V.M. Barragán

Permeation of electrolyte water-methanol solutions through a Nafion membrane

The volume flux through a cation-exchange membrane (Nafion 117) separating two equal electrolyte water-methanol solutions as a function of the pressure difference was determined under different experimental conditions. The results show that permeation rates through the membrane are strongly dependent on the methanol content of the solutions, thus the value of the flux increases when the methanol percentage increases. The effect of the electrolyte concentration of the solution on the membrane permeability is less important, although its influence becomes significant at low electrolyte concentration and high methanol content on solvent. This behavior is explained in terms of the amount of solvent sorbed by the membrane. Typical flux behaviors observed with pressure difference are linear at low pressures, exhibiting a positive deviation at higher pressure difference values.

Membrane potentials and electrolyte permeation in a cation-exchange membrane

A method has been developed which permits one to determine the electrolyte permeation velocities and the membrane system permeability from measurements of the membrane potential as a function of time. The method has been applied to a cation-exchange membrane separating two aqueous KCl solutions, at the same pressure and temperature, but of different concentrations. The experiments have been carried out in two concentration ranges and at different solution stirring rates. The obtained results showed that the membrane system permeability depends notably on the stirring rate, this dependence being greater for the highest concentration range. The intrinsic permeability of the membrane was determined from considerations about the concentration polarization effect, its value being greater in the lower concentration range.

Experimental estimation of equilibrium and transport properties of sulfonated cation-exchange membranes with different morphologies

Solvent uptake, hydraulic and electroosmotic permeabilities, true cation transport number, effective fixed charge concentration, and limiting current values have been determined in aqueous LiCl solutions for three commercial cation-exchange membranes with different morphologies and similar electric properties. The differences found in the equilibrium and transport properties of the membranes have been analyzed on the basis of their different structures. The experimental results show that the membrane morphology has an influence on the effect that the presence of an electrolyte has in the solvent uptake and in the liquid permeation. Differences have also been found in the polarization concentration effects, and on the loss of the membrane selectivity with the increase of the electrolyte concentration.

Osmotic behavior of a Nafion membrane in methanol–water electrolyte solutions

In this paper, an experimental study was carried out in order to investigate the osmotic transport of methanol-water electrolyte solutions through a Nafion membrane. The experimental data indicated that the Nafion membrane showed the typical anomalous osmotic behavior of charged membranes. The influence of some relevant parameters, such as electrolyte concentration difference, weight fraction of methanol on solution, and nature of cation was considered. The results showed that the osmotic volume flow was decreased with the presence of methanol on solvent, but did not alter the anomalous osmotic behavior of the membrane.

On current dependence of the electro-osmotic permeability in ion-exchange membranes

Abstract The apparent electro-osmotic permeability, W , of three ion-exchange membranes has been obtained under different experimental conditions. The behaviour of W as a function of the current density, I , shows the following characteristics: (i) at sufficiently low values of I , the apparent electro-osmotic permeability decreases when the current density increases; (ii) at an intermediate values of I , a local maximum appears; and (iii) at the highest values of I , the apparent electro-osmotic permeability seems to approach a roughly constant value. The value of I at which the local maximum takes place is very close to the so-called limiting current density.

Effect of unstirred solution layers on the thermal membrane potential through cation-exchange membranes

The thermal membrane potential through a cation-exchange membrane has been determined under different experimental situations. In all cases, a good linear dependence between the thermal membrane potential, Δψ, and the temperature difference, ΔT, established between both external electrolyte solutions has been observed. The value of Δψ decreases notably when the concentration of the electrolyte solution used is increased and it also depends on the type of the cation involved, so that Δψ decreases when the corresponding atomic number increases. The value of the temperature coefficient, ΔψΔT, is notably affected by the unstirred solution layers adjoining the membrane surfaces. A theoretical model is presented to determine the influence of the temperature polarization effect on the thermal membrane potential value.

Effect of an AC perturbation on a desalination electrodialysis process

The influence of an alternating (AC) sinusoidal perturbation, of known frequency and small amplitude, superimposed to the usual applied continuous (DC) signal in a desalination electrodialysis process has been studied. The experiments were carried out with two types of ion-exchange membranes and sodium chloride aqueous solutions of two initial concentrations and at different frequencies of the AC signal. The results show that the presence of the AC perturbation favours the desalination process, mainly at the lowest frequencies. This fact is considered as a consequence of that the electroosmotic transport accompanying ion transfer during the process is affected by the AC perturbation. This behaviour suggests that the presence of an AC perturbation may influence on the efficacy of the desalination process.

Methanol-Water Solution Transport in Nafion Membranes with Different Cationic Forms

Measurements of liquid transport were made with a Nafion membrane at different cationic forms. The experimental data are used to estimate the alcohol permeability when the membrane is separating water and water-methanol solutions. The obtained permeability coefficient values were useful for analyzing the influence of the substituted cations on the transport process in the membranes. In the present article, the permeability coefficient of methanol in Nafion substituted by Na+, K+, Cs+, Mg2+, Ca2+, Ba2+, Fe2+, Fe3+, and Al3+ were reported at different methanol concentration values. The analysis of the results revealed that, in general, for ions with the same period in the periodic table, the alcohol permeability decreases with increasing the valence. In contrast, when ions with the same valence are compared, the alcohol permeability decreases when the atomic mass increases, with the exception of the Mg2+. As a general trend, similar alcohol permeability variation with the concentration is observed for all the cationic forms of the membrane. There is an initial increase in the permeability, and, when the methanol concentration in the solutions is about 60%, the permeability decreases with the alcohol concentration. However, in the case of trivalent ions, the methanol permeability decreases with the methanol concentration.

On the dependence of the thermal membrane potential across cation-exchange membranes on the mean temperature

Abstract The thermal membrane potential across a cation-exchange membrane has been determined under different experimental situations. In all cases, a clear linear dependence between the thermal membrane potential, Δψ , and the temperature difference between both external electrolyte solutions, ΔT , has been observed. The value of Δψ depends on the mean temperature, T , of the membrane system, showing a maximum value at a certain value of T . Though the value of Δψ for each T is notably affected by the unstirred solution layers adjoining the membrane surfaces, the above-mentioned maximum has not been found to be a direct consequence of this effect.