Electrophoresis of pH-regulated, zwitterionic particles: Effect of self-induced nonuniform surface charge

Electrokinetics of the silica and aqueous electrolyte solution interface: Viscoelectric effects

The manipulation of biomolecules, fluid and ionic current in a new breed of integrated nanofluidic devices requires a quantitative understanding of electrokinetics at the silica/water interface. The conventional capacitor-based electrokinetic Electric Double Layer (EDL) models for this interface have some known shortcomings, as evidenced by a lack of consistency within the literature for the (i) equilibrium constants of surface silanol groups, (ii) Stern layer capacitance, (iii) zeta (ζ) potential measured by various electrokinetic methods, and (iv) surface conductivity. In this study, we consider how the experimentally observable viscoelectric effect - that is, the increase of the local viscosity due to the polarisation of polar solvents - affects electrokinetcs at the silica/water interface. Specifically we consider how a model that considers viscoelectric effects (the VE model) performs against two conventional electrokinetic models, namely the Gouy-Chapman (GC) and Basic Stern capacitance (BS) models, in predicting four fundamental electrokinetic phenomena: electrophoresis, electroosmosis, streaming current and streaming potential. It is found that at moderate to high salt concentrations (>5×10(-3)M) predictions from the VE model are in quantitative agreement with experimental electrokinetic measurements when the sole additional adjustable parameter, the viscoelectric coefficient, is set equal to a value given by a previous independent measurement. In contrast neither the GS nor BS models is able to reproduce all experimental data over the same concentration range using a single, robust set of parameters. Significantly, we also show that the streaming current and potential in the moderate to high surface charge range are insensitive to surface charge behaviour (including capacitances) when viscoelectric effects are considered, in difference to models that do not consider these effects. This strongly questions the validity of using pressure based electrokinetic experiments to measure surface charge characteristics within this experimentally relevant high pH and moderate to high salt concentration range. At low salt concentrations (<5×10(-3)M) we find that there is a lack of consistency in previously measured channel conductivities conducted under similar solution conditions (pH, salt concentration), preventing a conclusive assessment of any model suitability in this regime.

Hydrogel charge regulation and electrolyte ion-concentration perturbations in nanoparticle gel electrophoresis

Gel electrophoresis of spherical nanoparticles (NPs) is studied using an electrokinetic model that couples the ion conservation equations to the Poisson and fluid momentum equations, thus including the so-called polarization and relaxation processes. This model is therefore the charged gel electrophoresis analogue of the well-known O’Brien and White solution of the standard electrokinetic model for free-solution electrophoresis. Results are provided for the small NPs (size around 10 nm) to which gel electrophoresis is relevant, because particles must be small enough to permeate the gel: these include the particle drag coefficient (or Brownian diffusivity), which is subject to hydrodynamic screening and electroviscous effects, and the electrophoretic mobility, which is subject to nonlinear electrostatic and charge polarization influences. Also addressed are the influences of charge-regulating gels and the accompanying particle-induced immobile charge-density perturbations. Ion-concentration perturbations attenuate the electrophoretic mobility and enhance the drag coefficient according to the particle charge and the mobility of the most abundant counterion. However, dynamic regulation of the hydrogel charge—termed the secondary immobile charge-density perturbation—has a negligible influence on the particle mobility, and may therefore be neglected for most practical purposes.

Corona charge regulation in nanoparticle electrophoresis

Nanoparticle (NP) size and charge play key roles in bioconjugation chemistry, imaging and drug delivery. Although the electrophoretic mobility and hydrodynamic size are routinely measured, interpreting these data can be extremely difficult. Here, the challenge is addressed via an electrokinetic model for spheres bearing a soft amphoteric corona, the charge of which is regulated by a multi-component electrolyte. The model is applied to NPs with a metallic core to which are grafted poly(ethylene glycol) chains with either weak acid or amphiprotic end groups. The results elucidate the separate roles of electrolyte pH and ionic strength on the electrophoretic mobility and diffusion coefficient. In this study, the forces were evaluated directly, rather than from the Stokeslet velocity disturbances. While the second-order convergence was demonstrated by both methods, the direct approach, which uses only the inner part of the global solution, furnished superior accuracy and robustness. This may benefit future attempts to model the dielectric and electroacoustic properties of these complex nanoparticulates.

Electrophoresis of pH-regulated nanoparticles: impact of the Stern layer

A multi-ion model taking into account the Stern layer effect and the surface chemistry reactions is developed for the first time to investigate the surface charge properties and electrophoresis of pH-regulated silica nanoparticles (NPs). The applicability of the model is validated by comparing its prediction to the experimental data of the electrophoretic mobility of silica NPs available from the literature. Results show that if the particle size is fixed, the Stern layer effect on the surface charge properties of the NP is notable at high pH and background salt concentration; however, that effect on the particle mobility is significant when pH is around neutrality and the salt concentration is medium high (ca. 0.07 M) because of the double-layer polarization effect. Moreover, if pH and the background salt concentration are fixed, the Stern layer effect on the zeta potential and electrophoretic mobility of the NP becomes more significant for smaller particle size. Neglecting the Stern layer effect could result in the overestimation of the zeta potential, surface charge density, and electrophoretic mobility of a NP on the order of several times.

Influence of polyelectrolyte shape on its sedimentation behavior: effect of relaxation electric field

The sedimentation of an isolated, charged polyelectrolyte (PE) subjected to an applied field is modeled theoretically, taking into account the variation of its shape. In particular, the effects of double-layer relaxation, effective charge density, and strength of the induced relaxation electric field are examined. We show that the interaction of these effects yields complex and interesting sedimentation behaviors. For example, the behavior of the electric force acting on a loosely structured PE can be different from that on a compactly structured one; the former is dominated mainly by the convective fluid flow. For thick double layers, electric force has a local maximum as the Reynolds number varies, but tends to increase monotonically with increasing Reynolds number if the layer is thin. The drag factor is found to behave differently from literature results. The shape of a PE significantly influences its sedimentation behavior by affecting the amount of counterions attracted to its interior and the associated local electric field. Interestingly, a more stretched PE has a higher effective charge density but experiences a weaker electric force.

Simulation of polyelectrolyte electrophoresis: effects of the aspect ratio, double-layer polarization, effective charge, and electroosmotic flow

The electrophoresis of a deformable polyelectrolyte (PE) is studied theoretically by considering a Poisson-Nernst-Planck model coupled with modified Navier-Stokes equations, taking account of the effects of double-layer polarization, counterion condensation, and electroosmotic flow. The influences of the local electric field and the effective PE charge on the PE mobility are discussed, thereby providing a complete picture for the phenomenon under consideration. Our model explains successfully the presence of a local minimum in the mobility of a highly charged PE as the bulk salt concentration varies, as observed experimentally. Numerical simulation also reveals several interesting and important results. For example, the more a PE is stretched in the direction of electrophoresis, the larger is its mobility. As the double layer becomes thin, the local electric field becomes independent of the PE shape, and its behavior mainly depends upon its effective charge. We show that the force that stretches a PE is maximal when it is spherical and decreases with an increasing aspect ratio, which has not been reported previously.