Characterization of novel activated composite membranes by impedance spectroscopy

Journal of Electroanalytical Chemistry 451 (1998) 173 – 180

Characterization of novel activated composite membranes by impedance spectroscopy Juana Benavente a,*, Maria Oleinikova b, Maria Mun˜oz b, Manuel Valiente b b

a Departamento de Fı´sica Aplicada, Facultad de Ciencias, Uni6ersidad de Ma´laga, E-29071 Malaga, Spain Departamento de Quı´mica Analı´tica, Facultad de Ciencias, Uni6ersidad Autonoma de Barcelona, E-08193 Barcelona, Spain

Received 3 November 1997; received in revised form 5 February 1998

Abstract The paper reports the results obtained on the physicochemical characterization of activated composite membranes (ACM). Membrane samples containing different concentrations of di-2-ethyl-hexylphosphoric acid (DEHPA) as a carrier were prepared and in situ characterized by impedance spectroscopy (IS). The results obtained by this technique, based on the linear correlation between the electrical resistance of the membrane and the carrier content allows not only for an in situ characterization of the working membrane but also for a novel analytical method to direct determination of actual carrier content in the membrane. These results also correlate with those obtained by the application of other spectroscopic techniques to ACM samples. Thus, a linear correlation between the carrier content in the membrane and that in the casting solution was obtained from the results of the membrane analysis by using the inductively coupled plasma (ICP) technique. The carrier distribution in the membrane was evaluated by X-ray microanalysis (EDS), which allowed us to determine that DEHPA is trapped in the polymeric phase of the composite membrane. The results obtained by the indicated techniques correlate with good agreement and allow characterization of the membrane performance under different working conditions. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Activated composite membrane; Impedance spectroscopy; Membrane characterization; Inductively coupled plasma spectroscopy

1. Introduction Supported liquid membranes (SLM) have been developed and studied for the separation and concentration of toxic and/or valuable solutes [1,2]. The techniques based on liquid – liquid distribution processes have been implemented by using extracting reagents as carriers for the facilitated transport of targeted solutes. The SLMs have important advantages such as a high selectivity and diffusion rate compared to solid membranes. At the same time, their main disadvantage is the lack of stability, which is caused by different factors including the leakage of the organic extractant from the pores of the support, the formation * Corresponding author. Tel.: + 34 52 131929; fax: + 34 52 132000; e-mail: j – [email protected] 0022-0728/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S0022-0728(98)00070-9

of emulsions in these pores and the loss of the organic solvent [3–5]. Different approaches have been suggested to stabilize SLMs such as, for example, gel formation in the pores of the support [6]. Recently, Kemperman et al. [7] investigated and optimized the application of a thin polyamide layer covering the impregnated support, which partially avoided the loss of the carrier reagent from the membrane phase. Based on this method, a set of new activated composite membranes (ACM) have been developed by the incorporation of different concentrations of di-(2-ethylhexyl) phosphoric acid (DEHPA) as carrier. These membranes have shown good stability and a high rate for transport of rare earth metals [8]. One of the problems which arises when preparing and working with these ACMs, is the characterization

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of the carrier in the membrane and in particular the relationship between the active reagent present in the casting solution and its distribution in the membrane. Different destructive and non-destructive techniques such as inductively coupled plasma spectroscopy (ICP), X-ray energy dispersion spectroscopy (EDS) and scanning electron microscopy (SEM) can be used for this purpose. The characterization of activated membranes under different working conditions can also be done by measuring in situ the electrical resistance of a given membrane. Impedance spectroscopy (IS) is a non-destructive technique which is being used as a successful tool to determine the electrical properties of heterogeneous systems such as membrane/electrolyte systems, since it permits us to evaluate separately the electrical contribution of both the membrane and the electrolyte solution [9–11]. The main goal of the present study is focused on the physicochemical characterization of ACMs by determining the distribution of the active component and the heterogeneity of the membrane, and to relate such characteristics to the transport properties, establishing a methodology to improve the membrane capability and to predict the behaviour of a given membrane. The IS technique has the ability to provide data in this direction with the advantage of non-destructive in situ measurements.

data, measurements of the DEHPA content in the membrane using the ICP technique were performed. X-Ray microanalysis (EDS) was applied to determine the distribution of the carrier in the membrane, and SEM was used to generate information on the membrane surface characteristics.

