Thermodynamic Properties of N-Isopropylacrylamide in Water: Solubility Transition, Phase Separation of Supersaturated Solution, and Glass Formation

The solubility of N-isopropylacrylamide (NIPA) in water was found to discretely change at 25 °C. The highly concentrated NIPA solution separated into two solutions, the concentrations of which were higher than the solubility below 25 °C and lower than the solubility above 25 °C. The X-ray crystallographic analysis indicated that a NIPA crystal formed in the aqueous solution was H(2)O free. When the aqueous NIPA phase-separated solutions were cooled down to -90 °C at a rate faster than 35 °C/min, the glassy structure formed. On the other hand, the crystalline solid formation of NIPA and H(2)O were observed when the solutions were cooled to -50 °C at a slower rate than 3 °C/min. The DSC measurements of the phase-separated solutions revealed that the energy levels of NIPA were +15.2, +11.5, and -0.08 kJ/mol (regarding the crystalline solid state at 25 °C as the ground state) for the liquid state, the H(2)O-poor solution and the H(2)O-rich solution in the phase-separated state, respectively. The experimental results are explained in terms of the molecular assemblies of NIPA and H(2)O molecules in the solutions.