3.2.3 Radial Distribution Function (RDF)
RDF has proved as an important approach for the assessment of the
adsorption process type present during the inhibitor adsorption: either
a physical or chemisorption. In advancement to the MD simulations, RDF
is employed to measure the bond lengths between the heteroatoms and the
alloy surface, which dictates the interaction modes
(chemisorptions/physisorption).59 Further, this
analysis reveals the degree of resistance/ rigidity of the adsorbed
organics on the alloy surface.60 Emergence of the peak
in the RDF graph offers a simple distinction of the involved process.
The appearance of the peak from the distance 1 to 3.5 Å is an indication
of a chemisorption process, whereas in the case of physisorption RDF
peaks are expected at longer distances (> 3.5 Å). InFigure 6 , RDF values imparts the nature of intermolecular
interactions (stronger/weaker) that occurred between alloy surface and
the interacting atoms (nitrogen and oxygen atoms). Impressive findings
are perceived in the presence of PPD molecule where the bond lengths of
N-Ni-W & O-Ni-W shown less than 3.0 Å, from the surface plane of the
material illustrating the chemisorption process; i.e, stronger
interactive forces which aids in effective bounding of PPD molecule onto
the alloy surface.61 However, the bond length of
N-Ni-W & O-Ni-W for OPD molecule, recorded closer to 3.5 Å, pointing
out the physisorption; i.e, weaker interactive forces exhibiting a
tenuous affinity towards the alloy surface. Overall, the outcomes of RDF
results, speculate that PPD molecule exert the action of corrosion
inhibition through chemisorption, predominantly by the electrovalent
bonds55 inferring the higher tendency of PPD
adsorption onto the alloy surface, thus safeguarding the Ni-W alloy
against degradation. The schematic representation of adsorption of
additives onto the alloy surface was figured out in Figure 7.