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.