Copper(II)-oxaloyldihydrazone complexes: Physico-chemical studies: Energy band gap and inhibition evaluation of free oxaloyldihydrazones toward the corrosion of copper metal in acidic medium

2.2.1 Preparation of oxaloyldihydrazone ligands

The dihydrazone ligands, bis(salicylaldehyde)oxaloyldihydrazone (L₁), bis(2-hydroxy-1-naphthaldehyde)oxaloyldihydrazone (L₂), bis(2-hydroxyacetophenone)oxaloyldihydrazone (L₃) and bis(2-methoxybenzaldehyde)oxaloyldihydrazone (L₄), were prepared using the same procedure described in reference Hassan et al. (2015). Oxalicdihydrazide (0.01 mol) dissolved first in hot water (20 cm³) followed by adding methanol (40 cm³) was mixed with selected aldehyde [salicylaldehyde, 2-hydroxy-1-naphthaldehyde, 2-hydroxyacetophenone and 2-methoxybenzaldehyde] (0.02 mol) in absolute methanol. The resulting mixture was refluxed for 3 h under constant stirring. The product separated out on concentrating the solution to half of its volume and cooling. The crystals of the desired ligand were collected by filtration through a Buchner funnel and dried in the oven at 50 °C for 2 h. After that the ligand was recrystallized from DMF-MeOH_aq mixed solvent, collected, washed thoroughly on filter paper by acetone to remove any excess of dimethylformamide (DMF) and then dried in an electric oven at 50 °C for 4 h. The authenticity of the ligands was proved by elemental analyses in addition to FT-IR, mass and ¹H NMR spectroscopy (Tables 2 and 3). The spectral data sustained the chemical structure of the ligands as well as the correct molecular weights.

2.2.2 Preparation of solid complexes

Copper(II) complexes were synthesized by the conventional method reported in reference Hassan et al. (2015). The dihydrazone ligand (1 mmol) was dissolved in a minimum amount of DMF (20 ml) and then 50 ml methanol was added. The resulting solution was added slowly to a methanolic solution of Cu(II) acetate. The resulting mixture was heated under reflux for 4 h and then reduced to 15 cm³ by evaporation on hot plate. The resulting reaction mixture was cooled down to room temperature and the colored solid complexes were filtered off, washed several times with successive portions of hot solvents (DMF, methanol and acetone, respectively) to remove any excess of unreacted ligand and finally dried in an electric furnace at 80 °C for 7 h.

2.2.3 Weight loss measurement

In the weight loss experiment, 100 ml beakers containing 1 M HCl solutions were employed at room temperature (28 °C). Stock solutions of the inhibitors were prepared in DMF. All test solutions contained 10 ml (20 vol.%) of DMF to maintain inhibitors completely soluble. The total volume of the test solution is 50 ml. The coupons were suspended in the beakers with the aid of glass hooks. In the absence of inhibitors, the coupons were retrieved from their corrodent solutions at 60 min interval for 240 min. Further measurements were carried out after introduction of the additives (L₁–L₄) at concentrations 5 × 10⁻⁴, 1 × 10⁻⁴, 5 × 10⁻⁵ and 1 × 10⁻⁵ M in the beaker maintained at the room temperature for the same duration. It is evident that lower concentrations of ligand have been employed to avoid the ligand deposition. Coupons were dipped into saturated ammonium acetate solution at room temperature, to terminate the corrosion reaction. They were washed by scrubbing with a light brush and dried in acetone and finally in an oven maintained at 80 °C. The loss of weight of coupons is evaluated in grams as the difference in weight of the coupons before and after the test (Orubite-Okorosaye and Oforka, 2004). $$W=W_i-W_f$$ where W = weight loss of coupon, W_i = initial weight of coupon, and W_f = final weight of coupon.

