The branched PEI contain 35% of 1◦, 37% of 2◦and 28% of 3◦amines with pKa values of 4.5, 6.7, and 11.6, respectively. PEI is in the fully protonated form at pH < 5, however, in a basic environment, only the 3◦amines of PEI possesses positive charges [18,22].