Salt-induced volume phase transition of poly(N-isopropylacrylamide) gel

The salt effect on the phase transition of N-isopropylacrylamide (NIPA) gel was studied. The swelling behavior of the NIPA gel strongly depends on the salt concentration and is well described as a function of the chemical pot ential difference of water molecules in solution from that at the transitio n. From the analysis of the OH stretching, Raman spectra in water and in va rious aqueous solutions in terms of collective proton motions reveals that the presence of salts tends to disrupt or distort the water molecules in hy drophobic hydration shell around the NIPA gel. This leads to inducing the g rowth of the cluster shell around the salts, which leads to gel collapse. T he volume phase transitions due to the different types of perturbation (tem perature, salt) are induced by the same mechanism, hydrophobic hydration an d dehydration, and therefore can be described in a unified manner in terms of the chemical potential and the collective proton motions of water molecu les. (C) 2000 American Institute of Physics. [S0021-9606(00)52037-6].