Isobars, the coexistence curve, and the critical exponent β of N-isopropylacrylamide gels obtained using a simple experimental method.

We have obtained "iso-osmobars," which refer to lines with constant osmotic pressures, and coexistence curve of the volume phase transition of N-isopropylacrylamide gel using a very simple method: hanging gels above NaCl solution having a given concentration in a sealed container that was placed in a temperature controlled bath. Since the chemical potential of the water molecules in the gel is equal to that of the water molecules in the NaCl solution, the osmotic pressure of the gel became the same value with that of the NaCl solution. Thus, air that separated the gel and the NaCl solution played a role of infinitely flexible semipermeable membrane.