Electrophoretic mobility and charge inversion of a colloidal particle studied by single-colloid electrophoresis and molecular dynamics simulations

Abstract

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.

Single Molecular Demonstration of Modulating Charge Inversion of DNA

“…For DNA systems, only quadrivalent counterions of high concentration can lead to the CI, which is deviated from the theoretical prediction of strong correlation theory significantly. Recently, optical tweezers (OT) were employed to study the electrophoretic and the electro osmotic motion of a single colloid immersed in electrolyte solutions of different valences, and it is found that at low colloidal charge densities, ion correlation effects alone do not suffice to produce mobility reversal8. On the other hand, Lösche group demonstrated the CI of a lipid monolayer at ultralow electrolyte concentrations, much lower than the predicted of ion-ions correlation theories.…”

Computer simulations of single particles in external electric fields

“…This overcharging phenomenon can be explained by ion correlations [59] and has also been observed experimentally. In experiments, however, the effect is sometimes larger than predicted by numerical simulations, suggesting that it might be enforced by ion specific attractive forces [60]. Recent simulations by Raafatnia et al have shown that overcharging may even occur in electrolytes containing only monovalent ions if the colloids are coated by a suitable organic layer [61,62].…”

Optofluid-Based Reflective Displays

“…Electrophoresis causes the movement of charged particles through the stationary insulating liquid. The conventional Helmholtz–Smoluchowski equation is applied in EPDs [12]:$U=\frac{\epsilon {\xi }_{EP}{E}_x}{\mu }$ where U is the electrophoretic velocity of the particle, ε is the dielectric constant of the insulating liquid, ξ EP is the zeta potential of the particle, E x is the applied electrical field, and μ is the mobility of the particle. The electrophoretic zeta potential ( ξ EP ) is a property of the charged particle.…”