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.
Department of Chemistry, Faculty of Sciences, Kyushu University, 33 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan. shigeo-ssk@chem.kyushu-univ.jp
Shigeo
S
Sasaki
Satoshi
S
Okabe
Yuji
Y
Miyahara