Energy Decomposition Analysis of the Adhesive Interaction between an Epoxy Resin Layer and a Silica Surface

We investigate the adhesive interaction energy (ΔE_int) between an epoxy resin and a silica surface using pair interaction energy decomposition analysis (PIEDA), which decomposes ΔE_int into four components: electrostatic (ΔE_es), exchange repulsion (ΔE_ex), charge-transfer (ΔE_ct), and dispersion (ΔE_disp) energies based on quantum chemistry. Our previous study with PIEDA showed that synergistic effects of ΔE_es and ΔE_disp are critical at the interface between an epoxy resin fragment and a hydrophilic surface. The present study is designed to show in detail that the synergistic effects are significant at the interface between an epoxy layer model consisting of 20 epoxy monomers and a hydrophilic silica surface. The ratio of the dispersion energies to the total interaction energies of the layer model shows good agreement with experimental values, that is, the dispersion ratio of the work of adhesion (W_ad). The 20 epoxy molecules in the layer model are investigated individually to closely correlate the four decomposed energies with their structural features. Our energy-decomposition analyses show that H-bonding and OH-π interactions play important roles at the interface between an epoxy resin and a silica surface. PIEDA calculations for the epoxy layer model also show that the region 3.6 Å from the silica surface accounts for more than 99% of the total interaction energies.