Cation Effects on the Phase Transition of N-isopropylacrylamide Hydrogels.

Polymers formed from N-isopropylacrylamide (NIPAM) are highly water soluble and undergo a temperature-induced phase transition to an insoluble state. The phase behavior is determined by competing hydrophilic and hydrophobic forces. In this report, additional insight regarding the effect soluble metals have on the phase transition process is provided by showing that cation solvation aids with stabilization of hydrophobic forces. This reduces barriers to rehydration and decreases thermodynamic entropy and enthalpy, obtained with variable-temperature 1H nuclear magnetic resonance spectroscopy of NIPAM hydrogels in D2O, NaCl, MgCl2, and CaCl2. For the series of cations studied, it is observed that the order of increasing effect to facilitate the phase transition is Ca2+ < Mg2+ < Na+. NaCl and MgCl2 exhibited similar effects on the thermodynamics of the collapsing process. However, significant differences in the phase transition thermodynamics are observed between MgCl2 and CaCl2 salt solutions. The influence on Stage 1 enthalpy and entropy values for CaCl2 solutions is approximately half that of the MgCl2 solutions. This difference is likely related to their charge density of Ca2+, which is approximately half that of Mg2+.