Medium Effect on the Acid-Base Properties of Branched Polyethylenimine in Different Aqueous Electrolyte Solutions

The effect of different ionic media on the protonation of branched polyethylenimine (BPEI) was studied potentiometrically in aqueous solutions containing LiCl, KCl, and Me4NCl from (0.5 to 3) mol.L-1 and CaCl2 from (0.2 to 2) mol.L-1 at T = 298.15 K. Protonation constants of BPEI in alkali metal chlorides H are quite similar, and the trend is (alkali Cl) > CaCl2 > Me4NCl The dependence on ionic strength of K in different ionic media, was modeled using the Specific ion Interaction Theory (SIT) approach. Differences in protonation constants (in CaCl2 and Me4NCl, at low ionic strength, I < 1 mol.L-1) were also interpreted in terms of weak complex formation. These complexes are quite weak: log K (M + A = MA) = (24.9 and 24) L.mol(-1), for M = Ca2+ and Me4N+ (A = BPEI), respectively, at I = 1 mol.kg(-1). BPEI protonation enthalpies and entropies were determined by direct calorimetric titrations at different ionic strengths. The Delta H-i trend is KCl > NaCl > LiCl > CaCl2, for each protonation step of BPEL Dependence on ionic strength for Delta H-i was taken into account by the SIT model.