Ion-Specific Swelling of Poly(styrene sulfonic acid) Hydrogel

Abstract

Poly(styrene sulfonic acid) (PSSA) hydrogel was prepared by radiation crosslinking using methyl N,N-bis-acrylamide as crosslinker. Effects of ion species and concentration on the swelling behavior of PSSA hydrogel were investigated in aqueous solution of selected anions (F-, Cl-, Br-, SCN-), cations (Li+, Na+, K+, Ca2+), and hydrophobic ions (tetramethylammonium cation TMA+, tetrabutylammonium cation TBA+, and dodecyltrimethylammonium cation TAB+). The deswelling extent of PSSA hydrogel follows anion Hofmeister series, i.e., SCN- < Br- < Cl- < F-, in solutions containing selected anions and K+ as counterion up to a concentration of 2 mol.L(-1). On the contrary, the deswelling extent of PSSA hydrogel in solutions containing selected cations and Cl- follows the sequence of Li+ < Na+ < K+ < Ca2+, which is the reverse of the Hofmeister series except Ca2+. We have discussed the effects of ions on the hydrogen bonding through SO3- and phenyl ring in salt solutions at low and high concentrations. Other interactions, such as the cation-pi and hydrophobic interactions, also contributed to the ion-specific swelling of PSSA hydrogel. The proposed mechanism was further elucidated by FTIR and NMR analysis. A very specific deswelling-reswelling phenomenon of PSSA hydrogel in KF solution has been observed and ascribed to the F- binding to phenyl ring through a specific interaction.

Hofmeister series and specific interactions of charged headgroups with aqueous ions

“…The reversion of the ion series was observed also when measuring the cation affinity to ion-exchange resins [44][45][46], in studies of membrane potentials [44], of electrophoretic mobility of colloids [41], ion-transport phenomena [41], swelling of hydrogels [47] and counter-ion binding to long-chain fatty acids [35,[48][49][50][51][52][53]. For instance, the competition of cations for charged fatty-acid monolayer interfaces was studied by Rosano et al [54].…”

“…Xu et al [47] studied the swelling and de-swelling of PSSA (Poly (styrene sulfonic acid)) hydrogels in aqueous solution of selected alkali cations. The de-swelling extent of PSSA hydrogel was found to follow the sequence Li + b Na + b K + .…”

“…It relates the tendency of oppositely charged ions to spontaneously associate as inner sphere ion pairs in aqueous solution to matching absolute free energies of ion hydration. This is supposed to be due to the fact that the strength of interaction between Advances in Colloid and Interface Science 146 (2009) [42][43][44][45][46][47] the ions and the water molecules is correlated to the strength with which the ion interacts with other ions [8]. With this model an impressive number of phenomena and properties can be described.…”

Specific F− binding to phenyl ring of aromatic polymers

“…The deswelling of polymer gels in the presence of electrolyte ions is universal and is ascribed to the destruction of hydrogen-bonding by ions [9,10,15,20]. Both F À and Li + ions have strong hydration ability.…”

“…PSSA, HPMCP and P4VPh gel. However, specific reswelling of the gels in concentrated KF solutions had never been reported by other researchers and the mechanism was not fully elucidated [20,21]. In this work, we investigated the swelling behavior of a series of polymer gels containing phenyl ring, for instance, PSSA, P4VPh and HPMCP gel, in KF solutions.…”