Electrophoretic Retardation of Colloidal Particles in Nonpolar Liquids (original) (raw)
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Electrokinetics of colloidal particles in nonpolar media containing charged inverse micelles
Applied Physics Letters, 2008
We have compared optical tracking and electric current measurements to study the electrokinetics of colloidal particles in nonpolar media containing charged inverse micelles. Particle trajectories are measured in response to a voltage step, revealing spatial and temporal variations of the electric field when space-charge layers are created by charged inverse micelles. The particle trajectories and current measurements are in good agreement with simulations and analytical approximations based on drift and diffusion of charges. Electrohydrodynamic effects observed at high concentrations of charged inverse micelles are explained by injection of charged inverse micelles from the bulk into the space-charge layers.
Electrophoretic mobility of model colloids and overcharging: theory and experiment
Molecular Physics, 2003
Several theories claim that ion±ion correlations play an important role in the electric double layer of colloids. One of the most outstanding predictions is overcharging, which would take place at high electrolyte concentrations and surface charge densities. The counterion concentration next to the surface can become so large that the particle charge is overcompensated and the overcharging occurs. Sometimes this would also involve a reversal of the ± potential, but this phenomenon has been observed rarely through mobility measurements. This study explores the matter further. The electrophoretic mobility is measured for latex particles with moderate and extremely large surface charge densities at high ionic strengths (up to 2 M) in solutions of symmetric electrolytes. The results are analysed within the so-called hypernetted chain/mean-spherical approximation (HNC/MSA) and a Poisson±Boltzmann approach. In this way, the relevance of ion±ion correlations in practice and the occurrence of overcharging are probed experimentally.
Fast and precise measurements of particle charge with optical trapping electrophoresis
Complex Light and Optical Forces IV, 2010
We subject micrometer-sized, optically trapped colloidal particles in a non-polar liquid to a sinusoidally varying electric field, and measure their resulting movement. From this movement, we calculate the electrophoretic mobility and charge of the particle in the liquid. The use of high frequencies of the electric field (well above the corner frequency of the optical tweezers) allows us to estimate the electrical charge of colloidal particles with an accuracy of the order of the electron charge in a time interval of only 10 ms. This technique can be used to provide valuable information about the dynamics of the poorly understood processes that lead to the charge on colloidal particles in non-polar liquids.
Electrophoretic flow behaviour and mobility of colloidal fluids and crystals
2007
We report on measurements of the electrophoretic mobility μ of charged colloidal spheres in the deionized state, where the suspensions show fluid or crystalline order. In the fluid state, parabolic flow profiles are observed due to electro-osmotic solvent flow. In the crystalline state, complex flow profiles occur due to additional crystal cohesion. The mobility μ then may inferred from the flow velocity averaged over the complete cell cross section as performed in our home built super-heterodyne Doppler velocimeter. For two particle species of 68 and 122 nm diameter we measured μ as a function of particle concentration. Starting from a plateau value at low concentration, μ decreases approximately logarithmically with increased concentration. Interestingly, the decrease of μ is not affected by the phase transition, indicating that electro-kinetic properties may be viewed as single particle properties even in the case of structure formation. Moving further along this line of thought, we show that this behaviour may indeed be captured near quantitatively by an ad hoc combination of charge renormalization calculations and the Standard Electro-kinetic Model originally designed for isolated, non-interacting particles. From this coincidence, we may conclude that (i) counter-ion condensation behind the hydrodynamic slip plane is the dominant effect of finite particle concentrations and leads to a decrease of the effective particle charge and (ii) an increased colloid concentration can be understood in terms of an elevated total ion concentration and that (iii) it can thus be reduced to a modification of the screening parameter, such that the problem can be mapped onto a single particle theory for an infinite electrolyte.
Physical Review E - Statistical, Nonlinear, and Soft Matter Physics, 2013
Optical Tweezers are employed to study the electrophoretic and the electroosmotic motion of a single colloid immersed in electrolyte solutions of ion concentrations between 10 −5 and 1 mol/l and of different valencies (KCl, CaCl 2 , LaCl 3 ). The measured particle mobility in monovalent salt is found to be in agreement with computations combining primitive model molecular dynamics simulations of the ionic double layer with the standard electrokinetic model. Mobility reversal of a single colloid-for the first time-is observed in the presence of trivalent ions (LaCl 3 ) at ionic strengths larger than 10 −2 mol/l. In this case, our numerical model is in a quantitative agreement with the experiment only when ion specific attractive forces are added to the primitive model, demonstrating that at low colloidal charge densities, ion correlation effects alone do not suffice to produce mobility reversal.
