Electrophoresis of a charge-inverted macroion complex: Molecular-dynamics study (original) (raw)
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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.
The Journal of Chemical Physics, 2001
We report molecular dynamics simulation of the (overall neutral) system consisting of an immobile macroion surrounded by the electrolyte of multivalent counterions and monovalent coions. As expected theoretically, counterions adsorb on the macroion surface in the amount much exceeding neutralization requirement, thus effectively inverting the sign of the macroion charge. We find two conditions necessary for charge inversion, namely, counterions must be multivalently charged and Coulomb interactions must be strong enough compared to thermal energy. On the other hand, coion condensation on the multivalent counterions similar to Bjerrum pairing is the major factor restricting the amount of charge inversion. Depending on parameters, we observe inverted charge up to about 200% the original charge of the macroion in absolute value. The inverted charge scales as ∼ ζ 1/2 when ζ < 1 and crosses over to ∼ ζ for ζ > 1, where ζ = (A0/rs) 2 , rs is the Debye screening length in the electrolyte and A0 is the distance between adsorbed counterions under neutralizing conditions. These findings are consistent with the theory of "giant charge inversion" [
Electric and Electrophoretic Inversion of the DNA Charge in Multivalent Electrolytes
Soft matter, 2010
Counterion-induced inversion of the DNA charge was characterized through extensive molecular dynamics simulations. We observed reversal of the DNA motion in an external electric field upon increasing the concentration of trivalent or quadrivalent counterions. In the case of a divalent electrolyte, inversion of the DNA's electric charge was observed at high concentrations of the electrolyte but not reversal of the DNA' electrophoretic motion. We demonstrate that inversion of the DNA's electrophoretic mobility results from a complex interplay of electrostatics and hydrodynamics.
Colloquium: The physics of charge inversion in chemical and biological systems
Reviews of Modern Physics, 2002
The authors review recent advances in the physics of strongly interacting charged systems functioning in water at room temperature. In these systems, many phenomena go beyond the framework of mean-field theories, whether linear Debye-Hü ckel or nonlinear Poisson-Boltzmann, culminating in charge inversion-a counterintuitive phenomenon in which a strongly charged particle, called a macroion, binds so many counterions that its net charge changes sign. The review discusses the universal theory of charge inversion based on the idea of a strongly correlated liquid of adsorbed counterions, similar to a Wigner crystal. This theory has a vast array of applications, particularly in biology and chemistry; for example, in the presence of positive multivalent ions (e.g., polycations), the DNA double helix acquires a net positive charge and drifts as a positive particle in an electric field. This simplifies DNA uptake by the cell as needed for gene therapy, because the cell membrane is negatively charged. Analogies of charge inversion to other fields of physics are also discussed.
Screening of a macroion by multivalent ions: correlation-induced inversion of charge
Physical review. E, Statistical physics, plasmas, fluids, and related interdisciplinary topics, 1999
Screening of a strongly charged macroion by multivalent counterions is considered. It is shown that counterions form a strongly correlated liquid at the surface of the macroion. Cohesive energy of this liquid leads to additional attraction of counterions to the surface, which is absent in conventional solutions of the Poisson-Boltzmann equation. Away from the surface this attraction can be taken into account by a new boundary condition for the concentration of counterions near the surface. The Poisson-Boltzmann equation is solved with this boundary condition for a charged flat surface, a cylinder, and a sphere. In all three cases, screening is much stronger than in the conventional approach. At some critical exponentially small concentration of multivalent counterions in the solution, they totally neutralize the surface charge at small distances from the surface. At larger concentrations they invert the sign of the net macroion charge. The absolute value of the inverted charge densi...
The electrophoretic motion of cylindrical macroions inside a nanochannel using molecular dynamics simulation, 2023
This paper examines the electrophoretic motion of a cylindrical macroion floating in a solution of water and NaCl inside a silicon nanochannel using molecular dynamics simulation. The aim is to investigate the effects of parameters like free movement, initial orientation, shape, aspect ratio, surface charge density and initial electrical charge of macroion on the electrophoretic motion. Findings suggest that any increase in surface charge density could give a rise to electrophoretic velocity of the solution. Considering the macroion free to move around, initial orientation does not play significant role in final results. Results also imply that macroion's velocity does not change steadily by variations in surface charge density which is because of the multilayer structure of the Stern layer. Finally, average electrophoretic velocity of the solution for all scenarios in this study is witnessed to be the same. However, maximum electrophoretic velocity could be four times greater than average values. This must be taken into account in applications with limited maximum velocity.
