Journal of Colloid and Interface Science 285 (2005) 273–280 www.elsevier.com/locate/jcis Modification of polysulfone membranes with polyethylene glycol and lignosulfate: electrical characterization by impedance spectroscopy measurements J. Benavente a,∗ , X. Zhang b , R. Garcia Valls b a Grupo de Caracterización Electrocinética de Membranas e Interfases, Departamento de Física Aplicada I, Facultad de Ciencias, Universidad de Málaga, E-29071 Málaga, Spain b Grupo de Biopolímeros y Vegetales, Departamento de Ingeniería Química, Universitat Rovira i Virgili, E-43007 Tarragona, Spain Received 29 July 2004; accepted 15 November 2004 Available online 29 January 2005 Abstract Two sets of composite membranes having an asymmetric sulfonated polysulfone membrane as support layer have been obtained and electrically characterized (membranes SPS–PEG and PA–LIGS). The skin layer of the membrane SPS–PEG contains different percentages of polyethylene glycol in the casting solution (5, 25, 40, and 60 wt%), while lignosulfonate was used for manufacturing PA–LIGS membranes (5, 10, 20, and 40 wt%). Membrane electrical characterization was done by means of impedance spectroscopy (IS) measurements, which were carried out with the membranes in contact with NaCl solutions at different concentrations (10 −3  c(M)  5 × 10 −2 ). Electrical resistance and equivalent capacitance of the different membrane samples were determined from IS plots by using equivalent circuits as models. Results show a clear decrease in the membrane electrical resistance as a result of both polysulfone sulfonation and the increase of the concentration of modifying substances, although a kind of limit concentration was obtained for both polyethylene glycol and lignosulfonate (40 and 20%, respectively). Results also show a decrease of around 90% in electrical resistance due to polysulfone sulfonation, while the value of the dielectric constant (hydrated state) clearly increases.  2004 Elsevier Inc. All rights reserved. Keywords: Sulfonated polysulfone membranes; Polyethylene glycol; Lignosulfate; Thermal treatment; Impedance spectroscopy; Equivalent circuits 1. Introduction Membrane separation systems are currently employed in different industrial fields, mainly those related to the use of pressure and concentration gradients (ultrafiltration, nanofil- tration, or reverse osmosis and dialysis or hemodialysis, re- spectively), but those associated with an electrical potential gradient (electrodialysis) are also well established nowa- days [1–3]. Moreover, new membrane applications such as fuel cells must be considered, due to their industrial interest importance [4]. It is clear that the structure of membranes and the materials used in such diverse kinds of separation * Corresponding author. Fax: +34-952-132382. E-mail address: j_benavente@uma.es (J. Benavente). processes, as well as the characterization techniques, have to be rather different. In fact, to predict the membrane per- formance under pressure differences, it is often necessary to know the mean pore size and pore size distribution, or membrane (active layer) thickness and salt rejection [5–9]. However, for processes directly related to the transport of charged species, membrane electrical parameters such as ion transport numbers, bulk and surface charge concentra- tion, and membrane resistance (or conductivity) are para- meters of major interest [10–13]. On the other hand, with respect to membrane materials, polyamide, polysulfone, and regenerated cellulose are polymers commonly used in sep- aration under pressure and concentration gradients [1,2,7, 11], while sulfonated polymers are used as cation-exchange membranes in electrolysis and electrodialysis cells [14,15]. 0021-9797/$ – see front matter  2004 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2004.11.051