Each reading reported is an average of three experimental readings recorded to the nearest 0.000 l g on an Adam184 electronic weighing balance. The value of corrosion rate, ρ (in g cm⁻² min⁻¹) was calculated from the following equation (Mohammed-Dabo et al., 2011): $$\rho =\frac{W}{A\times t}$$ where W = weight loss to the nearest 0.0001 g, A = area of specimen to the nearest 0.01 cm² (2 × area of one surface/face) and t = exposure time (minutes).

The percentage inhibition efficiency (%IE), and a parameter surface coverage (θ) which represents the part of the surface covered by the inhibitor molecules, were calculated using the following equations (Mohammed-Dabo et al., 2011): $$\eta \%=\left[1-\frac{{\rho }_1}{{\rho }_2}\right]\times 100\%$$ $$\theta =1-\frac{{\rho }_1}{{\rho }_2}$$ where η% = corrosion inhibition efficiency, ρ₁ = corrosion rate in the presence of inhibitors, ρ₂ = corrosion rate in the absence of inhibitor at the same temperature and θ = surface coverage.

2.2.4 Kinetic treatment of weight loss results

Spontaneous corrosion reaction is a heterogeneous one, composed of anodic and cathodic reactions with the same rate. On this basis the kinetic analysis of the data is considered necessary. In the present study, the initial weight of steel coupon at time, t is designated W_i, the weight loss is ΔW and the weight change at time t is (W_i − ΔW) or W_f. When log W_f was plotted against time, a linear variation was observed, which confirms a first order reaction kinetics with respect to copper in HCl solutions, formulated as (Okafor et al., 2010) $$\log W_f=\log W_i-\mathit{Kt}$$ where W_i = the initial weight before immersion, K = slope (the rate constant), and t is time.

From the rate constant values obtained from the graphs, regarding the adsorption reaction on copper surface proceeds as first order, the half-life values (t_½) of the metal in the test solutions were calculated using the following equation (Okafor et al., 2010): $$t_{1/2}=\frac{\ln 2}{K}=\frac{0.693}{K}$$

3.1.1 IR spectra and bonding

The positions of the significant IR bands of dihydrazones and their copper complexes are recorded in Table 3 and clarified by Fig. 1.

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Figure 1

IR spectra of oxaloyldihydrazones and their copper(II) complexes.

