Thermodynamic Properties of O-Donor Polyelectrolytes: Determination of the Acid–Base and Complexing Parameters in Different Ionic Media at Different Temperatures

Abstract

A potentiometric study on the acid–base properties and formation of complexes with alkali metals (i.e., Na+ and K+) of some poly­(methyl vinyl ether-co-maleic) acids, namely, Gantrez AN169 (2000 kDa), S95 (220 kDa), and S97 (1200 kDa), was carried out under different experimental conditions consisting of the ionic medium, ionic strength, and temperature. Owing to the possible formation of micelles, the critical micelle concentration of Gantrez AN169 was determined by means of different techniques in pure water and at T = 298.15 K. The diprotic-like model was used for the elaboration of the experimental protonation data, and the best model was obtained by assuming that a monomeric unit consists of two methyl vinyl ether-co-maleic acid residues. Moreover, the protonation data of two poly­(acrylic comaleic acids), PCA 3 kDa and 70 kDa (published in a previous paper), under different experimental conditions were reanalyzed. The trend in the protonation constants of polyelectrolytes in different ionic media is (C2H5)4N+ ≫ K+ > Na+. The variation of the protonation and complex formation constants with the ionic strength was interpreted in terms of both the variation of the activity coefficients and the formation of Na+- and K+-polymer complexes.

Use of Gantrez Copolymers as Potential Chelating Agent for the Selective Sequestration of Metal Ions. Studies of the Interactions in Aqueous Solution at Different Ionic Strengths and Temperatures

“…As already reported in previous papers, , in the case of the Gantrez ligands, the best results were obtained by assuming a monomeric unit of GTZ formed by two methyl-vinyl-ether- co -maleic acid residues, and considering four carboxylic groups in our model. For this reason, from the acid–base and complexing point of view, GTZs were considered like ligands with charge −4.…”

“…The Högfeldt model requires the calculation of three stability constants calculated when α → 0, α → 1, and a third constant that accounts for the nonlinear variation of the stability constants at 0.1 ≤ α ≤ 0.9. These calculations are not simple for non expert researchers in this field, and for this reason, recently, a simplified approach called the diprotic like model ,, was proposed. This approach allows treating a polyelectrolyte, from an acid–base and complexing point of view, like a simple low molecular weight ligand, namely independent of the dissociation degree (α), taking into account the minimum number of protonation sites necessary to extensively describe the system.…”

“…The standard stock solutions of Gantrez copolymers were prepared under the critical micelle concentration, already determined in a previous paper, from the solid products [Ashland] and without further purification. The ligand concentrations were checked by alkalimetric titration, as already reported in ref . All the information about the physical and chemical properties of Gantrez ligands (i.e., formula, molecular weight, viscosity, purity, etc.)…”

“…These studies started from the information already available in the literature on the behavior of these copolymers in aqueous solution ,− and in particular on the acid–base properties, the critical micelle concentration, and the ability of these ligands to form weak complexes with the alkaline cations (Na + and K + ) …”

“…Owing to the difficulty to use the Henderson–Hasselbalch or Högfeldt approaches, − a simplified approach denominated the diprotic like model ,, was employed to study the thermodynamic properties in aqueous solutions of acidic or basic polyelectrolytes. This allows treating a polyelectrolyte, from an acid–base and complexing point of view, like a simple low molecular weight ligand, taking only into account the minimum number of protonation sites necessary to extensively describe the system, without considering the dissociation degree of the polyelectrolytes at the different pHs.…”