Thermodynamic study for the protonation of branched poly(ethylenimine) in NaClaq and its dependence on ionic strength.

In this paper, we investigated the interactions between poly(ethylenimine) [BPEI] and the proton by using (H+- glass electrode) potentiometry and titration calorimetry. The measurements were carried out in NaCl aqueous solutions, at T ) 298 K in a wide range of ionic strength (up to I 5.0 molâL-1). The protonation constants were calculated by following different approaches: (a) first, all the protonation constants were considered as true constants (considering BPEI as a simple low molecular weight tetramine); (b) second, the log KH 3 was considered as a linear function of pj (pj ) protonation degree); and (c) the log KH 3 was considered as a nonlinear function of pj. The difference in the standard deviation on the fit between (a) and (b) calculations is quite significant, while no further improvement is observed by using the (c) model. The protonation enthalpies were calculated from the calorimetric titrations. In these calculations, all the protonation steps of BPEI were considered as independent of pj. The dependence of protonation constants and enthalpies on ionic strength was modeled using the Specific ion Interaction Theory (SIT).