The Correlation between the Water Content and Electrolyte Permeability of Cation-Exchange Membranes.

The salt permeability through three commercial cation-exchange membranes with different morphologies is investigated in aqueous NaCl solutions. Ion-exchange membranes (IEMs) find application in different processes such as electrodialysis, reverse osmosis, diffusion dialysis, membrane electrolysis, membrane fuel cells and ion exchange bioreactors. The aim of this paper is the experimental determination of the electrolyte permeability in the following membranes: MK-40 membrane, Nafion N324 membrane and Nafion 117 membrane. The latter is selected as being a reference membrane. The effect of an increase in the NaCl concentration in the solutions on membranes transport properties is analyzed. With regard to membranes sorption, a decrease in the water content was observed when the external electrolyte concentration is increased. Concerning permeation through the membranes, the salt permeability increased with concentration for the Nafion 117 membrane and remained nearly constant for the other two membranes. A close relation between the degree of liquid sorption by the membranes and the electrolyte permeability was observed.

Millable polyurethane/organoclay nanocomposites: preparation, characterization, and properties.

Novel millable polyurethane (PU)/organoclay nanocomposites have been successfully prepared by conventional transformation techniques. One natural (C6A) and two organically modified (C15A and C30B) montmorillonites have been used as clays for preparing PU nanocomposites. The optimum dispersion of nanofiller at a nanometer scale in PU matrix was confirmed by X-ray diffraction patterns and transmission electron microscopy. A substantial improvement of the PU properties by addition of only a small amount of organoclay was observed. It is worthy to note that the organoclays show a different interfacial interaction with the PU matrix, which was reflected in different macroscopic properties. Thus, C30B organoclay seems to react with PU chains to form covalent bonds, while C15A only interacts physically with PU chains. Mechanical and barrier properties are analyzed.

Simultaneous electroosmotic and permeation flows through a Nafion membrane 2. Methanol-water electrolyte solutions.

The volume flow of methanol-water potassium chloride solutions through a Nafion membrane originated by the simultaneous action of electric potential and pressure gradients has been measured at different percentages of methanol. Measurements were conducted when both gradients act in the same and in the opposite directions under different experimental conditions. The results indicate that the simultaneous action of the pressure and potential differences originates a total flow different from the sum of the individual electroosmotic and permeation flows due to each force acting separately. The application of the irreversible thermodynamics theory, which includes second-order terms, allowed the study of the influence of the composition of the solutions on the determination of the different phenomenological coefficients.

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.

Simultaneous electroosmotic and permeation flows through a Nafion membrane. 1. Aqueous electrolyte solutions.

The volume flow through a Nafion membrane originated by the simultaneous action of an electric potential difference and a pressure difference has been measured using aqueous KCl solutions under different experimental conditions. The behavior has been analyzed when both gradients act in the same and in the opposite sense. The results indicate that the simultaneous action of the pressure and potential differences originates a total flow different from the sum of the electroosmotic and permeation flows due to each force acting separately. The application of irreversible thermodynamics, which includes second-order terms, allowed the determination of the phenomenological coefficients. Moreover, from these values, the equivalent pore radius was estimated on the assumption that the membrane is a porous medium filled with an internal solution.

Preparation and application of dense poly(phenylene oxide) membranes in pervaporation.

Dense flat-sheet membranes were prepared from poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) using the casting solvents chloroform and 1,1,2-trichloroethylene. X-ray diffraction, tapping mode atomic force microscopy (TM-AFM), and contact angle studies were used to characterize the membranes. The surface energy and the solubility parameters of the PPO membranes were determined from the measured contact angles and compared with the predicted ones from the group contribution method. Swelling experiments and pervaporation separation of methanol from its mixture with ethylene glycol over the entire range of concentration, 0-100%, were conducted using these membranes. Flory-Huggins theory was used to predict the sorption selectivity. The results are discussed in terms of the solubility parameter approach and as function of the morphological characteristics of the membranes. It was found that PPO membranes prepared with chloroform exhibited better pervaporation performance than PPO membranes prepared with 1,1,2-trichloroethylene.

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.

Study of the internal morphology of cation-exchange membranes by means of electroosmotic permeability relaxations.

The effect of an ac sinusoidal perturbation of known amplitude and frequency superimposed to the usual dc applied electric voltage difference on the electroosmotic flow through three cation-exchange membranes with different morphology has been studied. A dispersion of the electroosmotic permeability on the frequency of the applied ac signal has been found for the three membranes investigated, observing that the electroosmotic permeability reaches maximum values for some characteristic values of the frequency. These characteristic frequency values, which are related to relaxation processes in heterogeneous media, depend on the membrane system and permit to obtain information about the different structures of the membrane system. Thus, the study of the electroosmotic permeability relaxation can be used as a method to study the internal morphology of a cation-exchange membrane in a given electrolyte medium.