Dietrich Woermann

Electrochemical transport properties of a cone-shaped nanopore: high and low electrical conductivity states depending on the sign of an applied electrical potential difference

Siwy and Fulinski have reported (Phys. Rev. Lett., 2002, 89, 198 103) that an alternating electrical current (frequency, e.g., ν = 0.01 s−1) passing through a single cone-shaped pore which is embedded in a polymer foil causes a net transport of potassium chloride across the pore. The foil separates two aqueous potassium chloride solutions having the same composition or different compositions. Two silver/silver chloride electrodes are used to pass an electric current across the pore. In the present study these findings are analysed in terms of the “model of the membrane with narrow pores”. This analysis leads to a qualitative explanation of the reported transport phenomenon distinctly different from that given by Siwy and Fulinski.

Structure and transport properties of cation exchange gel membranes: facilitated transport of ethene with silver ions as carriers

Abstract The transition time t a of the rate of transport of gaseous ethene to reach a stationary value across two types of cation exchange gel membranes shows a characteristic dependence on the mole fraction x ( Ag + ) of silver counter ions present in the gel phase. Silver ions are carriers for ethene. The corresponding dependence of the characteristic time of adsorption t 0.9 of ethene by the same gels shows similar features. Both curves exhibit a maximum at about the same (small) value of x ( Ag + ) . The adsorption kinetics is studied using a gravimetric method. It is concluded that these phenomena reflect structural inhomogeneities of the gel matrix which have been detected in independent small-angle and ultra small-angle X-ray scattering (SAXS and USAXS) experiments.

Static and dynamic light scattering experiments on semidilute solutions of polystyrene in cyclohexane between the Θ-temperature and the binodal curve

Static and dynamic light scattering experiments are carried out with solutions of a high molar mass polystyrene (Mw=0.96×106 g mol-1) in cyclohexane at different concentrations of the polymer. At the Θ-temperature TΘ the critical concentration of the system is located in the semidilute regime. The temperature is varied between TΘ and the temperature of phase separation TP of the solutions. At TΘ the distribution of the amplitudes of the relaxation rates of the autocorrelation function of the electric field is bimodal (slow and fast relaxation rates) in the semidilute regime. It becomes monomodal approaching TP. The contribution of the fast relaxation rates disappears and the weak q-dependence of the slow mode changes over to a q-dependence similar to that of the critical relaxation rate in binary liquid mixtures of low molar mass components close to the critical temperature Tc. On approaching the temperature of phase separation by decreasing the temperature, concentration fluctuations with long range correlations develop. The change of the q-dependence of the slow relaxation rate is rationalised using the transient gel model of Brochard and de Gennes. Here it is assumed that approaching TP transient gels formed by interpenetrating polymer coils become part of the concentration fluctuations with long range correlations. The dynamics of the solutions in the vicinity of the binodal curve monitored by dynamic light scattering experiments is dominated by the dynamics of the concentration fluctuations with long range correlation.

Forces of inertia acting on the aqueous pore fluid of anionic polyelectrolyte gels

Pulses of an inertia electromotive force are generated during deceleration of stiff rods of polyelectrolyte gels with negatively charged ionic groups covalently bound to the matrix of the gel. The gels are loaded with Li+, Cs+, Ag+, H+ and Ba2+, respectively, and are in swelling equilibrium with water. The values of (m/q)exp, (m, mass and q, charge of one counterion in the pore field) calculated from the recorded voltage pulse ∫U dt(U, inertia electromotive force and t, time) during deceleration (time constant τ≈ 1 ms) are larger by a factor of at least five than that calculated from the mass and charge of a ‘naked’ single counterion. This is attributed to an effective mass of the counterions in the pore fluid of the gels which has a larger value than the mass of naked single counterion. During deceleration the hydrated counterions, together with the mobile water molecules in the pore fluid, are shifted relative to the ionic groups fixed to the stiff matrix of the gels. This generates an electric field causing the observed voltage pulse. The value of (m/q)exp of the H+ counterion is smaller by a factor of about five than that of the other counterion species (Li+, Cs+, Ag+). It is assumed that this is caused by the chain mechanism of proton migration found in aqueous solutions. Measurements of the electrical conductivity of gels loaded with different counterion species, including H+ ions, reveal that the ratios of the counterion conductivity in the gel phase to that in free solution at infinite dilution have approximately the same value. The inertia electromotive force and the electrical conductivity measurements indicate that the mechanism of H+ ion transport in the pore fluid is not modified considerably by the matrix of the gel.

Osmotic properties of a cation exchange membrane in contact with an aqueous solution of a non-electrolyte: influence of preferential solvation of counterions

A system in which a membrane separates two aqueous solutions of a non-electrolyte of different but constant compositions is used to demonstrate a continuous transition from positive to negative osmosis and from positive to negative retention coefficients in filtration experiments. The membrane is a cation exchange membrane loaded with two deliberately chosen counterion species which compensate the electrical fixed charges of the membrane matrix. The parameter of the experiments is the mole fraction of one of the mobile counterion species in the pore fluid of the membrane. The counterion species which is preferential solvated by the non-electrolyte acts as a carrier for that component across the membrane. Increasing the mole fraction of the carrier ion species in the pore fluid causes two effects: it increases the transport rate of the non-electrolyte across the membrane as well as its coupling with the osmotic volume flow across the membrane. This leads to the stated changes of the transport properties of the membrane.