The Effects of Salt On LCST of Pnipam

Classical molecular dynamics simulations were performed to investigate the effects of salt on the lower critical solution temperature (LCST) of Poly (N-isopropylacrylamide) (PNIPAM). PNIPAM is a temperature sensitive polymer which exhibits hydrophobic-hydrophilic phase transition at its LCST. The presence of salt in the solution will shift its LCST, typically to a lower temperature. This LCST shift follows the so-called Hofmeister series. Molecular dynamics (MD) simulations of PNIPAM in 1 M of NaCl, NaBr and KCl were carried out to elucidate the effects of different salt on LCST. Our results suggest that both direct and indirect interactions between the salt cations and the polymer play a critical role in the shift of LCST. Further, cations have a much stronger affinity with the polymer, whereas anions virtually have no binding with the polymer. Moreover, the cation-polymer binding affinity is inversely correlated with the cation-anion contact pair association constant in solution.