Hydration and Hydrogen Bond Network of Water duringthe Coil-to-Globule Transition in Poly(N-isopropylacrylamide)Aqueous Solution at Cloud Point Temperature.

Aqueous solutions of poly(N-isopropylacrylamide),P-NIPAAm, exhibit a noticeable temperature responsive change in molecularconformation at a cloud point temperature (Tcp). As the temperature rises above Tcp, theextended coil-like P-NIPAAm structure changes into a swollen globule-likeconformation as hydration levels decrease and hydrophobic interactionsincrease. Though water plays an important role in this coil-to-globuletransition of P-NIPAAm, the behavior of water molecules and the associatedhydrogen-bond (HB) network of the surrounding bulk water are stillveiled in uncertainty. In this study, we elucidate changes in thehydration state and the dynamical structure of the water HB networkof P-NIPAAm aqueous solutions during the coil-to-globule transitionby analyzing the complex dielectric constant in the terahertz region(0.25–12 THz), where bulk water reorientations and intermolecularvibrations of water can be selectively probed. The structural propertiesof the water HB network were examined in terms of the population ofthe non-HB water molecules (not directly engaged in the HB networkor hydrated to P-NIPAAm) and the tetrahedral coordination of the watermolecules engaged in the HB network. We found the hydration numberbelow Tcp(≈10) was decreased toapproximately 6.5 as temperature increased, in line with previousstudies. The HB network of bulk water becomes more structured as thecoil-to-globule phase transition takes place, via decreases in non-HBwater and reduction in the orderliness of the tetrahedral HB architecture.Together these results indicate that the coil-to-globule transitionis associated with a shift to hydrophobic-dominated interactions thatdrive thermoresponsive structural changes in the surrounding watermolecules. [ABSTRACT FROM AUTHOR]