Thermodynamic study of a pressure-temperature phase diagram for poly(N-isopropylacrylamide) gels

Abstract

The volume change, ΔVh, accompanying the hydrophobic hydration associated with the volume phase transition in Poly(N- isopropylacrylamide) gels was measured by a simple method. The hydration accompanies a negative ΔVh -2.5 cm3/mol. The P-T phase diagram, the coexistence curve, for the gels was determined from the swelling ratio-pressure curves up to 350 MPa for various constant temperatures. The contour of the coexistence curve is shaped like an ellipsoid on the P-T plain, which is a feature peculiar to the reversible pressure-temperature denaturation of a protein. The thermodynamic analysis of the Clausius-Clapeyron relation for the measured ΔVh elucidates that the obtained coexistence curve represents the phase boundary between thermodynamic different phases like the two phases, native and denatured, of a protein and gives the transition enthalpy, ΔH, 5.2kJ/mol by estimate, which well coincides with the transition heat measured by a calorimetric method. Considering the volume-dependent free energy, Δvmi · P, for the mixing free energy of the gel, we can fit the calculated curve to the measured swelling ratio-pressure curve of PNIPA gels. The value of Δvmi changes the sign from negative to positive above around 100MPa. © 2005 Wiley Periodicals, Inc.