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Ligands: oxloyldihydrazones (L₁–L₄) can exist either in the trans(staggered) configuration or in cis-configuration, Fig. 2, (Hassan et al., 2015). In cis-configuration, the dihydrazone can adopt either anti-cis-configuration or syn-cis-configuration. Infrared spectra gave strong bands at 1602–1617 cm⁻¹ assignable to the azomethine group (ν_C⚌N). The observation of these bands is unambiguously confirmation of the interaction of dihydrazides with aldehydes forming azomethine linkages. The bands of ν(OH)_{phenolic/naphthoic}, ν(NH) and ν(C⚌O) for L₁–L₃ were noticed at 3149, 3204 and 1666;3476, 3166 and 1705(m) + 1660(v.s); and 3448, 3293 and 1689(m) + 1651(v.s) cm⁻¹, respectively. Meanwhile, L₄ which does not contain o-hydroxy group revealed ν(NH) at 3202 cm⁻¹ and ν(C⚌O) at 1653 cm⁻¹. The appearance of both ν(C⚌O) and ν(NH) simultaneously in the IR spectra of L₁–L₄ indicates the presence of keto forms. The observation of carbonyl groups (C⚌O) near 1660 cm⁻¹ as one band in case of L₁ and L₄ and pair of bands near 1660 and 1700 cm⁻¹ for each L₂ and L₃ indicates the trans (staggered-structure, Fig. 2) configuration for L₁ and L₄ while mixture of [cis(syn/anti-cis-structure) + trans(staggered-structure), Fig. 2] isomers is for L₂ and L₃. This suggestion results from the field effect phenomenon which can be elucidated as follows. When the two carbonyl (C⚌O::) groups are in the same direction (cis configuration), the non-bonding electrons present on oxygen atoms cause electrostatic repulsion. This causes a change in the state of hybridization belongs to C⚌O group and also makes it to go out the plane of the double bond. Thus, the configuration diminished and absorption occurs at higher wave number. Subsequently, cis is absorbed (due to the field effect) at higher frequency compared to trans isomer. Subsequently, L₂ and L₃ showed two bands for C⚌O, one of them (at high frequency) associated with syn/anti-cis-symmetry while that at lower frequency related to staggered configuration. The low intensity of high frequency bands (near 1700 cm⁻¹) compared with that at low frequency (near 1660 cm⁻¹) indicates the domination of staggered structure. Worthy mention, NMR spectroscopy provided satisfactory agreement with this suggestion as shown later. The observation of OH (phenolic) group in L₁ at lower position (3149 cm⁻¹) is taken as evidence to the persistence of intramolecular H-bonding between the phenolic-OH and azomethine group (O—H⋯N) Mohammed-Dabo et al., 2011. The proposition of intermolecular H-bond (O—H⋯O—H) between ligand molecules is excluded owing to the sharpness of this band. The remarkable downward frequency shift indicates the strength of this bond. In fact, ¹H NMR did not distinguish this H-bond in its spectra because execution of NMR analysis in high polar solvent (DMSO) led to break these bonds. The phenolic-OH group did not form H-bonding in case of ligands L₂ and L₃ where it is observed after 3400 cm⁻¹. Further, the existence of two bands at 3278 cm⁻¹ (L₁) and 3227 cm⁻¹ (L₄) may be attributed to ν(OH)_enolic in H-bond bonding with azomethine group. The negative shift of these two bands compared with their normal positions asserts the weakness of this bond. Apparently the obscure of enolic OH in NMR spectra of L₁ and L₄ is attributed to two factors: (1) its lower concentrations,in which minute molecules may change one or two of its C⚌O groups into enolized configuration but the keto form is still dominated, (2) dilution by high polar DMSO disrupted these H-bonds returning the modified molecules into keto forms. The significant ν(C—O) groups associated with the aromatic ring of the dihydrazones L₁–L₄ were observed at 1275, 1287, 1246 and 964 cm⁻¹, respectively (Mohammed-Dabo et al., 2011). In addition to the above, all the ligands showed 3–4 bands in the range 1400–1600 cm⁻¹ related to ν(C⚌C) of the aromatic ring. Also, each ligand exhibited strong band in the region 700–800 cm⁻¹ corresponding to the out of plane deformation of the aromatic ring. The positions of other bands assigned to υ(CH)_aliphatic near 2900 cm⁻¹ and υ(CH)_aromatic near 3050 cm⁻¹ are demonstrated in Table 3.

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Figure 2

Proposed structures of oxaloyldihydrazone ligands, where L₁ (R = H, X = H, Y = H), L₂ (R = ph, X = H, Y = H), L₃ (R = H, X = H, Y = CH₃) and L₄ (R = H, X = CH₃, Y = H).

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Cu(II) complexes: The IR spectra of the Cu^II–L₁–L₄ complexes are represented in Fig. 1, meanwhile the assignment of the significant IR bands is depicted in Table 3. Indeed, it is difficult to unambiguously assign the bands due to foundation of various groups e.g., C⚌O, C⚌N, N—C⚌O, NH, OH, ph ring, C—N and C—H which absorb in the overlapping regions. On examining the IR spectra of the complexes in comparison with those of the free ligands, in these complexes, the dihydrazones (L₁–L₄) behave as tetra- and hexa-dentate ligands toward two centered copper(II) ions forming stable five or six membered chelate rings around each metal ion. This tremendous behavior of ligands can be explained by the aid of IR data as follows:

