Yuki Uematsu

Electrophoresis of electrically neutral porous spheres induced by selective affinity of ions

I investigate the possibility that electrically neutral porous spheres electrophorese in electrolyte solutions with asymmetric affinity of ions to spheres on the basis of electrohydrodynamics and the Poisson-Boltzmann and Debye-Bueche-Brinkman theories. Assuming a weak electric field and ignoring the double-layer polarization, I obtain analytical expressions for electrostatic potential, electrophoretic mobility, and flow field. In the equilibrium state, the Galvani potential forms across the interface of the spheres. Under a weak electric field, the spheres show finite mobility with the same sign as the Galvani potential. When the radius of the spheres is significantly larger than the Debye and hydrodynamic screening length, the mobility monotonically increases with increasing salinity.

Electro-osmotic flow of semidilute polyelectrolyte solutions

We investigate electro-osmosis in aqueous solutions of polyelectrolytes using mean-field equations. A solution of positively charged polyelectrolytes is confined between two negatively charged planar surfaces, and an electric field is applied parallel to the surfaces. When electrostatic attraction between the polymer and the surface is strong, the polymers adhere to the surface, forming a highly viscous adsorption layer that greatly suppresses the electro-osmosis. Conversely, electro-osmosis is enhanced by depleting the polymers from the surfaces. We also found that the electro-osmotic flow is invertible when the electrostatic potential decays to its bulk value with the opposite sign. These behaviors are well explained by a simple mathematical form of the electro-osmotic coefficient.

Analytical Interfacial Layer Model for the Capacitance and Electrokinetics of Charged Aqueous Interfaces

We construct an analytical model to account for the influence of the subnanometer-wide interfacial layer on the differential capacitance and the electro-osmotic mobility of solid-electrolyte interfaces. The interfacial layer is incorporated into the Poisson-Boltzmann and Stokes equations using a box model for the dielectric properties, the viscosity, and the ionic potential of mean force. We calculate the differential capacitance and the electro-osmotic mobility as a function of the surface charge density and the salt concentration, both with and without steric interactions between the ions. We compare the results from our theoretical model with experimental data on a variety of systems (graphite and metallic silver for capacitance and titanium oxide and silver iodide for electro-osmotic data). The differential capacitance of silver as a function of salinity and surface charge density is well reproduced by our theory, using either the width of the interfacial layer or the ionic potential of mean force as the only fitting parameter. The differential capacitance of graphite, however, needs an additional carbon capacitance to explain the experimental data. Our theory yields a power-law dependence of the electro-osmotic mobility on the surface charge density for high surface charges, reproducing the experimental data using both the interfacial parameters extracted from molecular dynamics simulations and fitted interfacial parameters. Finally, we examine different types of hydrodynamic boundary conditions for the power-law behavior of the electro-osmotic mobility, showing that a finite-viscosity layer explains the experimental data better than the usual hydrodynamic slip boundary condition. Our analytical model thus allows us to extract the properties of the subnanometer-wide interfacial layer by fitting to macroscopic experimental data.

Effects of strongly selective additives on volume phase transition in gels

We investigate volume phase transition in gels immersed in mixture solvents, on the basis of a three-component Flory-Rehner theory. When the selectivity of the minority solvent component to the polymer network is strong, the gel tends to shrink with an increasing concentration of the additive, regardless of whether it is good or poor. This behavior originates from the difference of the additive concentration between inside and outside the gel. We also found the gap of the gel volume at the transition point can be controlled by adding the strongly selective solutes. By dissolving a strongly poor additive, for instance, the discontinuous volume phase transition can be extinguished. Furthermore, we observed that another volume phase transition occurs far from the original transition point. These behaviors can be well explained by a simplified theory neglecting the nonlinearity of the additive concentration.

Interfacial layer effects on surface capacitances and electro-osmosis in electrolytes.

Many properties of the interfacial layer of water at surfaces differ significantly from those of bulk water. The consequences are most significant for the double-layer capacitance and the electrokinetic properties. We model the interfacial hydration layer by a modified dielectric constant and a modified local viscosity over a single interfacial width. Analytic expressions in the low-charge Debye–Hückel approximation are derived and shown to describe experimental surface capacitance and electro-osmotic data in a unified framework.

Crossover of the Power-Law Exponent for Carbon Nanotube Conductivity as a Function of Salinity

On the basis of the Poisson-Boltzmann equation in cylindrical coordinates, we calculate the conductivity of a single charged nanotube filled with electrolyte. The conductivity as a function of the salt concentration follows a power-law, the exponent of which has been controversially discussed in the literature. We use the co-ion-exclusion approximation and obtain the crossover between different asymptotic power-law behaviors analytically. Numerically solving the full Poisson-Boltzmann equation, we also calculate the complete diagram of exponents as a function of the salt concentration and the pH for tubes with different radii and p Ka values. We apply our theory to recent experimental results on carbon nanotubes using the p Ka as a fit parameter. In good agreement with the experimental data, the theory shows power-law behavior with the exponents 1/3 at high pH and 1/2 at low pH, with a crossover depending on salt concentration, tube radius and p Ka.