3.2.2 Molecular dynamics simulations
Further, the closer inspection of interfacial adsorption, strength of interaction51 and the relevancy between chemical species (OPD & PPD) and Ni-W alloy surface in the simulated solvent (H2So4) is discerned through MD simulations.52 Figure 5 displayed a flat-lying geometry with lowest energy configurations of OPD and PPD, respectively adsorbed on the Ni-W (110) surface in the simulated corrosion envirionment.. These results suggested that heteroatoms and spacer are responsible for the flat-lying geometry and denser adsorption (as supported by MAC and Fukui function). On closer inspection of the obtained snapshots (Figure 5) the flat-lying orientation of PPD molecules was greater with that of OPD molecules with distinctive slit. This variance in the orientation, ensures greater dispensation of electron density over the alloy surface, resulting in maximal blocking area with sturdy interactive forces.53,54 Further, an insight of faster movement of PPD additive molecules is seen towards the alloy surface through corrosive solution, comparative of OPD additive molecules. This faster motion, leads to greater relinquish of corrosive species55 evidencing the propensity of PPD molecules, in the robust barrier thin film formation on the alloy surface.
A notable two energetic outputs (Ebinding, Einteraction) were derived from MD simulations, which dictate the competitive adsorption potentiality of adopted additives onto the alloy surface.56 From Table 4, a larger negative value (−1132.24 kcal/mol) of interaction energy is an evident of higher interactivity between PPD molecules and alloy surface. It also signifies of its higher stability and spontaneity of the thin film formation over the alloy surface. In addition, the higher negative interaction energy obtained for PPD molecule marks the expel of sulphuric acid molecules and stronger adsorption capability over the alloy surface.57 Another noted energetic descriptor is binding energy (Ebinding) proposing the extent of adsorption of organic molecules onto the alloy surface (Ni-W). Higher binding energy obtained for PPD molecule (1132.241 kJ/mol) displays its higher binding affinity55 towards alloy surface than OPD molecule (1074.38 kJ/mol). Further, higher Ebinding (1132.241 kJ/mol) portrays its potential electro-donating ability of PPD additive by reason of larger isomeric spacer19 which mitigate the corrosion process. These energetic outputs ensure the ability of PPD additive molecules in greater aggregation, forming a dense molecular layer,58 limiting the access of corrosive species onto the surface of Ni-W alloy.