2. Experimental

2.4. Inducti6ely coupled plasma spectroscopy

2.1. Membranes

The amount of immobilized DEHPA was evaluated by the quantitative extraction of the carrier from the membranes using ethanol. After concentration of the ethanol solution by evaporation, the residual solution was dissolved in aqueous NaOH solution and the total concentration of DEHPA in the aqueous phase was determined by analyzing the phosphorus concentration by the ICP technique using an ARL Model 3410 inductively coupled plasma atomic emission spectrophotometer provided with a minitorch. The emission line used was 213.618 nm. Calibration solutions were prepared by sequential dilution of H3PO4 solutions (Panreac, Spain, 85%) of known concentration [15].

Polysulfone (PS) casting solution (15 mass%) was prepared by dissolving Udel P-3500 PS (Union Carbide) in A.R. grade N,N-dimethylformamide (DMF) (Fluka, Germany) by vigorous agitation during 12–14 h at 25°C. Non-woven fabric (Hollytex 3329, France) was used for making reinforced PS membranes which were obtained by the phase inversion technique as described elsewhere [12]. Thin top layers of polyamide containing DEHPA of different concentrations (from 25 to 500 mM) were obtained by interfacial polymerization [13]. An aqueous amine solution (1,3-phenylenediamine) was mixed with a water immiscible organic solution containing DEHPA. The excess solution was washed off the surface of the membrane with water. Finally, the top layer containing DEHPA was dried in an oven at 60°C for 10 min. The membrane, thus prepared is built out of two or more layers containing the carrier.

2.3. Impedance spectroscopy The experimental device used for the measurements of electrical impedance is similar to that described elsewhere [14]. The unit consisted of two half-cells, separated by the membrane, which was fixed between two rubber rings. Measurements were carried out with the membrane samples equilibrated with NaCl solutions of different concentrations ranging from 10 − 3 to 5× 10 − 2 M. The NaCl solutions in both compartments of the membrane cell (at both sides of the membrane) were of the same concentration. A frequency response analyzer FRA (Solartron 1255), controlled by a computer and connected with the solutions in each half-cell via platinum electrodes, was used. The experimental data were corrected by the software, taking into account the influence of connecting cables and other parasite capacitances. Measurements were made at 100 different frequencies in the range of 60 Hz–3×106 Hz at a maximum voltage of 0.01 V.

2.5. Scanning electron microscopy The surface texture of the membranes under study was examined by scanning electron microscopy (SEM) using a JSM-6300 scanning electron microscope. Flat samples were glued on the holder and coated with a thin layer of gold before examination.

2.2. Procedure 2.6. X-ray microanalysis The study has been carried out by systematic measurements of the membrane impedance with different contacting solutions of NaCl. To complement these

In order to investigate the DEHPA content in the membranes and its distribution along a cross-section,

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samples were used as standards for the quantitative determination of P and S.

3. Results and discussion In order to characterize the materials employed different techniques have been successfully applied.

3.1. Inducti6ely coupled plasma

Fig. 1. Concentration of DEHPA in different membrane samples plotted versus DEHPA concentration in the respective modifying solution (see text).

X-ray microanalysis was carried out on a JSM-6300 scanning electron microscope supplied with an energydispersive-X-ray spectrometer OXFORD. Flat membranes and their cross-sections were coated with carbon prior to determination of the elemental composition of the membrane material. The relative accuracy of the X-ray microanalysis was around 1%. GaP and FeS2

The data on direct determination of the amount of extractant incorporated into membranes by using different extractant concentrations in the casting solutions are presented in Fig. 1. This figure shows that the quantity of DEHPA incorporated into the thin polyamide film has a linear direct dependence of the extractant concentration in the casting solution. Such dependence indicates a homogeneous distribution of the carrier in the membrane and thus that DEHPA may have no chemical interaction with the polymer formed but is simply trapped in the net matrix of the polymer.

