Effect of different electrolytes on the swelling properties of calyx[4]pyrrole-containing polyacrylamide membranes Artur J.M. Valente a, * , Abı ´lio J.F.N. Sobral a , Alfonso Jime ´nez b , Sı ´lvia Patachia c , Ana R.C.B. Oliveira a , Victor M.M. Lobo a a Department of Chemistry, University of Coimbra, 3004-535 Coimbra, Portugal b Department of Analytical Chemistry, Nutrition and Bromatology, University of Alicante, P.O. Box 99, 03080 Alicante, Spain c Department of Chemistry, ‘‘Transilvania’’ University of Brasov, 29 Eroilor Str., 500036 Brasov, Romania Received 5 March 2006; received in revised form 26 March 2006; accepted 2 April 2006 Available online 19 May 2006 Abstract Calix[4]pyrrole (1) was synthesized and characterized and this macrocycle was incorporated in polyacrylamide gels. The presence of meso-octamethyl-porphyrinogen inside of gel was checked using infrared spectroscopy, differential scanning calorimetry, and swelling studies. The swelling degree of these hydrogels in equilibrium with different electrolytes (NaCl, LiCl, KCl, CaCl 2 and AlCl 3 ) was measured in a concentration range 0.1–0.5 mol dm 3 . Although no significant alterations in the swelling degree can be found for the different 1:1 electrolytes, when the cation charge of unsymmetrical electrolytes increases, the gel swells in a significant way. This swelling process is enhanced by the presence of calyx[4]pyrrole. The effect of alkaline hydrolysis of polaycrylamide-based hydrogels was also studied. The hydrolysed hydrogels collapse in the pres- ence of the electrolytes; this behavior is dependent on the hydrolysis degree, electrolyte charge and calyx[4]pyrrole presence and concentration; the latter leads to polyacrylamide with tailor-made properties. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Hydrogels; Polyacrylamide; Calyx[4]pyrrole; Swelling degree; Hydrolysis 1. Introduction Porphyrinogens, also known as calix[4]pyrroles, are excellent chelating agents for binding anions [1–5] and neutral molecules [6], host materials for inclusion crystals [7] and also compounds for metal complexation [8–11]. The past three decades have seen a growing inter- est in the use of neutral chelating agents to achieve ion permselectivity in membranes [12,13], which can be used for separation processes [14], or as sensing components in ion-selective electrodes [15]. Much work has been done incorporating neutral chelating agents, such as crown-ethers in hydrogels with low water content, in particular by physical immobilisation of crown-ethers in the water-swollen polymers [16]. Modified polyacrylamide hydrogels have also been developed to remove metallic ions (e.g., Cu 2+ ) 0014-3057/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2006.04.002 * Corresponding author. Tel.: +351 239854459; fax: +351 239827703. E-mail address: avalente@ci.uc.pt (A.J.M. Valente). European Polymer Journal 42 (2006) 2059–2068 www.elsevier.com/locate/europolj EUROPEAN POLYMER JOURNAL