Macromolecules, 1990
The effect of adsorbed polyelectrolytes and uncharged polymers on the 'diffusiophoresis" of charged colloids suspended in electrolyte gradients is investigated theoretically. It is known that the diffusiophoretic velocity of colloidal particles has two components: chemiphoretic and electrophoretic. Semianalytic expressions are derived for two parameters A(c) and A(e), which denote the weighting factors by which each of these velocity components is altered due to polymer adsorption. These factors are shown to depend on four dimensionless parameters: a, 8, B, and ((or Z), where a characterizes the distance to which the segments of the adsorbed polymer extend into the solution (and is the same as the "shielding ratio" defied by Varoqui and Dejardin),' 0 characterizes a dimensionless Debye l_ength, B corresponds to the degree of dissociation of the ionizable groups of the adsorbed polyelectrolyte, and { (or 2) is the zeta potential (or surface charge density) of the particle. The alteration in the particle's velocity due to polymer adsorption is estimated for various values of these dimensionless parameters. It is observed that polyelectrolyte adsorption, in certain situations, could lead to (i) particle migration velocities higher than those possible with 'clean" particles under otherwise identical conditions and (ii) a reversal in the direction of migration. The adsorption of uncharged polymers, on the other hand, leads to a reduction in the particle velocity, if the two parameters { and @-a normalized dimensionless mobility difference of the two ions of the electrolyte-have the same sign. However, when and j3 are of opposite signs, a reversal in the direction of particle migration is possible even with uncharged polymers under some conditions. Finally, an offshoot of the present analysis is the additional result that A(s) also represents the factor by which adsorbed polyelectrolytes/uncharged polymers alter the electrophoretic mobilities of colloidal particles subjected to electrophoresis by an externally applied electric field.
Electrophoretic mobility of a charged colloidal particle: a computer simulation study
Journal of Physics: Condensed Matter, 2004
We study the mobility of a charged colloidal particle in a constant homogeneous electric field by means of computer simulations. The simulation method combines a lattice Boltzmann scheme for the fluid with standard Langevin dynamics for the colloidal particle, which is built up from a net of bonded particles forming the surface of the colloid. The coupling between the two subsystems is introduced via friction forces. In addition, explicit counterions, also coupled to the fluid, are present. We observe a non-monotonic dependence of the electrophoretic mobility on the bare colloidal charge. At low surface charge density we observe a linear increase of the mobility with bare charge, whereas at higher charges, where more than half of the ions are co-moving with the colloid, the mobility decreases with increasing bare charge.
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2003
Electrokinetic phenomena are considered in a dilute suspension of spherical colloidal particles in a salt-free medium containing counter-ions only. Simple derivation is given for the electrophoretic mobility of the particles. Expressions for the electrical conductivity and the sedimentation potential are also obtained. An Onsager relation is found to hold between electrophoretic mobility and sedimentation potential. It is shown that if the particle charge Q is lower than a certain critical value, then the electrophoretic mobility and the electrical conductivity due to counter-ions increase linearly with increasing Q . If Q is higher than the critical value, then these quantities become constant independent of Q because of counter-ion condensation effects. The sedimentation potential, however, is not subject to the counter-ion condensation effects, since these effects upon the electrical conductivity and the concentration of the counter-ions cancel out with each other. #
Colloid Electrophoresis for Strong and Weak Ion Diffusivity
Physical Review Letters, 2011
We study the electrophoretic flow of suspensions of charged colloids with a mesoscopic method that allows to model generic experimental conditions. We show that for highly charged colloids their electrophoretic mobility increases significantly and displays a mobility maximum on increasing the colloidal charge for all salt concentrations. The electrophoretic mobility of highly charged colloids is also enhanced significantly when ion advection is dominant, leading to a strong heterogeneity in the local electrophoretic response especially at low salt concentration, when ion diffuse layers overlap.
Determination of the effective charge of individual colloidal particles
Journal of Colloid and Interface Science, 2006
An optical method is presented that allows simultaneous determination of the diffusion constant and electrophoretic mobility of individual charged particles with radius down to 0.2 µm. By this method the size dependency of the effective charges and zeta potentials of individual particles can be investigated, as well as interparticle interactions and Brownian motion in confined geometries. The diffusion constant and mobility are determined from the power spectrum of the particle speed in a sinusoidal electrical field. The accuracy of the method was tested on PMMA spheres of known size in water. Experiments have been carried out on charged pigment particles with low concentration in a nonaqueous medium containing a charging agent. The mobility is found to be independent of the particle size.