Simple theory for volume phase transition of hydrated gels

A simple theory to describe the volume phase transition of hydrated gels is presented. The theory takes a coupling of the volume change and the dehydration of the gel chain into consideration. The change of the excluded volume of chain segments with the dehydration is taken into account in evaluating the free energy of the chain conformation. The mixing free energy of the hydrated and dehydrated chain segments is evaluated by the quasichemical partition function method. The volume change is described as a function of the chemical potential of water molecules. The theory reproduces well the temperature and NaCl salt additive induced volume phase transition behavior of N-propylacrylamide gel observed.