For I and II (In comparison with free corresponding ligands) the mode of bonding is proposed on the following basis: (1) The splitting (1616, 1612 cm⁻¹, complex II) or broadening (1609, 1600 cm⁻¹, I) of the azomethine group suggested the involvement of this group in bonding (Hassan et al., 2007) considering two dissimilar azomethine groups (Okafor et al., 2010). One of these C⚌N groups is coordinated with copper(II) ion while the other is free. This happened due to the enolization of left or right half of the dihydrazone molecule assuming keto-enol skeleton. (2) The weakness of NH (shoulder) and C⚌O bands accompanied with changes in their locations is most probable due to the obscure of one NH and one C⚌O group by virtue of enolization. Foundation of C⚌O as one band in II instead of two bands in free L₂ besides its absorption at somewhat lower frequency is most probable due to the presence of the two oxygen atoms related to the carbonyl groups in (a) trans direction because of the field effect, or (b) cis direction but with occupation of the free electrons on oxygen by coordination with the central metal. In fact, magnetic data (shown later) supported the second probable for high spin II complex considering that quenching of the spin moments of the copper ions (spin coupling) leads to decrease the μ_eff value. (3) Unnoticeable of the ν(OH) of o-hydroxyl group supported deprotonation of the above group through the coordination. This result is evidenced by the appearance of new bands attributed to ν(M—O_{phenolic/naphthoic}) as well as the positive shift observed for ν(C—O) vibrations (Mohammed-Dabo et al., 2011; Abd El-Wahab et al., 2015).

For III (in comparison with free L₃), the mode of bonding is proposed on the following basis: (1) The splitting of the azomethine group (1607 → 1605, 1631 cm⁻¹) suggested the involvement of this group in bonding (Mohammed-Dabo et al., 2011) assuming the existence of coordinated and uncoordinated azomethine groups. As previously clarified, this occurred due to keto-enol skeleton. (2) The weakness of NH (3293 cm⁻¹, weak shoulder) and C⚌O (1670 cm⁻¹, medium) bands is most probably due to the obscure of one NH and one C⚌O group by virtue of enolization. Also, persistence of C⚌O as one band in III instead of two bands in free L₃ coincides with the presence of the two oxygen atoms related to the carbonyl groups in cis direction but with occupation of the free electrons on oxygen by coordination with the central metal. As mentioned in II, magnetic data coincide with the second probable for high spin III complex considering a quenching of the spin moments of the copper ions. (3) Observation of the ν(OH) phenolic as two bands (3447, 3320 cm⁻¹) instead of one band in L₃ (3448 cm⁻¹) referred to the existence of two kinds of phenolic OH suggesting coordinated and free OH group. This probable is endorsed by foundation of ν(M—O_phenolic) at 560 cm⁻¹ and splitting of νC—O (phenolic) at 1300, 1235 cm⁻¹. Moreover, existence of OH enolic at 3356 cm⁻¹ illustrated that deprotonation of this group did not occur. This conclusion is evidenced by the appearance of new band assignable to ν(M—OH_enolic) at 480 cm⁻¹ (Kumar et al., 2011).

In case of IV, half of the L₄ molecule was altered to enol form and coordination with two metal ions took place via two OMe, one C⚌O, and one protonated enolized carbonyl (C—OH) groups. This behavior is assumed on the basis: (a) the observation of ν(NH) and ν(C⚌O) at the same locations but with weak intensity indicates that half of L₄ molecule was enolized. The positive shift of ν(C—OMe) with its appearance as one band illustrated the coordination of methoxy group with Cu(II) ion. The observation of ν(C⚌N) at the same position (1600 cm⁻¹) without any shift elucidated that C⚌N group did not contribute in bonding. The broadening of this band may be due to the interference absorption of various kinds of C⚌N groups (Salapathy and Sahoo, 1970).

