Effect of added salt on the stability of hydrogen-bonded interpolymer complexes

Association of macromolecules in aqueous media through hydrogen bonding results in the formation of discrete interpolymer complexes (IPCs). In this work, the effect of added salt on the stability of IPCs consisting of polyacrylic acid and either a flexible polymer (polyethylene oxide or polyvinylpyrrolidone) or a semirigid polymer (hydroxy propylcellulose) is examined by a combination of spectrophotometry, viscometry and potentiometry. Addition of a neutral salt (e.g., NaCl) typically results in IPC aggregation. The response of the IPCs to salts that promote salting-in (i.e., LiSCN, NaSCN and GuSCN, where Gu denotes guanidine) is strongly cation-dependent, since Li⁺ cations induce an increase in solution pH, whereas Gu⁺ cations compete with polyacrylic acid for complexation sites. The degree of complexation (), calculated from potentiometric data, is greatest for IPCs in the absence of salt and is found to decrease in the order NaCl>NaSCN>GuSCN. Addition of salts that induce salting-in at polymer solution concentrations favoring complexation is found to enhance IPC solubility (and reduce ) until the IPC particles are either completely solubilized or dissociated.