The effect of salt (NaCl, MgCl₂, LiCl, NH₄Cl, Na₂SO₄, MgSO₄, (NH₄)₂SO₄) on the adsorption properties of thermosensitive N-isopropylacrylamide hydrogel was studied using hydrophobic organic molecules as adsorbate. The effect of salt on the adsorption properties of the gel was evaluated in terms of the phase transition temperature T_p of the gel and the structure of water in the solution.
The addition of salt increased the amount of adsorption of organic molecules onto the gel, while decreasing the phase transition temperature T_p of the gel. The addition of salt containing Cl^– ions gave the gel larger adsorption capacity without decreasing the T_p of the gel significantly. On the contrary, the addition of salt containing SO₄^2– ions increased the amount of adsorption, while decreasing the T_p of the gel substantially. The addition of salt affected enormously the structure of water in the salt solution. The amount of adsorption was enhanced with increasing fraction of bound water in the salt solution. The fraction of bound water in the solution containing Cl^– ions was larger than that in the solution containing SO₄^2– ions so that the amount of adsorption increased effectively in the solution containing Cl^– ions. The effect of salt on the adsorption properties of thermosensitive polymer hydrogel is determined by both the T_p of the gel and the structure of water in the salt solution.