Salt-Induced Counterion-Mobility Anomaly in Polyelectrolyte Electrophoresis
Physical Review Letters, 2008
We study the electrokinetics of a single polyelectrolyte chain in salt solution using hydrodynamic simulations. The salt-dependent chain mobility compares well with experimental DNA data. The mobility of condensed counterions exhibits a salt-dependent change of sign, an anomaly that is also reflected in the counterion excess conductivity. Using Green's function techniques this anomaly is explained by electrostatic screening of the hydrodynamic interactions between chain and counterions.
Screening of a charged particle by multivalent counterions in salty water: Strong charge inversion
The Journal of Chemical Physics, 2000
Screening of a macroion such as a charged solid particle, a charged membrane, double helix DNA or actin by multivalent counterions is considered. Small colloidal particles, charged micelles, short or long polyelectrolytes can play the role of multivalent counterions. Due to strong lateral repulsion at the surface of macroion such multivalent counterions form a strongly correlated liquid, with the short range order resembling that of a Wigner crystal. These correlations create additional binding of multivalent counterions to the macroion surface with binding energy larger than kBT . As a result even for a moderate concentration of multivalent counterions in the solution, their total charge at the surface of macroion exceeds the bare macroion charge in absolute value. Therefore, the net charge of the macroion inverts its sign. In the presence of a high concentration of monovalent salt the absolute value of inverted charge can be larger than the bare one. This strong inversion of charge can be observed by electrophoresis or by direct counting of multivalent counterions.
Electrostatics in macromolecular solutions
An overview of the interaction between charged macromolecules in aqueous solution is presented. The starting point is the dielectric continuum model and the Debye-H¨uckel equation. The usefulness of the simple theory is emphasized in particular for biological macromolecules, whose net charge or surface charge density often is low. With more highly charged macromolecules or aggregates it may be necessary to go beyond the simple Debye-H¨uckel theory and invoke the non-linear Poisson-Boltzmann equation or even to approach an exact solution using Monte Carlo simulations or similar techniques. The latter approach becomes indispensable when studying systems with divalent or multivalent (counter)-ions. The long range character of the electrostatic interactions means that charged systems of varying geometry - spheres, planes, cylinders... - often have many properties in common. Another consequence is that the detailed charge distribution on a macromolecule is less important. Many biological macromolecules contain titratable groups, which means that the net charge will vary as a consequence of solution conditions. This gives an extra attractive contribution to the interaction between two macromolecules, which might be particularly important close to their respective isoelectric points. The treatment of flexible polyelectrolytes/polyampholytes requires some extra efforts in order to handle the increasingly complex geometry. A theoretical consequence is that the number of parameters - chain length, charge density, polydispersity etc - prohibits the presentation of a simple unified picture. An additional experimental, and theoretical, difficulty in this context is the slow approach towards equilibrium, in particular with high molecular weight polymers. A few generic situations where polyelectrolytes can act both as stabilizers and coagulants can, however, be demonstrated using simulation techniques.
Molecular mechanics and electrostatic effects
Biophysical Chemistry, 1994
Continuum solvent models predict a quadratic charge dependence (linear response) of the free energy of a system of charged solutes. The relation between this prediction and the structure of the salvation shell around the solutes is discussed. Studies of the derivative of the free energy with respect to the charges for different reference states are shown to be a convenient way of testing the linear response assumption without resorting to the standard free energy perturbation method. We illustrate this with a system of two oppositely charged ions in aqueous solution, where nonlinearities are observed before the full charging process is completed. Since molecular mechanics (MM) simulations preserve the full nonlinearity of the problem, they are well suited to the investigation of the conditions under which linear response accurately reflects the behavior of the system. The error when using linear response theory to calculate the free energies of charging is estimated to be as large as lo-20%.