Doubtless, the appearance of new M—O bands in all copper complexes (Table 3) gives a clue to the formation of discrete complexes. Strong broad absorptions in the 3300–3550 cm⁻¹ region for metal complexes substantiated the presence of crystalline and coordinated water in all copper species. Other bands ascribed to negative acetate ion (OAc) were located at 1345–1381 and 1410–1440 cm⁻¹, respectively, suggesting ν_as and ν_s carboxylic modes. The larger separation between the ν_as and ν_s frequencies confirmed the coordination of acetate as a unidentate anion through the C—O moiety of the carboxylic group (Nakamoto, 1970). Also, the slight changes in the positions of ν(C⚌C)_ph upon chelation are mainly due to metal–ligand interaction.

3.1.2 ¹H NMR spectra

The assignment of the main signals in ¹H NMR spectra of all ligands (Hassan et al., 2015) is shown in Fig. 3 along with Table 1, where copper complexes have not been scanned owing to their paramagnetic nature. ¹H NMR spectra of all ligands exhibited multiple signals of the aromatic protons in the 6.5–8.5 ppm region. The signals of equal integration observed in L₁, L₂ and L₃ at δ (12.6, 11), (12.8, 12.6) and (12.9, 11.8) ppm downfield of TMS have been assigned to NH and ortho-OH protons, respectively. On the other hand, L₄ revealed a signal at 12.3 ppm attributed to secondary NH group. Upon the addition of D₂O then OH and NH signals were obscured. Further, the existence of the δOH (phenolic/naphthoic) at its normal frequency excluded any intramolecular hydrogen bonding operating between ortho-OH and CH⚌N group (CH⚌N⋯H—O). The azomethine signals, δ(CH⚌N), observed only in L₁, L₂ and L₄ have been assigned at 8.75, 9.73 and 8.95 ppm, respectively. As reported in the literature (Kumar et al., 2011), if the dihydrazone exists in the syn-cis-configuration or staggered configuration, the δOH, δNH and δCH⚌N, resonances, each should appear as a singlet. However, the appearance of these signals in the form of six signals (doublet of doublet) indicates anti-cis (chair) configuration. Actually, the features of the ¹H NMR spectra of the dihydrazones are compatible with syn-cis- or staggered symmetry. According to IR interpretation mentioned above, the staggered configuration is well defined/dominated for all ligands (L₁–L₄). ¹H NMR spectra of L₃ and L₄ differ from other ligands spectra where they showed signals at 2.55 and 3.9 ppm downfield of TMS due to the methyl (CH₃) and methoxy (OCH₃) protons, respectively (Sherif and Ahmed, 2010; Hassan et al., 2015; Nakamoto, 1970).

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Figure 3

¹H NMR spectra of the oxaloyldihydrazones in (a) DMSO d₆ and (b) DMSO d₆ + D₂O.

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3.1.3 Electronic spectra

The assignments of the observed electronic absorption bands of the oxaloylhydrazones and their metal complexes in Nujol mull (Sutton, 1968; Lever, 1984) as well as the magnetic data of the formed chelates are depicted in Table 3.

The electronic data of the studied ligands exhibited five absorption bands at λ_max (nm) equals 371–395 (n–π^∗, C⚌N), 345–390 (n–π^∗, C⚌O), 320–370 (π–π^∗, C⚌N), 290–348 (π–π^∗, C⚌O) and 270–320 (π–π^∗, aromatic ring) (Ahmed et al., 2015).

Spectrum of I showed an absorption band at 666 nm suggesting a trigonal distorted octahedral structure. Meanwhile, each of other complexes (II–IV) exhibited an absorption band within the range 700–750 nm which is taken as a conclusive evidence for the pseudo tetrahedral environment. The band observed at 420–500 nm for I–IV is mainly assigned to charge-transfer L → Cu on the basis of its high intensity. This ligand–metal charge transfer (L → MCT) transition may be associated with, most probably, an electronic excitation from the HOMO of ortho phenolate/naphtholate/methoxy oxygen to the LUMO of copper(II) ion Sutton, 1968. Any band due to d-d transition in this region is masked by this charge transfer transition band.

