Interplay between Water Structure, Electrostatic Interactions, and Polymer Conformation in the Hydration Properties of Poly(acrylic acid)

Polyelectrolytes, zwitterionic polymers, and other charged polymeric systems are frequently used as surface functional groups to impart antifouling properties. Such charged polymers are hypothesized to resist fouling via tightly bound interfacial water; however, this notion of a “tightly bound” hydration layer and, more generally, the precise molecular determinants of antifouling properties remain poorly quantified at the molecular level. In this work, we utilize atomistic molecular dynamics simulations of poly(acrylic acid) (pAA), at various chain lengths and pH (polymer charge state), to establish the relationships between its hydration dynamics, structure, and thermodynamics. Specifically, we compute hydration water diffusivities and survival probability relaxation times which, interestingly, reflect distinct time scales governed by electrostatic interactions and polymer conformation, respectively. Subsequently, we use water’s triplet angle distribution to show that increasing pH (increased polymer charge density) leads to icosahedral structuring of hydration water and a decrease in more-ordered tetrahedral structuring. Through a principal component analysis of variations in the angle distribution, we find that an increase in the magnitude of the principal component representing highly coordinated water reflects an entropic response of the hydration water in these systems. By computing the Coulomb interaction potential energy between pAA and water, we also find an enthalpic response, reflected in increasing water–polymer attractions as pH increases. Interestingly, we find positive linear relationships between diffusivity and both tetrahedrality and Coulomb interaction potential energy, which differs from prior studies of uncharged polymers where tetrahedral enhancement alone tracks decreases in diffusivity. Last, we calculate the diffusion activation energy, finding that it increases with pH, demonstrating kinetic evidence for a tightly bound hydration layer. Overall, this study suggests that hydration around pAA is not simply a “tightly bound” water layer but is described by a more complex entropic-enthalpic-kinetic interplay, in which water structure and restructuring entropy, direct electrostatic interactions, and the activation barrier for diffusion all contribute to the picture.