Solvation Behaviors of N-Isopropylacrylamide in Water/Methanol Mixtures Revealed by Molecular Dynamics Simulations

Abstract

Solvation behaviors of N-isopropylacrylamide (NIPAM) in water/methanol mixtures are investigated by molecular dynamics simulations. The results indicate that NIPAM-solvent interactions are weakened with the increase of methanol mole fractions (x(methanol)) from 0.25 to 0.80, rationalizing the reentrant coil-to-globule-to-coil transition behaviors of poly(N-isopropylacrylamide) in the mixed solvents. Interestingly, hydrogen-bonded water/methanol clusters are abundant in binary mixed solvents, leading to the decrement of NIPAM-solvent interactions. To better understand the intermolecular interactions in the water/methanol complex clusters, the structures of pure water and methanol clusters are also studied for a comparison. Although the amount of water clusters decreases with an increase in x(methanol), the structure of water clusters stays stable, and hydrogen-bonded networks are not essentially disrupted. As for methanol molecules, they prefer to form short nonbranched chainlike hydrogen-bonded clusters. However, most of the chainlike hydrogen-bonded methanol clusters are broken in water-rich solutions, leaving the little probability for the formation of dimeric and trimeric methanol clusters.