For the obtained magnetic studies, the values of μ_eff at room temperature (Table 2) for the consequential binuclear copper(II) complexes (I–IV), the magnetic moment values are less than those expected per molecular formula regarding the existence of two metal ions in the proposed structures. This can be explained on the basis of metal–metal interaction. It is suggested that these complexes have high spin but presence of two metals near to each other in the same molecule may cause partial quenching of the spin moments of the metal ions (spin coupling) decreasing the magnetism (Mahapatra et al., 1991). This means that the supposition of two metal ions in the molecular structure is not excluded and the obtained μ_eff values may be not conflicted with the proposed structures.

3.1.4 TG studies of Cu(II)-complexes

Thermal stability of all investigated complexes (I–IV) was studied.

The TGA data of complex (I), revealed six stages of decomposition. The first stage (50–90 °C) related to the loss of (found/calculated = 4.7/5.5)% of the original weight of complex corresponding to the loss of two crystalline water molecules. The second step of degradation (90–170 °C) corresponds to the elimination of one acetate group (found/calculated% = 7.7/8.7). After the last step of decomposition (700–800 °C), a residue of 2CuCO₃ was formed (found/calculated% = 36.8/37.8).

The TGA data of complex (II) exhibited five stages of disintegration. The first stage corresponds to expelling of two crystalline water molecules (found/calculated = 5.6/5.4)% at the temperature range 50–110 °C. The second step of decomposition corresponds to the loss of (found/calculated = 9.1/8.6)% within the range (175–230 °C) which can be attributed to the loss of one coordinated acetate group. The third stage (230–275 °C) corresponds to the loss of coordinated water molecule (found/calculated% = 3.6/2.7). After that, a residue of (36.5/37.2)% of the original weight of the complex was observed which corresponds to the loss of 2CuCO₃ molecules.

The TGA curve of the complex III showed three steps of decomposition. The first step observed at 50–140 °C corresponds to the elimination of three crystalline water molecules (found/calculated% = 7/8). The second step observed in the range 140–330 °C refers to expelling of four acetate groups (found/calculated% = 29.4/29.6). After that, a residue of (found/calculated = 33.8/32.0)% of the original weight of the complex was obtained assigned to 2CuCO₃ molecules.

The TGA curve of VI complex exhibited four stages of decomposition started at 50, 150, 300, and 400 °C. The first stage (50–150 °C) corresponds to the loss of two crystalline water (found/calculated = 4/4.8)%. The other three steps of decomposition point to the gradual disintegration of the complex. At the final step, a residue of CuCO₃ and CuO mixture (found/calculated = 27/26)% was formed.

3.1.5 ESR spectra

The ESR spectra of complexes I, II and IV showed broad signals (Fig. 4) with two “g” values (g_//, g_⊥) depicted in Table 2. The spectra of I and II have symmetrical shape while IV is not. For all complexes the g_// > g_⊥ > 2.0023, characteristic of complexes with ²B₁(d_x²_–y²) orbital ground state. The average g values were calculated according to the following equation: $$g_{\text{av}.}=\frac{1}{3[g_{//}+2g_{\perp }]}$$ where the calculated values are presented also in Table 2. The g_// is a moderately function for covalent nature.

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Figure 4

Suggested structures of the Cu^II-oxaloyldihydrazone complexes.

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I, II and IV complexes exhibited g_// at 1.36, 1.93 and 1.99, respectively which are less than 2.3, suggesting covalent character of copper–ligand bonds in the present complexes, where, g_// > 2.3 concerns to ionic metal–ligand bond (Lever, 1984; Kivelson and Nieman, 1961). Fairly high values of g are in conformity with the oxygen and nitrogen coordination in these compounds (Lal et al., 2008). The splitting in case of IV bands may be due to the additional interactions of the magnetic moment of the electron with local magnetic fields originating from non-zero nuclear spins. This coupling is known as the hyperfine interaction. The comparable g values of I and IV may arise from the synonymous structures. The above two complexes have pseudo tetrahedral structure as indicated from the electronic absorption spectra.

