Research Article Influence of Co-Solvent Concentration on the Properties of Dope Solution and Performance of Polyethersulfone Membranes Polyethersulfone (PES) dope solutions were prepared from mixtures of two sol- vents containing N,N-dimethylformamide (DMF) as core solvent and acetone as co-solvent (CS) in a closed heating system. PES asymmetric membranes were cast by a dry/wet phase inversion process. Complete miscibility of PES with the fixed mixture of acetone and DMF under atmospheric pressure could be achieved. The kinetic and thermodynamic properties indicated that interaction of DMF and acetone is strongest when their mole ratio is unity, pointing to the phenomenon of true co-solvency for PES dissolution. These results were supported by determi- nation of the water uptake, contact angle measurement, and SEM analyses. Mem- brane performance, pore volume, and total pore volume on the membrane sur- face were also investigated. Keywords: Asymmetric membrane, Co-solvent, Dope solution, Polyethersulfone, Solvents Received: April 13, 2013; revised: June 02, 2013; accepted: July 01, 2013 DOI: 10.1002/ceat.201300235 1 Introduction The development of innovative polymeric membrane materials is one of the most significant contributions to progresses in membrane technology. Among the amorphous polymers, the sulfone family polymer, polyethersulfone (PES), is the most promising membrane material for water and gas separation applications [1–3]. Some earlier studies reported that various concentrations of PES in aprotic solvent (N,N-dimethylform- amide (DMF), N-methyl-2-pyrrolidone (NMP), N,N-di- methylacetamide (DMAc), dimethylsulfoxide (DMSO)) cast solutions were used to produce membranes with denser and less porous top surface structures. However, this was far from expectations where non-solvent components are vital in pre- paring high-performance membranes with required morphol- ogy [1, 4, 5]. Significant progress has been made in improving PES membrane liquid and gas separation characteristics by formulating the right polymer solution in two solvent system compositions [6, 7]. Numerous studies demonstrated that the addition of a co-solvent (CS) or non-solvents (NS) to polymer solutions could significantly improve the performance and structure of asymmetric membranes for liquid separation [1, 8]. It was also determined that each solvent/non-solvent in PES has a considerable influence on the structure of the mem- brane skin layer and the sub-layer which resulted in lower water fluxes [1, 6–8]. Traditionally, the formation of PES by means of Lewis acid- base complexes with aprotic solvent has been used, however, several researchers also employed solvent/aliphatic alcohol as co-solvent or non-solvent and characterized the PES asym- metric membrane for dialysis, ultrafiltration (UF), and micro- filtration (MF) [8–10]. Li and Jiang [10] investigated the influ- ence of PES concentration in PES/NMP, PES in NMP/H 2 O, and series of NS (1-butanol, ethylene glycol, diethylene glycol) concentrations in NMP/PES on rheological properties of UF membrane-forming systems. Their results revealed that the viscous property was dominant in PES/NMP and PES/NMP/ CS membrane-forming systems which exhibited Newtonian properties [10]. Acetone was extensively employed but mostly as a non-sol- vent in amounts of less than 5 wt % [1, 11] while Barth et al. investigated the physical properties of polysulfone (PSf)- DMF-acetone polymer solution [11]. In a previous study [12], the influence of a lithium bromide (LiBr) additive on PES membranes was studied. The amount of PES/DMF was kept constant, whereas the weight ratios of LiBr (1–5 wt %) to ace- tone were varied. The produced membrane exhibited good performance. In this study, the PES concentration is kept at 20 wt %, and the weight ratios of acetone to DMF are varied in the range of 20–25wt %. To the best of our knowledge, to date there exist no reports on the effect of maximum concentration of acetone Chem. Eng. Technol. 2013, 36, No. 10, 1–9 © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.cet-journal.com Iqbal Ahmed 1 Ani Idris 2 Ahmad Hussain 3 Zainal A. M. Yusof 4 Muhammad Saad Khan 1 1 Universiti Teknologi Petronas, Chemical Engineering Department, Tronoh, Perak, Malaysia. 2 Universiti Teknologi Malaysia, Faculty of Chemical Engineering, Department of Bioprocess Engineering, Skudai, Johor, Malaysia. 3 King Abdulaziz University, Department of Nuclear Engineering, Jeddah, Kingdom of Saudi Arabia. 4 Polymer Engineering Technology, UniKL-MICET, Taboh Naning, Malacca, Malaysia. – Correspondence: Dr. I. Ahmed (iqbal.ahmed@petronas.com.my), Universiti Teknologi Petronas, Chemical Engineering Department, Bandar Seri Iskandar, 31750 Tronoh, Perak, Malaysia. Asymmetric membrane 1