Effect of external salt solution concentration on carboxyl dissociation degree (α) and pKa of weak polyelectrolyte membranes for sustainable technologies

Abstract

Understanding the dissociation process of weakly charged polymers under varied external salt conditions is critical to develop innovative charged polymer membranes with desirable transport properties for sustainable technologies. We previously designed a series of weakly charged polymer membranes, i.e., cross-linked acrylic acid (AA)–poly(ethylene glycol) diacrylate (PEGDA) (AA–PEGDA) random copolymer networks with a wide ion-exchange capacity (IEC = 0–4 mequiv. per g) range and limited water swelling. Here, we report the dissociation process in the representative chemical structure of AA–PEGDA series, i.e., 10-2 AA–PEGDA network (PEGDA cross-linker length n = 10, mIEC = 2 mequiv. per g) in different external salt concentration solutions (0–1 M NaCl (aq)). When titrated with a strong base (NaOH), both POT titration (detecting a solution phase) and ATR–FTIR analysis (probing a polymer phase) well describe the dissociation process and yield similar ranges of dissociation parameters (α, pK_a). The dissociation behavior follows the modified Henderson–Hasselbalch equation, showing lower pK_a values with increasing external salt concentrations. The governing molecular factor for dissociation was determined by comparing four length scales (r_c, r_ion, l_B, and r_D) in the system, including (1) charged group distance in a polymer (r_c), (2) distance between salt ions in an external solution (r_ion), the respective (3) Bjerrum length (l_B) and (4) Debye screening length (r_D). At the lower external salt concentration (0 M ≤ C_Nacl ≤ 0.01 M), the relative standing of these length scales (r_c < l_B ≪ r_D < r_ion) indicates that the enhanced electrostatic interaction in dilute conditions suppresses the dissociation in the network and thus increases pK_a. At the higher salt concentration (0.1 M ≤ C_Nacl ≤ 1.0 M), the different order of these length scales (r_D ≪ r_c ≲ l_B ≲ r_ion) represents that the screened electrostatic interaction via the added external salts promotes the dissociation and thus decreases pK_a. As the external salt concentration increases (0–1.0 M NaCl), water swelling in the network slightly decreases due to osmotic deswelling. However, the lower water swelling has very little effect on effectively decreasing the charged group distance in the polymer (r_c), and thus, leads to little substantive influence on the electrostatic interaction (consequently, the dissociation). Therefore, the screened electrostatic interaction by the added external salts dictates the dissociation and pK_a in our system. Our multiscale analysis of dissociation across varying external salt concentrations provides a pathway to tune the dissociation behavior of weakly charged polymers and achieve desired transport properties for target applications.