F. Tejerina

Pore size distributions in microporous membranes. A critical analysis of the bubble point extended method

Abstract An extended bubble point method has been used to examine the porous morphology of several track-etched microporous polycarbonate membranes with nominal pore sizes ranging from 0.1 to 5.0 μm. The technique has been carefully analyzed and corrected to take into account the diverse non-ideal factors in flow along with the prevalence of Knudsen flow over the Hagen-Poiseuille one in the smaller pores. The pore size distributions were obtained in terms of flow or pore numbers. Bulk porosity and pore densities were in good agreement with nominal and SEM values for each filter. Performancewise, measured water permeabilities agreed well with those obtained from pore density distributions, assuming Knudsen air flow through the narrowest pores.

Mechanisms of protein fouling in cross-flow UF through an asymmetric inorganic membrane

Abstract Flux decline and retention of aqueous solutions of several proteins (pepsin, BSA, lipase, γ-globulin and invertase) with molecular weights of 36, 67, 80, 150 and 270 kDa are studied when they are tangentially filtered through an inorganic microporous membrane with a nominal pore size of 0.02 μm made by Anopore under a constant applied pressure of 5.0 kPa. The fouling process of this ultrafiltration membrane is investigated as a function of the protein concentration, the tangential velocity at the membrane surface and the nature and size of the protein. Finally, the deposition mechanism on the membrane surface and/or into its porous structure is analyzed in terms of several kinetic models. It is seen that an initial particularly intense flux decline due to external blockage is followed by an internal deposition (partially retained proteins) or the formation of a cake (totally retained proteins). The kinetics of these fouling mechanisms are proved to follow reasonable trends versus feed concentrations, recirculation velocity and the protein molecular weight.

Structural characterization of an UF membrane by gas adsorption-desorption and AFM measurements

Abstract In this work, the pore size distributions referred to volume, surface and number of pores, along with the internal surface area and the surface roughness parameters are determined for a capillary microporous symmetric membrane with a nominal diameter of 150 A. This has been realized by using two specially interesting techniques: nitrogen adsorption-desorption at 77 K and atomic force microscopy (AFM). Adsorption isotherms, combined with the BET theory for multilayer adsorption allowed us to obtain the internal surface area of the membrane, while the volume, surface and pore number distributions were calculated from the Kelvin equation both in the adsorption and desorption processes. In this way the existing asymmetry between both adsorption and desorption processes was analyzed and it has been related to the dead-end pores (pores closed to flux). The results of image analysis on AFM micrographs allowed the determination of the surface roughness parameters of the membrane, along with the pore sizes on the membrane surface. Finally, the results have been thoroughly analyzed and compared with other characterization results previously obtained for these membranes from different characterization techniques by other authors.

Quantitative microscopic study of surface characteristics of track-etched membranes

Abstract The surface morphology of various types of Nuclepore® track-etched membranes (with applications in the areas of micro- and ultrafiltration) has been investigated quantitatively. As experimental techniques scanning electron microscopy (SEM) and computerized image analysis (IA) have been employed. From the results it is inferred that, in these membranes, surface pore density is an acceptably controlled parameter; however, the presence of a considerable amount of multiple pores leads to true distributions of pore sizes and shapes which deviate from their nominal values. Porosity is reduced and is related to the relative quantity of multiple pores by a linear correlation. These findings suggest a strong dependence of the characteristic parameters on method of preparation and reveal that the idealized description sometimes assumed for these membranes is not always justified.

Streaming potential through and on ultrafiltration membranes influence of salt retention

Abstract The pore size distribution of a polysulfone membrane has been obtained from computerised image analysis of scanning force microscopy pictures. The streaming potential on the active layer of this membrane has been measured when it is in contact with aqueous NaCl solutions of different concentrations. Finally, NaCl retention has been taken into account to allow the translation of measured electric potential to the actual streaming potentials through the membrane. It has been shown that it is possible if the diffusion potentials appearing through both the concentration polarisation and the membrane itself are taken into account. By a fitting procedure, the effective transport numbers are obtained, which prove to be adequate not only for high concentrations, when there is no significative electrical double layer overlapping, but also for all the concentrations range used leading to the same streaming potential than measured on the membrane. The so-obtained effective cation transport numbers are not equal to the free solution true transport numbers that are defined when no pressure is applied. They are very similar for the cation and the anion, what should be caused by the applied pressure that should make their mobilities almost equal. Only a little contribution from the diffusion potentials, caused by retention without a relevant separation of the ions, should remain.

