle stability. In a first step we established a method to assess the repulsive interaction energy of dispersed colloids based on the measurement of the rate of slow coagulation with light scattering. Due to an energy resolution in the order of magnitude of the thermal energy, the method was termed microsurface potential measurements (MSPM). We then used the MSPM to measure the potential at the outer Helmholtz plane (oHp), the diffuse potential, which determines the electric double layer of surface charged colloidal particles. The MSPM were performed on anionic particles in the presence of di- and trivalent counterions as a function of the bulk electrolyte concentration. We found that the the diffuse potential does only weakly depend on the magnesium but strongly on the lanthanum ion concentration. In both cases the absolute value of the the diffuse potential decreases with increasing electrolyte concentration. The absolute values of the the diffuse potential are always lower for the trivalent counterions as compared to the divalent results. To supplement the results of the MSPM, we measured the zeta-potential of the particles under similar conditions. Here we detected charge reversal in the experiments with the di- and trivalent counterions. In the salt concentration range of the MSPM the zeta-potential and the the diffuse potential were closely related for both ion species but could not be described by Poisson-Boltzmann based models. In the case of the trivalent counterions, we could experimentally verify the strong influence of counterion adsorption in the destabilization of the surface charged colloids. Furthermore, we showed that the zeta-potential is not suited for calculating the particle stability in the experiments involving trivalent counterions and found strong experimental indications for counterion correlations. We also used MSPM to investigate an anionic SPB in the presence of trivalent counterions. For this purpose we measured the interaction force of two planar polyelectrolyte brush layers across an aqueous medium containing trivalent counterions with the surface forces apparatus. We found that steric repulsion does not occur. The repulsion only arises from residual charges inside the brush layers. From the resulting force curves we were able to deduce an interaction profile of SPB particles in aqueous solution containing multivalent counterions. Thus, we were able to measure the effective repulsive energy of SPB particles using MSPM with an accuracy of the thermal energy. Due to the increase of confined lanthanum counterions in the brush layer the electrostatic repulsion decreased with rising lanthanum concentration. Furthermore, the experimental results were well predicted by a mean-field model. For the first time, we described the means to measure and predict the repulsive energies of SPB particles in aqueous solution in the presence of multivalent counterions. In a next step we refined the theoretical basis of the MSPM and expanded the electrolyte concentration range of the stability experiments. We also measured the form factors of the SPB doublets and found pronounced deviations between the data points and the predictions of the Rayleigh-Debye approximation. We showed that the MSPM are now accurate enough to measure the effective charges per SPB particle with a sub millimolar concentration resolution. Furthermore, we used the mean-field model to predict the particle stability and the effective charge per SPB particle. In both cases we found the deviations between the experimental data and the model to be within an error margin of 20%. Therefore we predicted the particle stability of SPBs in aqueous solution for the first time. In conclusion, this thesis provides a deeper insight into the mechanisms of particle stability and coagulation of electrostatically and electrosterically stabilized dispersions. It offers a new method to investigate the repulsive interactions between colloidal particles which is applicable to a wide variety of colloidal systems. Moreover, we made the first steps toward a more complete understanding of the stability of SPB particles, which is important for potential industrial applications of these kind of systems.