Water role in hydrogels: from solvent-monomer interactions to water states in poly-HEMA systems

Despite the numerous experimental papers devoted to hydroxyethyl methacrylate (HEMA)-based physical and chemical hydrogels, still poor data regard theoretical studies on both the HEMA monomer and on poly-HEMA systems. Due to their wide application in biomedical fields, transport properties of such hydrogels and the system dynamics might deserve deeper investigations. For this reason, this work was divided in two parts, the first one elucidating the monomer-solvent interactions and the second one dealing with water states in hydrogels. Regarding HEMA-solvent interactions, we decided to study (i) the monomer structural and spectroscopic properties through Density Functional Theory (DFT), to confirm cis- and trans- conformers coexistence; (ii) hydrogen bonding description through molecular dynamics (MD) simulations with various water models (TIP3P, SPC/E, TIP3P-FB, TIP4P-FB); (iii) vibrational and electronic properties. It resulted that cis- or trans- HEMA conformation does not affect solute-solvent interactions, which occur mainly through the carbonyl and hydroxyl groups in the T range between 230 and 360 K. Concerning poly-HEMA systems, the water role was investigated differentiating three states in physical systems from water content (WC) 10 to 40% w/w. The distinction was performed on a distance basis from the polymer, defining: (i) “bound” water within 3.5 Å of polymeric chains; (ii) “intermediate” water between 3.5 and 6 Å; (iii) “free” water further than 6 Å. Radial distribution functions were calculated for evaluating the solute-solvent and solvent-solvent interactions. Water diffusion coefficients were computed at various T (273, 298, 360 K, respectively). Moreover, pairwise distances were calculated to verify which molecules were effectively remaining in the same shell during the diffusion coefficient evaluation time. Classifying water states according to solvation shells, there is almost no free water in hydrogel systems up to WC 20% w/w, in agreement with experimental data. From WC 20 to 40%, there are almost 2 water molecules around each hydroxyl and just 1 near the carbonyl group at 1.8 Å distance, in agreement with experimental findings. By considering the system at WC 40% w/w, all the water states are well represented. The average diffusion coefficients get larger while increasing WC; this is in compliance with the rising free water fraction, which presents a bulk-like water behavior.