PORE SIZE DISTRIBUTIONS OF TRACK-ETCHED MEMBRANES; COMPARISON OF SURFACE AND BULK POROSITIES

Abstract Here we compare surface and bulk porous morphologies for several track-etched membranes. The experimental techniques include approaches with different proper supplementary pore size ranges. In order to analyze bulk porosities we used: a mercury intrusion apparatus, an extended bubble point method and a gas adsorption–desorption technique which allows to detect pores with decreasing sizes. Referring to the surface structure, scanning electron and atomic force microscopies have been used. Finally, a solute retention method has been used to evaluate pore size distributions from operative conditions of operation. In particular from a comparison of bulk and surface pore size distributions, it is noted that permeability is really determined by the bulk pore size distributions as shown for several track-etched membranes. For these membranes, pore entrance sizes are narrower than those determined by permeability which in turn agree with the bulk pore size distribution. Gas adsorption–desorption results are compared with the atomic force microscopy (AFM) data and solute retention ones for membranes with narrower pores. AFM is especially interesting as far as it is the best technique to analyze the surface roughness. It is worth noting that, among the methods leading to bulk porosity those detecting only thoroughly open pores give always more realistic estimations of the actual permeation performances.

Study of some electrokinetic phenomena in charged microcapillary porous membranes

Abstract A detailed study of some transport properties of charged capillary membranes (concentration potential and solute permeability) is presented. The used transport model is based upon the Nernst-Planck and Poisson-Boltzmann equations. It is solved by an original numerical method which gives the degree of convergence. An experimental verification of the model is carried out for some small pore size microporous capillary membranes (Nuclepore).

Effects of structure and charge on the diffusive salt permeation across microporous membrane filters

Abstract Diffusion of LiCl through various types of polycarbonate track-etched membranes has been studied at different temperatures between 25°C and 50°C. From these results, the salt permeability, Pms, of the membrane system formed by the membrane itself and the two adjacent boundary layers has been obtained. The results conform to a simple relation between 1/Pms and l/∈, l being the thickness of the membrane and ∈ its porosity. From this relation an estimation of the thickness of the boundary layers is obtained. Taking into account these results, a capillary charged membrane model has been used in order to estimate the charge density inside the pore in the range of concentrations studied.

MEASUREMENT OF THE HYDRAULIC PERMEABILITY OF MICROPOROUS MEMBRANES FROM THE STREAMING POTENTIAL DECAY

Abstract The standard methods used to measure the hydraulic permeability of microporous membranes are revised. It is proposed that the relaxation of the streaming potential be used once an initial pressure difference is established and the system is free to evolve. This method is compared with those that use pressure or height decay, and its simplicity and easy automatization is pointed out. The results are compared with those obtained by measuring the volume flow for several constant pressures (as suggested by the definition) for four different Nuclepore membranes and LiCl-water solutions (1 × 10−3 mol/L) at 298.0 ± 0.5 K. These results coincide within the error range.

Study of Electrolyte Conductivity in Charged Microcapillary Porous Membranes

Abstract The electrolyte conductivities of several microporous membranes, when bathed by an electrolyte solution, have been measured by the dc method. With membranes of the polycarbonate track-etched thin film type (Nuclepore), the mean pore diameter ranged from 0.015 to 0.100 μm. Concentrations of the studied electrolyte (LiCl, NaCl, and MgCl2) range from 5 × 10−4 to 2 × 10−2 mol/L. The results obtained have been compared to those deduced from a space-charge model for the electrolyte transport in charged capillary pores. This allows an estimate of the charge density on the pore walls which agrees with those obtained earlier for these types of membranes and solutes from a study of other electro-kinetic phenomena.