In view of recorded data and from covering arguments made above, structures of the copper complexes can be represented by Fig. 5.

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Figure 5

ESR spectra of solid Cu^II-oxaloyldihydrazone complexes.

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3.3.1 Kinetic considerations

Regarding the experimental section, when log W_f was plotted against time, Fig. 8, for copper sheets in the presence of oxaloyldihydrazone ligands, a linear variation was observed, which indicated first-order reaction kinetics with respect to the abovementioned sheet in HCl solutions (Hassan et al., 2015). Fig. 7 gives the rate constants and half-life values. There is a general decrease in the rate constants with increasing concentrations of oxaloyldihydrazone inhibitors (L₁–L₄), Fig. 7. The enhancement of half-life (t_1/2) with increase in concentration of hydrazone ligands sustains the inhibition of the copper in 1 M HCl by the additives. The increase in half-life reflects more protection of the metals by the oxaloyldihydrazone ligands (Hassan et al., 2015).

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Figure 8

Variation of log W_f with time for copper coupons in HCl solution containing oxaloyldihydrazones.

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3.3.2 Adsorption isotherms and Gibb’s free energy

Values of θ, degree of surface coverage, calculated from weight loss measurements have been used to study the adsorption characteristics of the inhibitors on the surface of copper metal. The data obtained at room temperature (nearby 303 K) were tested to fit curves for different adsorption isotherms including Freundlich, Temkin, Langmuir, Frumkin and Flory–Huggins, and the correlation coefficients (R²) were used to determine the best fits. The Gibb’s free energy of adsorption ΔG_ads at room temperature for all tested isotherms was calculated according to the following relation (Rodosevic et al., 1995): $$\mathrm{\Delta }{G}_{\text{ads.}}=-\mathit{RT}55.5K_{\text{ads.}}\text{,}\text{where}T\text{is the temperature}(\text{in}\text{K})$$

3.3.2.2

3.3.2.2 Langmuir adsorption

This isotherm may be formulated as (Shockry et al., 1998) $$\frac{C}{\theta }=\frac{1}{K}+C$$ where K is the equilibrium constant for adsorption process.

Fig. 9 represents the plot of log C/θ versus log C for different compounds relative to copper metal. The obtained linear plots with high correlation coefficients indicate that the adsorption of the inhibitors onto different metal surface follows Langmuir adsorption isotherm. From the resulting data one can postulate that there is no interaction between the adsorbed species. The thermodynamic parameters for adsorption process, K_ads and ΔG_ads, associated with the investigated compounds are displayed in Fig. 10.

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Figure 9

Plots of (a) Freundlich ln θ vs. ln C, (b) Langmuir Log (C/θ) vs. Log C, and (c) Frumkin Log C[θ/(1 − θ)] vs. θ for copper coupons in HCl solution containing oxaloyldihydrazones.

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Figure 10

Values of ΔG_ads (kJ/mol), adsorption constant (K), Freundlich heterogeneity factor (n), lateral interaction term describing the interaction in adsorbed layer (α), and correlation coefficient (R²) of inhibitors for copper coupons.

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3.3.3 Adsorption consideration

The inhibition effect of inhibitor compound is ascribed to adsorption of the molecule on metal surface. This adsorption may be physical or chemical depending on the adsorption strength. For a physical adsorption mechanism, the value of the free energy (ΔG_ads) of adsorption should be less negative than −40 kJ/mol. The negative values of ΔG_ads set out in Fig. 10 suggest spontaneous adsorption of the inhibitor and characterize their strong interaction with the metal surface (Bilgic and Sahin, 2001). The magnitudes of ΔG_ads(>−40 kJ/mol) indicate that adsorption on metal surface is compatible with physical adsorption mechanism (Bilgic and Sahin, 2001). Accordingly, the corrosion inhibition may be justified due to the formation of a protective film on the metal surface where the additive covered both the anodic and cathodic sites.