Ion-specific swelling behavior of poly(N-vinyl-2-pyrrolidone) gel: Correlations with water hydrogen bond and non-freezable water

Poly(N-vinyl-2-pyrrolidone) (PVP) gels were prepared by irradiating aqueous solutio ns with gamma-rays. Swelling behavior was investigated as a functi on of salt concentration in the immersing solution. The observed anion spec ificity for the swelling ratio in the higher concentration region of potass ium salts (greater than or equal to ca. 1 M) was SO42- < F- < Cl- < SCN- le ss than or equal to I-, and the cation specificity of chlorides, which was less marked than that of the anion, was Cs+ < K+ less than or equal to Na< Li+ less than or equal to Ca" < Mg2+. The hydrogen bond enthalpy of water (Delta H-HB) in the gel was estimated by IR measurement. The Delta H-HB va lues showed a clear correlation with the swelling ratios; the water hydroge n bond is stronger for systems of higher swelling ratios except for iodides and thiocyanates. The non-freezable water (NFW) content of the gel was eva luated by DSC measurement. The amount of NFW increases for chlorides that s how higher swelling ratios. All the results, except for those for I- and SC N- ions which probably are bound to PVP, are reasonably interpreted on the basis of a mechanism for ion specific swelling proposed in the previous stu dy. (C) 1998 Elsevier Science Ltd. All rights reserved.