Effect of inorganic and organic salts on the thermogelling behavior of poly(organophosphazenes)

The effect of various inorganic (NaCl, KCl, NaBr, LiCl and NaI) and organic (Et4NBr, n-Pr4NBr and n-Bu4NBr) salts on the thermogelling behavior of poly(organophosphazenes) with alpha-amino-omega-methoxy-poly(ethylene glycol) (AMPEG) and amino acid ester side groups was studied. The salting-out or salting-in effects of inorganic salts were in good agreement with the so-called "Hofmeister series". In the presence of inorganic salts other than NaI, the association temperature (T-ass) and the temperature at the maximum viscosity (T-max) shifted to lower temperatures. This indicates that the inorganic salts facilitated the thermosensitive gelation of the poly(organophosphazenes). The viscosity (V-max) at T-max was greatly augmented by the addition of inorganic salts, implying an increase in the number of hydrophobic association points. In contrast, NaI and organic salts dramatically suppressed the thermosensitive gelation of poly(organophosphazenes). The addition of these salts increased both T-ass and T-max, and decreased V-max. In the inorganic salt systems, both the radius and the charge density of the anion seem to be the main decisive factors with regard to salting-out and salting-in effects. In the organic salt systems, the suppression of thermosensitive gelation may be attributed to the ionization of the hydrophobic side groups of the poly(organophosphazenes) upon binding of the large cations of the organic salts through hydrophobic interactions.