Salt concentration and particle density dependence of electrophoretic mobilities of spherical colloids in aqueous suspension.
ABSTRACT Using laser Doppler velocimetry in the superheterodyne mode, we conducted a systematic study of the electrophoretic mobility of dispersions of small silica spheres (a=18 nm) suspended in water at different salinities and particle concentrations. The concentration of NaCl was varied from 40 microM up to 16 mM, while the particle concentrations were varied between 4.2x10(18) and 2.1x10(20) m-3. We find a decrease of mobility with increasing salt concentrations and an increase with increased particle number densities. The latter observation is not backed by the standard cell model of electrophoresis with Shilov-Zharkikh boundary conditions. Rather, if the experimental data are interpreted within that model, an unexpected change of the zeta potential at constant added salt concentration results. Interestingly, all experimental data collapse onto a single master curve, if plotted versus the ratio C* of particle counterions to added salt ions. We obtain a logarithmic increase of mobility for C*<1 and a plateau for C*>1. This may indicate a change of the Stern layer structure not yet included in the theoretical model.
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ABSTRACT: We study the electrophoretic mobility of spherical particles and the electrical conductivity in salt-free concentrated suspensions including finite ion size effects. An ideal salt-free suspension is composed of just charged colloidal particles and the added counterions that counterbalance their surface charge. In a very recent paper [Roa et al., Phys. Chem. Chem. Phys., 2011, 13, 3960-3968] we presented a model for the equilibrium electric double layer for this kind of suspensions considering the size of the counterions, and now we extend this work to analyze the response of the suspension under a static external electric field. The numerical results show the high importance of such corrections for moderate to high particle charges, especially when a region of closest approach of the counterions to the particle surface is considered. The present work sets the basis for further theoretical models with finite ion size corrections, concerning particularly the ac electrokinetics and rheology of such systems.Physical Chemistry Chemical Physics 09/2011; 13(43):19437-48. · 3.83 Impact Factor
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ABSTRACT: On a superhydrophobic surface a liquid is exposed to a large air-water interface. The reduced wall friction is expected to cause a higher electro-osmotic mobility. On the other hand, the low charge density of a superhydrophobic surface reduces the electro-osmotic mobility. Due to a lack of experimental data it has not been clear so far whether the reduced wall friction or the reduced charge density dominate the electrokinetic mobilities. To separate the relative contributions of electrophoresis and electro-osmosis, the mobilities of colloids on a negatively charged hydrophilic, a superhydrophobic (Cassie) and a partially hydrophilized superhydrophobic (Cassie composite) coating were measured. To vary the charge density as well as its sign with respect to those of the colloids the partially hydrophilized surfaces were coated with polyelectrolytes. We analyzed the electrokinetic mobilities of negatively charged polystyrene colloids dispersed in aqueous medium on porous hydrophilic and superhydrophobic surfaces by confocal laser scanning electron microscopy. In all cases, the external electric field was parallel to the surface. The total electrokinetic mobilities on the superhydrophobic (Cassie) and negatively charged partially hydrophilized (Cassie composite) surfaces were similar, showing that electro-osmosis is small compared to electrophoresis. The positively charged Cassie composite surfaces tend to 'trap' the colloids due to attracting electrostatic interactions and rough morphology, reducing the mobility. Thus, either the charge density of the coatings in the Cassie composite state or its slip length is too low to enhance electro-osmosis.Journal of Physics Condensed Matter 11/2012; 24(46):464110. · 2.36 Impact Factor
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ABSTRACT: In this work we describe the fundamentals of the phenomenon of dielectric dispersion in aqueous colloidal suspensions, as well as the most recent advances on the subject. We begin by establishing the admitted definitions of the permittivity of a heterogeneous system consisting of a certain volume of a material dispersed in the form of identical spheres (the particles) in a liquid medium (an electrolyte solution). Attention is also paid to the relationship between the electric permittivity of the suspension and the strength and frequency dependence of the dipole moment induced by the external field.A thorough historical revision is provided, describing the key contributions, both experimental and theoretical, to the development of this field of electrokinetics and interface physics. In fact, elucidation of the mechanisms responsible for the values of the permittivity of disperse systems over a wide enough frequency range is a rich exercise in electromagnetism, fluid mechanics and electrochemistry of interfaces.Three mechanisms are typically responsible for the dielectric dispersion of the suspension. The gamma dispersion is a manifestation of the frequency dependence of the permittivity of the aqueous electrolyte solution where the particles are suspended. It is mainly determined by the polar nature of the water molecules and its characteristic frequency is in the GHz range. The delta dispersion (typically in the MHz range) is determined by the Maxwell–Wagner–O'Konski relaxation mechanism: it occurs because of the different permittivities and conductivities of the particle and the surrounding medium. Finally, the alpha- or Low Frequency Dielectric Dispersion (LFDD) is a phenomenon characterized by a huge increase of the permittivity at very low frequencies (kHz range). Its relationship with the phenomenon of concentration polarization is carefully discussed, as it is an essential feature of the electric permittivity of suspensions.The mathematical treatment of the problem is rather complex, and analytical solutions are only available in a limited number of cases. Attention is hence also devoted to describing and comparing the numerical approaches that can be used. Experimental determination of the (particularly low frequency) dielectric dispersion is complicated mainly because of the phenomenon of the polarization of the electrode–solution interface. In this contribution we describe the solutions reported to this problem, both in the frequency and in the time domains.An interesting aspect of dielectric dispersion determinations, not shared by other techniques, is their applicability to concentrated suspensions. The modifications of the theory of the permittivity of suspensions, required to account for the hydrodynamic and electrical interactions are also described, stressing the fact that suspensions often considered as dilute are actually far from being so.The review is finished with a description of the most recent advances, namely the consideration of suspensions of soft particles and extensions of the standard electrokinetic model in order to reach a better agreement between theory and experiments. The conclusion of the work refers to the expected developments, particularly in the field of experimental determinations (mainly in the high frequency side of the dispersion), and of descriptions of the solid/liquid interface with corresponding extensions of the standard electrokinetic model.Current Opinion in Colloid & Interface Science - CURR OPIN COLLOID INTERFACE S. 01/2010; 15(3):145-159.