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⁺.