Primary amines are strong bases, but vicinal diamines have, due to charge repulsion between vicinal ammonium groups, a pronouncedly lower 2nd pKa.

For ethylene diamine, pKa1 and pKa2 are 9.92 and 7.11. To my knowledge, this is related to ideal solutions in water.

If the solution of ethylene diamine is in a medium of relatively high ionic strength (in presence of salts), does binding of the first ammonium group (pKa1) to an anion lead to an apparently higher second pKa (pKa2_app), due to shielding effects of the anion bound to the first ammonium group?

E.g., would the binding affinity of ethylene diamine to phospholipid membranes at slightly acidic pKa be underestimated (based on pKa2, dissociation degree less than 50%) based on pKa2 of 7.11 instead of pKa2_app?