3.2. Scanning electron microscopy The surface texture of the polyamide top layer of the membranes was examined by SEM. Fig. 2 shows a

Fig. 2. SEM photographs of the membrane surface at different carrier concentrations. (a) 50 mM DEHPA and (b) 200 mM DEHPA ( × 10000× ).

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Fig. 3. EDS spectrum of a membrane sample prepared by using a 200 mM DEHPA casting solution.

Fig. 4. P/S ratio in membrane samples with different carrier contents plotted versus DEHPA concentration in the modifying solution (see text).

typical view of the membrane surface. No remarkable defects are observed on the microphotographs obtained. It may indicate a homogeneous distribution of the carrier (DEHPA) molecules in the bulk membrane including the pores of the polysulfone film. A typical rippled surface pattern was always observed at low carrier concentrations (0 – 50 mM) in the casting solution (see Fig. 2a). When a higher DEHPA concentration was used (200 mM and more), the structural features of the polyamide top layer of the membrane were not developed (see Fig. 2b). Such a difference can be explained by the gradual loading of the top membrane layer with the carrier at higher DEHPA concentrations in the casting solution used for the membrane manufacturing.

to the maximum of the phosphorus peak (see Fig. 3). Typical distribution curves are shown in Fig. 5. The zero point on the abscissa axis corresponds to the left edge of the non-woven support of the membrane. The left edge of the polysulfone layer corresponds to 80 mm. As seen in Fig. 5, the modifier incorporated into the membranes is mainly concentrated in the upper part of the polysulfone layer in contact with the polyamide layer (right edge of the membrane). It is supposed that the thickness of the polyamide layer does not exceed 1 mm [10]. From these observations one may conclude that the incorporated carriers are mainly trapped in the polyamide net with probably some diffusion into the polysulphone pores.

3.3. X-ray microanalysis

3.4. Impedance spectroscopy

The chemical analysis of freshly prepared membranes was performed by scanning electron microscopy measuring the energy distribution of the X-ray signal generated by a focused electron beam. The typical EDS spectrum of the surface of the membrane modified with 200 mM DEHPA solution is presented in Fig. 3, confirming the presence of the elements of the membrane components. Fig. 4 shows the ratios of phosphorus to sulfur peak heights measured by the EDS technique in freshly prepared membranes plotted versus the concentration of DEHPA in the casting solution. The observed linear relationship also confirms our previous results of ICP analysis that DEHPA has homogeneous distribution in the membrane and that no irreversible chemical reactions take place on the incorporation of DEHPA. The distribution of DEHPA across the membrane was evaluated by scanning a cross section of the freshly prepared membranes at the wavelength corresponding

In situ characterization of membranes under experimental conditions mimicking different operating situa-

Fig. 5. Distribution of phosphorus along the cross-section of different membrane samples.

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Fig. 6. Bode plot for the activated composite membrane with 400 mM DEHPA +electrolyte solution (cNaCl =0.002 M).

tions (i.e. different NaCl concentrations), was carried out by impedance spectroscopy measurements. Figs. 6 and 7 show two typical impedance plots (Bode and Nyquist plots, respectively) for an activated membrane prepared using 400 mM DEHPA casting solution and a 2× 10 − 3 M NaCl solution in the cell compartments for the test experiments. Similar curves were obtained with other membrane samples (containing different DEHPA concentrations) in contact with NaCl solutions of different concentrations. Two different dielectric relaxations can be observed in Figs. 6 and 7. Also, the Bode plot (Fig. 6) permits us to assign the range of frequencies corresponding to each sub-system: the membrane, m, (60 Bf(Hz)B 104), and the electrolyte solution, e, (105 Bf(Hz)B 3×106). Similar plots have been previously reported for different membrane/electrolyte systems [9,16]. Analysis of ac data is usually carried out by the complex plane method, which involves plotting the impedance imaginary part ( −Zimg) versus the real part (Zreal). When plotted on a linear scale, the equation for a parallel resistance-capacitor circuit (RC) rises to a semi-circle in the Z* plane, which has intercepts on the Zreal axis at R (v“ ) and R0(v “ 0), where (R0 − R ) is the resistance of the system [17]. The maximum of the semi-circle equals 0.5(R0 −R ) and occurs at a frequency v(v=2pf ) such that vRC =1, RC being the relaxation time. Complex systems may present different relaxation times and the resulting plot is a depressed semi-circle, such as shown in Fig. 7. In these cases, a non-ideal capacitor, known as a constant phase element (CPE) is considered; the CPE admittance is expressed by [17]

