Effect of Inorganic and Organic Salts on the Thermogelling Behavior of Poly(organophosphazenes)

Abstract

Summary: 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 α‐amino‐ω‐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 (Tass) and the temperature at the maximum viscosity (Tmax) shifted to lower temperatures. This indicates that the inorganic salts facilitated the thermosensitive gelation of the poly(organophosphazenes). The viscosity (Vmax) at Tmax 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 Tass and Tmax, and decreased Vmax. 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. magnified image

Effects of salts in the Hofmeister series and solvent isotopes on the gelation mechanisms for hydroxypropylmethylcellulose hydrogels

“…A similar trend was reported by Cho, An, and Song 41 in their study of the effects of salts on the viscosity of polyorganophosphazenes. 41 As discussed earlier, the thermally induced gelation of HPMC solutions mainly involves hydrophobic association, which leads to a three-dimensional network. Therefore, gel strength is governed by the hydrophobic associations.…”

Salting-in effect of ionic liquids on poly(N-vinylimidazole) hydrogels

“…However, this phenomenon must be more complicated because the magnitude of the salting-out effect depends on the nature of the polymer and salt as well as on the ionic strength and polymer concentration. The Hofmeister series sorts anions and cations of common salts by their relative salting-out efficiency in descending order [13,14,17]. Anions appear to have a greater influence than cations and their effects seem to be independent and additive [7,11,15,20].…”

“…Some electrolytes improve the solubility of linear nonionic polymers or the swelling ability of nonionic gels in aqueous media relative to pure water, which is called the salting-in effect [8,11,[14][15][16][17]21]. In general, salts inducing salting-in effects are expected to be at the end of the Hofmeister series but anti-Hofmeister behavior has also been reported for some gels [22].…”

“…The solubility of nonionic polymers in water is greatly influenced by the addition of salts [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20]. Frequently, the solubility reduces sharply upon salt addition, which is known as the salting-out effect.…”

Porous ultrafine fibers via a salt-induced electrospinning method

“…As we know, addition of salts into electrospinning system can control fiber properties [22]. For example, the fiber will have some novel catalytic, optical, and magnetic properties by adding some salts [23][24][25]. It has been reported that highly porous nylon-6 fibers can be obtained by introducing salt [22] to the electrospinning system, but in this method, pores were still formed by selective dissolution of the salt in the as-spun fibers.…”