Density functional complexation study of metal ions with poly(carboxylic acid) ligands. Part 1. Poly(acrylic acid) and poly(a-hydroxy acrylic acid) Henna Pesonen a,c,1 , Atte Sillanpa¨a¨ a,1 , Reijo Aksela b,2 , Kari Laasonen c, * a Department of Chemistry, University of Naples ‘Federico II’, I-80126 Naples, Italy b Kemira Oyj, Espoo Research Center, P.O. Box 44, FIN-02271, Espoo, Finland c Department of Chemistry, University of Oulu, P.O. Box 3000, FIN-90014 Finland Received 2 September 2005; received in revised form 12 October 2005; accepted 12 October 2005 Available online 2 November 2005 Abstract We have studied metal ion complexation with poly(carboxylic acid) ligands using density functional methods and a continuum solvation model (COSMO). Geometry optimisations have been carried out for metal complexes of poly(acrylic acid) (PAA) and poly(a-hydroxy acrylic acid) (PHA) oligomers. The complexation energies for Mg 2C , Ca 2C , Mn 2C , Fe 3C , and Zn 2C with one oligomer ligand (pentamer), and two ligands (dimer and trimer) have been calculated. The nature of hydrogen bonding in PHA has been studied both in free oligomers and in metal complexes. The obtained binding degrees are in good agreement with experimental results although with quite small energy differences. The complexation energies were found to be very sensitive to changes in internal hydrogen bonds. The role of hydroxyl oxygen atoms in metal ion binding was observed to be more important for Ca 2C than for other metals. q 2005 Elsevier Ltd. All rights reserved. Keywords: Metal ion complexation; Ab initio calculations; Binding energies 1. Introduction Complexation is a very efficient way to inactivate and remove metal ions. Many heavy metals, e.g. lead and chromium are directly toxic to humans [1], but they can also be a problem in different industrial processes. From the viewpoint of the paper industry, the most problematic metal ions are manganese and iron, which catalytically decompose hydrogen peroxide and peracids which are used in the pulp bleaching processes [2]. Pulp contains not only these harmful metal ions but also magnesium and calcium. Mg 2C has actually been found to be beneficial to the bleaching process, and Ca 2C is inactive and its complexation only consumes the complexating agent. The widely used and efficient complexing agents, such as EDTA (ethylene diamine tetraacetic acid) and DTPA (diethylenetriamine pentaacetic acid), display low biodegrad- ability [3]. Furthermore, they are able to mobilise heavy metals from sediments in natural waters. Thus, it is important to develop and study more environmentally friendly complexing agents. One interesting group of these includes (nitrogen free) poly(carboxylic acids), e.g. poly(acrylic acid) and poly(a- hydroxy acrylic acid). The most common poly(carboxylic acids), e.g. poly(acrylic acid), are weak acids and their pK a values are linearly dependent on the degree of proton dissociation, a [4]. This has been explained by the electrostatic interaction between negatively charged carboxylate groups in the polymer [5]. For the polymers with higher charge density, e.g. poly(maleic acid), on the other hand, a sharp increase in pK a has been reported at ca. aZ0.5. This interesting property has been thought to be related to intramolecular hydrogen bonding between the dissociated and undissociated carboxyl acid groups [6]. According to these previous experimental studies, internal hydrogen bonds have been shown to have a very clear effect on properties and behaviour of polyelectrolytes. The dissociation behaviour of weak polyacids has been studied widely by experimental methods [4,7]. Metal ion binding properties of poly(acrylic acid) (PAA) have been studied by equilibrium dialysis [8], thermogravimetry, and spectroscopic methods [9]. PAA has been found to be the most Polymer 46 (2005) 12641–12652 www.elsevier.com/locate/polymer 0032-3861/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2005.10.069 * Corresponding author. Tel.: C358 8 533 1640; fax: C358 8553 1603. E-mail addresses: henna.pesonen@oulu.fi (H. Pesonen), atte.sillanpaa@ csc.fi (A. Sillanpa¨a¨), reijo.aksela@kemira.com (R. Aksela), kari.laasonen@ oulu.fi (K. Laasonen). 1 Tel: C39 081 674 212; fax: C39 081 674 090. 2 Tel: C358 1061 516; fax: C358 10862 2000.