title: Competitive Cd2+/H+ complexation to polyacrylic acid described by the stepwise and intrinsic stability constants.
abstract: Stepwise constants can be used to describe competitive proton and metal binding to macromolecules with a large number of sites. With the aim of accessing information on the microscopic binding model, we report an expression that connects the stepwise constants to the site-specific metal constants. This expression holds for a very general complexation model including heterogeneity, interactions, and chelate complexation. Assuming bidentate binding of the Cd ions to adjacent carboxylate groups in poly(acrylic acid), stepwise and intrinsic stability constants for proton and cadmium binding were estimated from the experimental data. Intrinsic values were split into specific and electrostatic contributions (by means of the Poisson-Boltzmann equation under cylindrical geometry). Free of the electrostatic contribution, the remaining Cd binding energy showed almost no dependence on the coverage and ionic strength, and the corresponding average values allowed for a reasonable reproduction of raw binding data. Small systematic discrepancies from the homogeneous behavior are critically discussed.
authors.affiliation: Departament de Química, Universitat de Lleida (UdL), Avenida Rovira Roure, 191. E-25198 Lleida, Spain.
authors.forename: Calin
authors.initials: C
authors.lastname: David
authors.forename: Encarnació
authors.initials: E
authors.lastname: Companys
authors.forename: Josep
authors.initials: J
authors.lastname: Galceran
authors.forename: Josep Lluís
authors.initials: JL
authors.lastname: Garcés
authors.forename: Francesc
authors.initials: F
authors.lastname: Mas
authors.forename: Carlos
authors.initials: C
authors.lastname: Rey-Castro
authors.forename: José
authors.initials: J
authors.lastname: Salvador
authors.forename: Jaume
authors.initials: J
authors.lastname: Puy