Monomeric 2-hydroxyethyl methacrylate (HEMA) and acrylic acid (AA): structural influences on solute-solvent interactions and spectroscopic properties

2-hydroxyethyl methacrylate (HEMA) and acrylic acid (AA) are widely used co-monomers for building polymeric hydrogels applied in various fields. Because of poor literature data supporting the existence of different stable HEMA conformers in solution, this work is focused on (i) the study of the structural and spectroscopic properties of monomers through Density Functional Theory (DFT) calculations; (ii) the investigation of the hydrogen bond interactions of solute in water by performing molecular dynamics (MD) simulations at different temperatures and with four water models (TIP3P, SPC/E, TIP3P-FB, TIP4P-FB); (iii) the comparison the spectroscopic properties of selected model to experimental UV-Vis and IR data. The main outcomes of these investigations were that: (a) the co-existence of two stable HEMA conformers was confirmed, even if switching temperature from 230 K to 360 K; (b) the solute-solvent interactions are not affected by the monomer conformation, and hydrogen bonds with water mainly involve the carbonyl and hydroxyl oxygens; (c) the first solvation shell of both HEMA and AA is characterized by hydrogen bond interaction with solvent; (d) the vibrational spectra allow to discriminate between the HEMA conformers, while no appreciable differences have been observed in UV-vis spectra. The asymmetric shape of the carbonyl stretching band observed in IR spectrum of HEMA in water has been rationalized by considering both the computed IR spectra and the dipole moments of two conformers; these results allow a differentiation and estimation of the conformers ratio, which results to be HEMA trans : cis equal to 56.5 : 43.5 at ambient conditions. Current results are the starting point to extend the investigation to more complex polymeric chains so to simulate the structure and dynamics in real hydrogels.