Structure and dynamics of water surrounding the poly(methacrylic acid): a molecular dynamics study

All-atom molecular dynamics simulations are used to study a single chain of poly(methacrylic acid) in aqueous solutions at various degrees of charge density. Through a combination of analysis on the radial distribution functions of water and snapshots of the equilibrated structure, we observe that local arrangements of water molecules, surrounding the functional groups of COO- and COOH in the chain, behave differently and correlated well to the resulting chain conformation behavior. In general, due to strong attractive interactions between water and charged COO- via the formation of hydrogen bonds, water molecules tend to form shell-like layers around the COO- groups. Furthermore, water molecules often act as a bridging agent between two neighboring COO- groups. These bridged water molecules are observed to stabilize the rodlike chain conformation that the highly charged chain reveals, as they significantly limit torsional and bending degrees of the backbone monomers. In addition, they display different dynamic properties from the bulk water. Both the resulting oxygen and hydrogen spectra are greatly shifted due to the presence of strong H-bonded interactions.