3.1. Electronic-scale results
The optimized structures, highest occupied molecular orbital (HOMO),
lowest unoccupied molecular orbital (LUMO) and molecular electrostatic
potential (MEP) surfaces of the adopted molecules are shown inFigure 1. It reveals that electron densities of the OPD and PPD
in HOMO are distributed along the entire rings of the additives and it
is most dense on the part of the ring that contains nitrogen (N) and
oxygen (O) atoms. These structural units of the additives have the
greater ability to distribute their electron density with the surface of
Ni-W alloy. This sharing of electrons has a consequence of formation of
an organic film that prevents the corrosion of the surface by acting as
a barrier.25,29 Electron densities of both additives
in LUMO are occupied on the spacer phenylene ring nearby the N and O
atoms; implying that this is the region where the electron acceptation
from the electron rich surface takes place. The HOMO and LUMO electron
density distributions of both additives played an equal significance in
the surface adsorption of additive onto the surface of Ni-W alloy via
electro-donation/acceptation pathways.
In general, the higher value of EHOMO of the inhibitor
have greater tendency to share their electrons. Herein, PPD would
reflect an somewhat enhanced adsorption on the surface of Ni-W alloy via
non-bonding electrons located on N and O atoms to the vacant d-orbitals
of Ni and W surface atoms.29,30 With respect to the
energy gap (ΔE), it is one of the crucial indices, which signifies the
intensity of chemical reactivity towards corrosion inhibition.
Furthermore, this parameter reveals the donating ability, chemical
stability, softness and polarizibility of
additive/inhibitor.17,31,32 From Table.1, it is
reflective that separation energy (or) energy gap of PPD molecule
(-2.202) is lower in comparative with that of OPD molecule (-2.586).
This outcome implies that a lesser amount of energy is needed for a PPD
molecule inorder to remove an outer orbital
electrons,33 which ennobles its inhibition capacity.
The various parameters with their respective values gained from DFT
analysis are shown in Table 1.23,25,28 The adsorptions
of OPD & PPD onto the alloy surface are further backed from their
relatively soft electron affinities/high ionization potentials,
which–providing an identical proficiency to swap electrons to the Ni-W
alloy surfaces (Table 1).23,28
Furthermore, inhibitory effectiveness of an organic molecule is well
related with its global softness (σ) and hardness (η). The chemical
descriptor global hardness, which is an inverse of global softness,
describes the level of molecular resistance to the charge transfer and
electron cloud polarization and its reactivity in combat of acidic
corrosion of metals.34-36 The sizable values of high
chemical softness (0.736) and low global hardness (1.360) obtained for
PPD molecule, emphasis its burly reactivity towards corrosion inhibition
and lower resistance towards charge transfer and electron cloud
polarization. Conversely, OPD molecule resulted in high hardness (1.369)
and a very low softness (0.730) values with that of PPD molecule.
Maximum hardness of OPD molecule, flaunt its larger kinetic
stability31 and higher resistance to charge transfer,
thereby illustrating its reluctance to react.18Moreover, a sizable and low values of the chemical softness (0.736) and
hardness (1.360) are suggests a a strong adsorptive tendency of PPD
molecules on the Ni-W surface.
Global electrophilicity index (ω) is one of the prime quantum chemical
descriptor, which deduces the information on the stabilization of
molecular system through electron donation or
acceptance.37 Greater value of (ω) has higher electron
donating ability of organic moiety to the empty metal
d-orbitals.38 The obtained (ω) values (Table 1)
validate that PPD additive with higher ω= 4.316, had the most reactivity
to attach onto the alloy surface, providing greater corrosion
inhibition. It is worth noting that the greater electron donating
ability affidavits the stronger adsorption mechanism of PPD molecule
onto the alloy surface.39
Polarization is one of the polarity related quantum descriptor, which
describes the distribution and distortion of electron
density.40 Moreover, it clearly facilitates the extent
of polarization, which determines the degree of experimental corrosion
efficiency.41 Here in the present study the following
results were reported with respect to the polarizability; α=572.971 a.u
(OPD) and α=650.707 a.u respectively. Interesting assets can be inferred
from the computed results. Higher value of polarizability obtained for
in the case of PPD additive, stresses the easy electron density
distribution from the molecule onto the metal surface, which means that
the strong electron affinities with alloy surfaces. Further, the higher
polarizability value (α=650.707 a.u) in comparative with that of OPD
(α=572.971 a.u) explicate the higher surface coverage and sturdy
adsorption for Ni-W alloy protection40,42 representing
a more effective interaction as the isomeric spacer differ in its
position.19 The trend in the enhancement of
polarizabilities for compounds (PPD>OPD) is consistent with
the order of experimental corrosion efficiency
results.19
Mulliken atomic charges (MAC) and Fukui function have been employed as
vital tools for evaluating the accountable atomic sites in adsorbed
materials.27,28 Based on this approach, OPD and PPD
additives showed the highest negative values of MAC procured on N and O
atoms. Similarly, the Fukui indices provide the valuable information
about reactive sites, nucleophilic and electrophilic properties of the
inhibitor molecules. The Fukui values on these atoms as revealed by MAC,
atoms imply that they provide the electron density for the Ni-W surface
coordination and back-donation (showing their dual character in this
process). These parts of molecules with a prevalence of the electron
densities are noticeable (in red) in Figure 1, and with respect to MAC
(presented jointly with the Fukui indices values in the Figure 2). The
full computed condensed Fukui functions and the results of the local
reactivity indices of OPD and PPD additive molecules are tabulated in
Table 2 & 3.