Q(v)=Y0(jv)n

(1)

where Y0 and n are two empirical parameters (05n 5 1). In these cases an equivalent capacitance, C eq, can be determined by the relationship [18] C eq = (R0Y0)1/n/R0

(2)

The experimental data for the whole range of frequencies were fitted to a circuit which consists of two elements in series, namely the membrane and the electrolyte solution as shown in Fig. 7: the membrane contribution is represented by a parallel (RmQm) circuit, while the electrolyte part is a parallel (ReCe) one. These results allow us to determine the resistance and capacitance values of the electrolyte and the membrane by means of a non-linear program [19]. A good agreement between experimental and calculated values was obtained in all cases within an uncertainty B 5%. Only the membrane contribution will be considered in the following discussion. The membrane equivalent capacitance values were obtained by Eq. (2) and are practically independent of the salt concentration. Their average values slightly increase when the DEHPA concentration in the membrane rises (by a factor of 10 when the concentration of DEHPA in the casting solution varies from 0 to 400 mM). This indicates a higher charge adsorption in the membrane when the concentration of carriers increases. However, the membrane resistance values are strongly dependent on the salt concentration as seen in Fig. 8, which is mainly attributed to the effect of the electrolyte taken up by the membrane [20]. Taking into account the following assumption: (i) the resistance of the membrane matrix, R0, is a characteristic of the

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Fig. 7. Nyquist plot for the system activated composite membrane with 400 mM DEHPA/electrolyte solution (CNaCl =0.002 M).

material and does not depend on the electrolyte concentration; (ii) the contribution of the electrolyte taken up by the membrane depends on the NaCl concentration, the following expression is proposed: Rm(cNaCl)= R0 − ac bNaCl

(3)

Correlation coefficients higher than 0.956 were obtained in the fitting of the results obtained by studying five membrane samples. The correlation between mem-

brane matrix resistance, R0, and concentration of the carrier in the membrane is shown in Fig. 9. As seen from this figure the electrical resistance versus carrier concentration plot is satisfactorily approximated by a straight line. This relationship also validates our previous data by pointing out that the electrical resistance has an univocal dependence of the compound DEHPA and possible transformations either do not affect it or are not taking place. On the other hand, a practical

Fig. 8. Membrane resistance versus NaCl concentration, cNaCl for different carrier contents: (“) 0 mM DEHPA; () 100 mM DEHPA; () 200 mM DEHPA; () 300 mM DEHPA; ( ) 400 mM DEHPA.

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Fig. 9. Membrane samples’ resistance versus DEHPA concentration, cDEHPA in respective modifying solutions.

application of the results obtained can be carried out as an analytical method to determine the carrier content in the membrane by measuring the membrane resistance.

4. Conclusions The ICP technique allowed us to correlate the carrier content in the membrane with its initial concentration in the modifying solution. X-ray microanalysis can be used as a non-destructive analytical tool for the determination of the concentration and the distribution of the carrier in activated membranes. Impedance spectroscopy measurements have permitted us to obtain the electrical response of an activated composite membrane in contact with salt solutions, and to determine the electrical resistance of different membrane samples. The linear relationship between the electrical resistance of the membrane matrix and the concentration of DEHPA in the membrane allows to use this physical parameter as a tool to determine the actual content of active carrier. This is a direct method to characterize activated composite membranes under working conditions. Finally, it is remarkable that different techniques applied to the characterization of membrane samples have resulted in a good agreement with each other and, hence, allow us to determine the composition of activated membranes in a unique way.

Acknowledgements The authors thank the Comisio´n Interministerial de Ciencia y Tecnologı´a (CICYT) (projects number: MAT97-0970-C03 and QUI96-1025-C03-01) and the Direccio´n General de Ciencia y Tecnologı´a (DGCYT) (project number PB95-048). MO is a recipient of a fellowship from CIRIT (Comisio´n de Ciencia y Tecnologı´a de Catalunya).

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