3.1 Oil yield
Figure 1 below shows the schematic flow used in this study for the extraction of Okuobaka aubrevillei seed oil. Okoubaka aubrevillei seeds produced 46.4% of their weight in oil, which is comparable to other oil-rich seeds including rapeseed, sunflower, and soybean. This suggests that the seeds of Okoubaka aubrevillei are a potential source of edible oil that can be used for a variety of purposes.
3.2 Chemical composition of Okoubaka aubrevillei seed oil
Table 1 displays the chemical make-up of Okoubaka aubrevillei seed oil. Linoleic acid was discovered to be the most prevalent unsaturated fatty acid in the oil, making up 50.19% of the total oil. Most of the fatty acids were saturated, with palmitic acid (12.3 9%) being the most prevalent as shown in the GC-MS chromatogram of the edible seed oil as shown in Fig. 2. Eicosanoic acid (1.78%), stearic acid (1.07%), and octadecanoic acid (2.03%) were also present in trace concentrations in the oil. Okoubaka aubrevillei seed oil has a high linoleic acid concentration, which means that it has strong oxidative stability and nutritional value [16]. Linoleic acid is known to improve insulin sensitivity, lower blood cholesterol levels, and reduce inflammation, among other positive impacts on human health [17]. The oil contains important fatty acids that humans cannot generate and must be acquired through dietary regimen, as shown by the presence of linoleic acid and linolenic acid [17], [18]. These fatty acids play a number of physiological roles, including those related to immune response, hormone synthesis, cell membrane formation, and visual ability [19].
The mass-spectrometry (MS) peaks of the most active metabolites found in n-hexane extracts of Okoubaka aubrevillei edible seed oil are as shown in Figs. 3a - d.
Table 1
Active phytochemical compounds present in Okoubaka aubrevillei seed oil.
S/N
|
Name of Compound
|
Molecular Formula
|
Molecular Weight
|
Retention Time (min)
|
% Abundance
|
1
|
2,3,6,7-tetrahydro-3a,6-methano-3aH-indene
|
C10H12
|
132.09
|
2.793
|
1.98
|
2
|
[(1E)-2-methyl-1-butenyl] benzene
|
C11H14
|
146.11
|
3.096
|
1.31
|
3
|
1-(1-methyl(ethenyl)-benzene
|
C12H16
|
160.13
|
3.399
|
1.43
|
4
|
1,2,3,4-tetrahydro-1,4-methanonaphthalen-9-ol
|
C11H12O
|
160.09
|
3.713
|
1.16
|
5
|
Β-methylnaphthalene
|
C11H10
|
142.08
|
3.879
|
6.68
|
6
|
3,8-dihydroxy-3,4-dihydronaphthalen-(2H)-one
|
C10H10O3
|
178.06
|
5.491
|
3.53
|
7
|
Caryophyllene oxide
|
C15H24O
|
220.18
|
5.656
|
1.99
|
8
|
Tetradecanoic acid
|
C14H28O2
|
228.21
|
6.628
|
1.83
|
9
|
Hexadecanoic acid (palmitic acid)
|
C17H34O2
|
270.26
|
7.914
|
9.79
|
10
|
9,12-octadecadienoic acid
|
C19H34O2
|
294.26
|
8.257
|
1.03
|
11
|
(Z,Z)-9,12-octadecadienoic acid (linoleic acid)
|
C18H32O2
|
280.24
|
8.880
|
39.67
|
12
|
Methyl stearate
|
C19H38O2
|
298.29
|
8.388
|
1.07
|
13
|
Octadecanoic acid
|
C18H36O2
|
284.27
|
9.017
|
2.03
|
14
|
β-sitosterol/γ-sitosterol
|
C29H50O
|
414.39
|
17.647
|
2.43
|
15
|
Diosgenin
|
C27H42O3
|
414.31
|
17.379
|
3.65
|
16
|
Eicosanoic acid
|
C20H40O2
|
312.30
|
9.600
|
1.78
|
Without derivatization, the GC-MS analysis of Okoubaka aubrevillei seed oil identified the existence of other components in addition to fatty acids. Sterols and phenolic compounds are two of them. The majority of the sterols found in Okoubaka aubrevillei seed oil are phytosterols, which are sterols originating from plants and resemble cholesterol in structure [20], [21]. According to reports, phytosterols can decrease cholesterol by preventing its absorption through the intestinal wall and increasing its elimination [22]. In Okoubaka aubrevillei seed oil, β-sitosterol, which makes up 3.07% of the total oil, is the most prevalent phytosterol. Additionally, studies have demonstrated the anti-inflammatory, anti-cancer, and immunomodulatory capabilities of β-sitosterol [23], [24].
