Electrodynamics of colloids (1998)

The goal of the present study is to deepen the insight into the non-equilibrium properties of the electric double layer of colloidal systems. Of basic interest are the ionic mobilities in the different regions of the electric double layer as well as the potential at the plane of shear, i.e., the electrokinetic potential (ζ-potential). These parameters determine the colloidal behaviour under non-equilibrium conditions when the double layer is perturbed, for instance if external fields are applied and in particle-particle interaction during coagulation.One of the experimental methods utilized in this study is the measurement of the conductivity and the streaming potential of close-packed plugs of particles. From the resulting data we retrieved the dzeta.gif -potential, the surface conductivity, and the mobility of the counterions behind the plane of shear. The results are well comparable to those from the experimental low-frequency (LF) dielectric response of dilute dispersions of latex particles.The electrodynamic parameters can be influenced by adsorbing neutral polymer onto the surfaceIt is shown that the ζ-potential as well as the mobilities of the ions behind the plane of shear are decreased by the polymer film.The data in the above studies were successfully interpreted under the assumption of local equilibrium between the (complete) electric double layer and the adjacent electrolyte. However, there are double-layer conditions where this assumption is violated. In order to study these, we theoretically investigated the influence of relaxation of the compact part of the double layer (occupied inner-Helmholtz Stern layer) on the LF dielectric response and electrophoretic mobility. Possible relaxation mechanisms are retarded adsorption/desorption and ion migration along the surface. Along the same lines, the stability of the sol against coagulation was expressed in terms of the relaxation characteristics of the Stem layer.Chapter 2 dealt with the determination of plug conductivities and streaming potentials of a close-packed porous plug of latex particles for a number of indifferent electrolytes and ionic strengths. From these, the dzeta.gif -potentials and surface conductivities were computed. Monodisperse sulphate latex is an ideal model system since the surface charge consists of strong acidic groups so that a constant surface charge density is maintained throughout all the experiments. It was shown that the surface conductivity is insensitive to the ionic strength and that a large part of the countercharge is situated behind the shear plane. Furthermore, it was demonstrated that the ions in the double layer have a mobility close to the bulk mobility.<In chapter 3 practical expressions were developed for the low-frequency (LF) dielectric response of dilute dispersions of spherical particles suspended in a binary electrolyte. The LF dielectric response of dilute sulphate latex dispersions was experimentally determined in the frequency range of 500 Hz to 500 kHz as a function of the ionic strength of suspending KCI. The resulting surface conductivities are insensitive to the ionic strength and practically identical to the values obtained by steady state methods (chapter 2). It was proposed that counterion motion can be retarded by specific interaction with the surface and by neutral polymer hairs present on the surface. In order to test the latter effect, the influence of the adsorption of uncharged polymer poly(ethylene) oxide onto the latex surface was investigated by means of LF spectroscopy, plug conductivities and streaming potentials of plugs in chapter 4. It was found that the polymer film on the surface reduces the surface conductivity. The drag on the ions in the polymer film can be described by considering the polymer layer as an inhomogeneous Brinkman fluid, characterised by a Darcy permeability which depends on the local polymer volume fraction. The polymer and counterion distributions were calculated from statistical self-consistent field lattice models.<In order to investigate the influence of the surface charge density on the streaming potential and static conductivity, plugs of monodisperse spherical Stöber-silica particles were studied in chapter 5. Contrary to the latex, the surface charge density of silica can be controlled by pH. The high-charge silica plug showed more surface conduction than the low-charge plug since more mobile counterions are present in the double layer of the former. Stöber-silica particles are highly porous. For the relatively large particles under consideration, the major part of the countercharge is situated in the micropores of the particles. It was shown that these counterions do not contribute to the plug conductivity because of their low mobility.Chapter 6 analysed the dynamic aspects of particle electrophoresis. It was shown theoretically as well as experimentally that colloidal particles respond to an applied electric field much faster than does the liquid inside a measuring capillary. Therefore, it is possible to apply an alternating electric field with such a frequency that unwanted electroosmosis, induced by charge on the capillary wall, is suppressed, whereas the particles are still able to follow the field according to their dc mobility. This study illustrates that knowledge of the dynamics and the corresponding relaxation times is not only of purely scientific interest, but that it also offers solutions to very practical problems.In chapter 7 the influence of polarization of surface charge (or charge in an inner-Helmholtz layer) on the particle mobility, static conductivity, and low-frequency dielectric response was studied within the framework of the thin double-layer theory. It was shown that the characteristic times of relaxation processes in the Stern layer are accessible from dielectric spectroscopy. The relaxation phenomena under consideration are Stern-layer polarization via retarded adsorption/desorption and polarization via lateral transport in the Stem layer. The two processes may occur simultaneously. Since these relaxation processes are also relevant for particle-particle interaction, chapter 8 considered the implications for colloidal stability. In the situation of small transient disequilibrations of the surface charge, the stability could be expressed in terms of the characteristic times of surface charge relaxation. This allows the use of electrodynamic data obtained by dielectric spectroscopy in the interpretation of colloidal stability. On an even more rigorous level, the free energy of particle-particle interaction was also considered in the space of the two variables surface charge and separation. This formalism opens the way to investigate coagulation far from equilibrium.