The effect of different aqueous ionic media on the acid-base properties of some open chain polyamines.

Acid-base properties of some open-chain polyamines (ethylenediamine, diethylenetriamine, triethylenetetramine, spermine, tetraethylenepentamine and pentaethylenehexamine) were studied at different ionic strengths in different aqueous ionic media at 25 °C. Measured were: (i) the protonation constants of triethylenetetramine, tetraethylenepentamine and pentaethylenehexamine from potentiometric measurements [0 ≤ I ≤ 2.5 mol·L−1 in NaCl and (CH3)4NCl)]; and (ii) protonation enthalpies of ethylenediamine, diethylenetriamine, and spermine from calorimetric measurements [NaCl: 0 ≤ I ≤ 1 mol·kg−1 for ethylenediamine, diethylenetriamine, 0 ≤ I ≤ 2 mol·kg−1 for spermine; (C2H5)4NI: 0 ≤ I ≤1 mol·kg−1; (CH3 )4NCl: 0 ≤ I ≤ 2.5 mol·kg−1 only for diethylenetriamine]. Previously published protonation data for these polyamines in aqueous NaCl, (CH3)4NCl and (C2H5)4NI, were also examined. The general trends for the Gibbs energy and entropic contributions are, for G: NaCl > (CH3)4NCl > (C2H5)4NI, and for TS: (C2H5)4NI > (CH3)4NCl > NaCl. This trend is more pronounced for the first protonation step. The dependences of these quantities on ionic strength were modeled with the SIT (Specific ion Interaction Theory) equations, and differences found among the different media were interpreted in terms of weak complex formation.