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

摘要

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 ℃, 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 Hoegfeldt. 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 1~(-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.