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.