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This article was written by Volker Knecht . He is a research group leader at the Max-Planck Institute of Colloids and Interfaces at Golm, Germany and an expert in Molecular Dynamics Simulations. Electrokinetic phenomena are characterized by a coupling of an electric field parallel to two phases in contact and a relative tangential movement of the two phases. The most known electrokinetic phenomenon is electrophoresis which was first discovered by Reuss in 1809. Electrophoresis is the migration of a dispersed particle in a static and homogeneous electric field, the migration rate being proportional to the field applied. Electrophoretic activity is generally assumed to arise from a net charge on the particle due to ions at the interface. Continuum theories predict the charge and the electrophoretic mobility of a particle (i.e. its migration rate as a function of field intensity) to be equal in sign. Based on this theory, electrophoresis is used as an experimental means to probe the charge of colloidal particles which is of major interest in colloid and electrochemistry. Remarkably, oil droplets exhibit negative electrophoretic mobilities. This is explained in terms of adsorption of hydroxyl ions to the interface giving rise to a negative surface charge. Recent molecular dynamics simulations, however, showed electrophoresis of oil droplets in the absence of hydroxyl or any other ions, challenging the current theory of electrokinetic phenomena (Journal of Colloid and Interface Science 318, p. 477, 2008 ).  Electrophoresis of oil droplets (heptane with chemical formula CH3(CH2)5CH3) in water (H2O) in the absence of ions in molecular dynamics simulations. Left: (Periodic) simulation box (black frame) with direction of the external electric field. Middle: Heptane droplet of diameter 9 nm (dark gray) in water (white) moving in negative field direction. Right top: Heptane-water interface. Right bottom: Single heptane and water molecule. CH3 and CH2 groups in heptane were treated as compound atoms. For heptane, covalent bonds are depicted as white lines, and one of the bond torsion angles is indicated. For water, partial charges on oxygen and hydrogen atoms are indicated.
Nevertheless, due to the small time steps required and the large number of degrees of freedom involved, such calculations involve an enormous computational expense such that only time and length scales of the order of nanoseconds and nanometers, respectively, can be assessed. For the study of electrophoresis, this meant that field intensities several orders of magnitude larger than those employed in typical electrophoresis experiments had to be used to induce significant electrophoretic motion on the timescales of the simulations. However, the migration rate scaled linearly with field intensity, indicating that nonlinear effects are not significant and the electrophoretic mobilities observed at high field intensities can be related to available experimental data. Due to the limited length scale, either oil droplets two orders of magnitude smaller than observed in experiment or only a cutout of a larger system including a planar water/oil interface parallel to an external electric field could be simulated. The same qualitative behavior though, a relative movement of the oil relative to the water phase in negative field direction, was observed in both cases. Simulation results also critically depend on the models used. Tests were thus performed to study possible effects from the water model or the boundary conditions of electrostatic interactions employed. All those simulations yielded similar results. The simulation results suggest that an electrophoretic force may arise from the specific molecular structure of interfacial water giving rise to gradients in water dipole energies. The precise mechanism, however, remains unclear. Electrophoresis as all electrokinetic phenomena is generally explained in terms of net charges on phases in contact based on continuum theory. The molecular dynamics study of oil droplets, however, reveals that charge is not always required for electrophoretic activity. This suggests that deducing charges of colloidal particles from electrophoretic mobilities may be misleading and that a fundamental understanding of electrokinetic phenomena necessitates a molecular description. |
Molecular Dynamics 
