Polyacrylates in aqueous solution. The dependence of protonation on molecular weight, ionic medium and ionic strength

Abstract

The protonation constants of polyacrylates with different molecular weights ( W=2000–750 000 Da) were determined in different ionic media (alkali metal chlorides and nitrates, tetraalkylammonium chlorides), at 25 °C, by potentiometric measurements (H +–glass electrode). Literature data were also considered. Different models used to analyse protonation data were compared: the first was the modified Henderson–Hasselbalch two-parameter equation, and the second was the three-parameter equation proposed by Högfeldt. The dependence on the ionic strength of the different supporting electrolytes and all the protonation parameters involved in the two models showed the trend Et 4N +≫Li +>Na +≥K +, in accordance with the tendencies of these cations to interact with polycarboxylates. The dependence on molecular weight shows a slightly increasing effect with N (number of monomer units in the polyelectrolyte). Empirical relationships were found for the dependence of the protonation parameters on both ionic strength (I/mol l −1) and N. The use of protonation data was considered for polyacrylates in speciation studies, and specific interaction parameters were calculated according to the SIT (Specific Ion Interaction Theory) model.