Salt Effects on Complexes of Oppositely Charged Macromolecules Having Different Affinity to Water

The influence of salt concentration on the size and on the thermodynamic stability of interpolymer complexes composed of oppositely charged macroions having different affinity to solvent was studied from a theoretical viewpoint. It was shown that increasing salt concentration causes changes in the structure of complex particles. At low salt concentration, the particles preserve their structure and size. At a critical salt concentration, n(S)(cr), the particle size rises sharply to a slightly larger dimension. From this concentration, the macroions forming the interpolymer complex start to separate, and the complex is fully destroyed at a salt concentration n(S)*. After separation, the macroions coexist in solution and with further increase in salt concentration reduce their sizes according to the screening of polyion charges by salt ions. n(S)(cr) and n(S)* depend on physical parameters such as the degree of polymerization of macroions, their degree of ionization, and macroion-solvent interaction parameters. Experimental data collected in the particular cases of PLL-PLCA and PLL-PLCAI complexes with polylysine qualitatively agree with the trends indicated by the theoretical approach.