Influence of the counter-ion on the effective charge of polyacrylic acid in dilute condition

Osmotic pressure measurements were satisfactorily used to compute the effec tive charge of poly(acrylic) acid (PAA) at different pHs and with different monovalent and divalent counterions. This experimental method is sensitive to the osmotically active species (polymer and counterions). After correct ion for the polymer contribution to the total osmotic pressure (solvency an d excluded volume), the remaining pressure can be attributed to the polyele ctrolyte counterions and processed with Donnan and equation of state in ord er to compute the effective charge number per polymeric chain (Z(eff)). The behavior of Z(eff) against the chain concentration and pH was investigated after neutralization of PAA with LiOH, NaOH, and TMAOH. The results clearl y indicate that the nature of the monovalent counterion has no effect on Z( eff) leading to the conclusion that the interaction between monovalent coun terions and the acrylate functionality is purely electrostatic in agreement with conductimetric and potentiometric results reported in the literature. The behavior of Z(eff) against the degree of ionization of the polymer and its concentration is also in good agreement with the theoretical expectati ons of the theory of ionic condensation. Osmotic measurements were also use d in order to understand the influence of divalent cations (Mg2+, Ca2+, Ba2 +) on Z(eff) of the sodium salt of PAA at pH 9 and at different divalent/ac rylate molar ratio. All the divalent cations depress Z(eff) each of one at different degrees, confirming a specific divalent/polymer interaction. The energy of hydration of cations can explain most of the observed results wit h divalents. (C) 2001 Elsevier Science Ltd. All rights reserved.