Preparation and characterization of polyacrylonitrile membranes modified with polyelectrolyte deposition for separating similar sized proteins Filiz Yasar Mahlicli a , Sacide Alsoy Altinkaya a,n , Yilmaz Yurekli b a Department of Chemical Engineering, Izmir Institute of Technology, Gulbahce Kampusu, 35430, Urla, Izmir, Turkey b Department of Chemical Engineering, Ondokuz Mayıs University, Kurupelit Kampusu, 55139, Kurupelit, Samsun, Turkey article info Article history: Received 10 October 2011 Received in revised form 10 May 2012 Accepted 11 May 2012 Available online 22 May 2012 Keywords: Ultrafiltration Polyelectrolytes Polyacrylonitrile Myoglobin Lysozyme abstract One of the challenges faced by ultrafiltration membranes is to separate proteins with a small difference in their molecular weights. Recently, some researchers tried to overcome this problem by using charged membranes. This study examined the use of layer by layer deposition of polyelectrolytes on the chemically-modified polyacyronitrile membrane to increase the selectivity of the ultrafiltration. The membranes were prepared by wet-phase inversion technique and polyethylenimine (PEI) and alginate (ALG) were chosen as cationic and anionic polyelectrolytes for the modification of the surfaces. Sieving coefficient data were obtained with myoglobin and lysozyme as model proteins. The influences of solution pH, ionic strengths of the protein and polyelectrolyte solution and the number of polyelectrolyte bilayers on both selectivity and throughput were investigated. The highest selectivity and throughput were achieved with the 1-bilayer PEI-ALG coated polyacrylonitrile (PAN) membrane. Increasing the number of coating bilayers or the ionic strength of the protein solution or adding salt into the polyelectrolyte coating solution decreased both the maximum selectivity and throughput of the modified membranes. & 2012 Elsevier B.V. All rights reserved. 1. Introduction Previously, ultrafiltration systems have been limited to separat- ing proteins that differ in molecular mass by at least a factor of 10. In recent years, it was shown that charged ultrafiltration mem- branes can overcome this limitation, since the process in this case involves both size and charge-based exclusion. Several studies have demonstrated that electrostatic interactions between the proteins and membrane play an important role in obtaining high-resolution separations. It was recognized that significant improvements in the ultrafiltration of the proteins can be obtained by controlling the pH and ionic strength of the protein solution. Pujar and Zydney (1994) found that the transmission of bovin serum albumin (BSA) through a 100 kDa membrane increased by more than two orders of magni- tude as the salt concentration was increased from 1.5 mM to 150 mM [1]. This was attributed to the increased electrostatic exclusion of the charged BSA from the membrane pores at low ionic salt concentration. By careful adjustment of pH and conduc- tivity, Saksena and Zydney (1994) found a significant increase in the selectivity for fractionation of BSA from immunoglobulin G [2]. Similarly, Rabiller–Baudary et al. (2001) also reported that, with an increase in ionic strength, lysozyme transmission rates through zirconia and grafted membranes also increased [3]. Nakao et al. (1988), Saksena and Zydney (1994), Balakrishnana and Agarwal (1996), Lucas et al. (1998) and Burns and Zydney (1999) reported maximum protein transmission at the isoelectric point of the protein [4–7]. Several studies have also shown the importance of membrane surface charge characteristics on the high-resolution separation of proteins [8–10]. In these studies, reduced protein transmission rates were reported under conditions where the membrane and protein have similar charges, and were attributed to the resulting electrostatic repulsion between the protein and membrane. van Reis et al. (1999) have found higher selectivity for fractionation of BSA and Fab proteins by using charged membranes compared to neutral membranes [10]. In the literature different methods were proposed for placing a charge on the surface of ultrafiltration and reverse osmosis membranes, such as treatment with chemical reagents, irradia- tion [4,9,11–16] and proteins [17–19]. The layer-by-layer (LBL) assembly technique [20] was also used as an alternative method for creating either positively or negatively charged surfaces [21–23]. In this technique, a membrane with a charged surface is immersed in a solution of a macromolecule carrying opposite charges to those of the membrane surface. Commercial mem- branes with negative charges on their surface can be directly used [24,25] or classical membranes such as polysulfone and poly- acrylonitrile can be functionalized with charged groups by surface modification. Polyelectrolyte multilayer (PEM) show selective Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/memsci Journal of Membrane Science 0376-7388/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.memsci.2012.05.028 n Corresponding author. Tel.: þ90 2327506658; fax: þ90 2327506645. E-mail address: sacidealsoy@iyte.edu.tr (S. Alsoy Altinkaya). Journal of Membrane Science 415–416 (2012) 383–390