Gallic acid methyl ester and vanillic acid methyl ester were found to be the phenolic components in Okoubaka aubrevillei seed oil, accounting for 1.03% and 1.83% of the total oil, respectively. These substances are produced when tannins are hydrolyzed [25], which are prevalent in the bark of Okoubaka aubrevillei and are polyphenolic substances. Numerous biological functions of phenolic compounds, including antibacterial, anti-inflammatory, anti-cancer, and antidiabetic properties, have been outlined [26]–[29]. Additionally, they function as antioxidants through the transfer of hydrogen atoms or electrons to metal ions or free radicals [30].
3.3 Molecular Dynamic (MD) simulations
The MD simulations of seed oil from Okoubaka aubrevillei on mild steel surfaces revealed that the oil molecules adhered to the surface through physical interactions such as Van der Waals forces and electrostatic interactions as depicted in Figs. 4a and b. A high adsorption strength is indicated by the computed adsorption energy of -353.55 Kcal/mol on the mild steel surface by the inhibitor molecules as seen in Figs. 5 and 6. The surface coverage of the oil molecule was calculated to be 0.43, which indicates that they covered almost 43% of the mild steel surface. By determining the angle between the long axis of the molecule and the normal vector of the steel surface, the molecular orientation of the oil molecules was examined.
The average angle was discovered to be 33°, implying that the oil molecules were slanted at a 33° angle from the surface. The results of MD simulations were compared with those of oleic acid, a conventional corrosion inhibitor for mild steel. The adsorption energy of oleic acid has been determined to be -304.2 Kcal/mol [31] in an aqueous environment, indicating a lower adsorption than Okoubaka aubrevillei seed oil.
These findings imply that Okoubaka aubrevillei seed oil has some mild steel corrosion inhibition potential. Okoubaka aubrevillei seed oil may have higher adsorption energy and surface coverage than oleic acid due to its complex composition and structure [32]. The higher molecular orientation angle of Okoubaka aubrevillei seed oil may also increase its interaction with the mild steel surface [33].
Optimization was used to estimate electronic properties such as energy of the highest occupied molecular orbital (EHOMO), energy of the lowest unoccupied molecular orbital (ELUMO), and energy gap (ΔE) between LUMO and HOMO on the backbone atoms. Figure 7a shows the optimized molecular structures, while Table 2 shows the electronic characteristics. The HOMO and LUMO energies are proportional to the percent inhibition efficiency. If the molecules have greater HOMO energies and lower LUMO energies, the percentage inhibition efficiencies are enhanced [34], [35]. The percent inhibition efficiency is increased with decrease in energy gap as observed in this study.
When the inhibitor molecule and bulk iron metals are combined, electrons will flow from the inhibitor molecule with lower-electronegativity value to the iron atom with higher-electronegativity until the chemical potential is equal [35]. Following that, the proportion of electrons transferred from the inhibitor molecule to the iron atoms can be determined by [35],
Where is the fraction of electrons transferred from the inhibitor molecule to the iron atom, χ is the absolute electronegativity values of iron and the inhibitor molecules, and η is the absolute hardness of iron and the inhibitor molecule (linoleic acid), respectively. The number of electrons transferred reveals whether a molecule is capable of donating or absorbing electrons. The electron transfer from the metal surface to the inhibitor molecule occurs if ΔN < 0 [36] in which case, the value of the fraction of electron transfer (ΔN = 2.218), indicating that the electron transfer occurred from the interaction of the electrons from the inhibitor molecule to the metal surface which is best for corrosion inhibition potency. The HOMO density is of great importance for the mentioned transfer. Localization HOMO for O. aubrevillei seed oil was highly distributed around the molecules as given in Fig. 7b and sparsely distributed in the LUMO localization as shown in Fig. 7c. The electrophilicity of the inhibitor molecule as depicted in Fig. 7d, further shows that the inhibitor molecule had a high electrophilic index which indicates stronger contacts with the metal surface and, as a result, a higher potential to inhibit corrosion [37]. Additionally, it created a shield on the metal surface and retarded the rate at which iron atoms dissolve. The findings in Table 2 also demonstrate that the spots with the most potential for an electrostatic interaction have higher negative net charges[38].
Table 2
Quantum chemical parameters calculated at DFT level for O. aubrevillei seed oil.
Inhibitor
|
EHOMO (eV)
|
ELUMO (eV)
|
ΔE (eV)
|
ΔN
|
O. aubrevillei seed oil (linoleic acid)
|
-5.377
|
-0.824
|
4.553
|
2.218
|
The strong attraction of the oil on the metal surface, which forms a barrier preventing corrosive species from accessing the metal atoms, is another factor contributing to its high inhibitory efficiency [39]. It was therefore discovered that the carboxylic groups of the oil molecules, which reacted electrostatically with the positively charged metal surface, were primarily responsible for the binding of the oil molecules.