Medium effects on the protonation enthalpies of linear diamines in NaCl aqueous solutions, at 25°C

Protonation enthalpies of ethylenediamine, 1,6-hexanediamine, 1,8-octanediamine and 1,10-decanediamine were determined at 258C in NaCl aqueous solutions, 0 < I 3 mol dm ÿ3 (0 < I 5 mol dm ÿ3, for 1,6-hexanediamine). The dependence on ionic strength of DH0 can be expressed by the equation ÿDH0 ÿDH0 I 0 a1I a2I2. Values of enthalpy changes at different ionic strengths made it possible to determine the temperature gradients of the speci®c interaction parameters of Pitzer equations. The functions DG0 and DH0 versus ionic strength are very similar and the resulting T DS0 values are quite low and fairly constant. Other empirical relationships for the dependence of both DH0 and DG0 on alkyl chain length and ionic strength are reported.