rXXXX American Chemical Society A dx.doi.org/10.1021/ie102034c | Ind. Eng. Chem. Res. XXXX, XXX, 000–000

ARTICLE

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Anticorrosion Potential of 2-Mesityl-1H-imidazo[4,5-f][1,10]-phenanthroline on Mild Steel in Sulfuric Acid Solution:Experimental and Theoretical StudyI. B. Obot,*,† N. O. Obi-Egbedi,‡ and A. O. Eseola§

†Department of Chemistry, Faculty of Science, University of Uyo, P.M.B 1017, Uyo, Akwa Ibom State, Nigeria‡Department of Chemistry, University of Ibadan, Ibadan, Nigeria§Chemical Sciences Department, Redeemer’s University, Redemption City, Km. 46 Lagos - Ibadan Expressway, Nigeria

bS Supporting Information

ABSTRACT: A new phenanthroline derivative, 2-mesityl-1H-imidazo[4,5-f][1,10]phenanthroline (MEIP), was synthesized andcharacterized by elemental analysis, FT-IR, 1HNMR, and 13CNMR spectra. MEIP was evaluated as corrosion inhibitor for carbonsteel in 0.5 M H2SO4 solution using gravimetric and UV-visible spectrophotometric methods at 303-333 K. Results obtainedshow that MEIP is a good inhibitor for mild steel in H2SO4 solution. The inhibition efficiency was found to increase with increase inMEIP concentration but decreased with temperature, which is suggestive of physical adsorption mechanism. Activation parametersandGibbs free energy for the adsorption process using Statistical Physics were calculated and discussed. The UV-visible absorptionspectra of the solution containing the inhibitor after the immersion of mild steel specimen indicate the formation of a MEIP-Fecomplex. The calculations of global reactivity indices of MEIP such as the localization of frontier molecular orbitals, EHOMO, ELUMO,energy gap (ΔE), dipole moment (D), hardness (η), softness (σ), the fractions of electrons transferred (ΔN), electrophilicity index(ω), total energy change (ΔET), and Mulliken charge distributions together with local reactivity by means of Fukui indices wereused to explain the electron transfer mechanism between the MEIP molecules and the steel surface. The quantum chemicalcalculations were performed at the density functional theory (DFT) level using B3LYP functional with the 6-31G (d) basis set for allatoms using Spartan’06 V112 program package.

1. INTRODUCTION

Mild steel is a common constructional material for manyindustrial units because of its excellent mechanical properties.They are used widely in industries as reaction vessels, pipelinesfor petroleum industries, machinery, storage tanks, and chemicalbatteries.1 It is a well-known fact that acids are used in manyindustrial operations such as pickling, cleaning, descaling and oilwell acidizing, etc. Because of their aggressiveness, inhibitors areused to reduce the rate of dissolution of metals, and it constitutesone of the most economical ways to preserve industrial facilities.2

Molecular design has become a very useful tool for the syn-thesis of corrosion inhibitors that not only allows one to controlthe corrosion rate but at the same time also fulfills environmentalprotection standards. In this regard, the treatment of mild steelcorrosion in an acidic environment through organic compoundsof low toxicity which do not contain heavy metals and organicphosphates has resulted in considerable savings to the oil and gasindustry.3 Most of the efficient inhibitors used in industry areorganic compounds which mainly contain oxygen, sulfur, nitro-gen atoms, and multiple bonds in the molecule through whichthey are adsorbed on metal surface.4 Moreover, many N-hetero-cyclic compounds have been used as effective inhibitors for thecorrosion of metals and alloys in aqueous media.5-10 It has beenreported that many organic inhibitors usually promote the for-mation of a chelate on the metal surface, which includes thetransfer of electrons from the organic compounds to metal

forming coordinate covalent bond during such chemical adsorp-tion process.11 In this way, the metal acts as an electrophile,whereas the nucleophile centers of inhibitor molecule are nor-mally heteroatoms with free electron pairs which are readily avai-lable for sharing, to form a bond.

Phenanthroline derivatives are of interest due to their potentialactivity against cancer, bacterial, virial, and antifungal infections.Recently, Roy et al.12 has reported on the use of phenanthrolinederivatives with improved selectivity as DNA-targeting anticanceror antimicrobial drugs. Imidazo[4,5-f][1,10]phenanthrolines areversitile ligands because they can form stable complexes withvarioud d-block transition metals.13 Metal complexes of phenan-throline derivative (example, Mn(II) complex of 2H-5-hydroxy-1,2,5-oxadiazo[3,4-f][1,10]phenanthroline) has been reported toshow antitumor activity.14 The choice of 2-(6-methylpyridin-2-yl)-1H-imidazo[4,5-f][1,10]phenanthroline (MEIP) for the presentinvestigation was based on the following considerations: (i) it canbe synthesized easily from relatively cheap materials; (ii) it con-tains two heterocyclic moiety in one compound i.e. imidazole andphenanthroline ringswith severalπ-electrons and aromatic systemscontaining four N atoms, which can induce greater adsorption of

Received: October 6, 2010Accepted: December 28, 2010Revised: December 28, 2010

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the inhibitor molecule onto the surface of mild steel comparedwith compounds containing only imidazole ring already used ascorrosion inhibitor in industry;15 (iii) the compound has a highermolecular weight (311.34 g mol-1) than most N-heterocycliccompounds already studied as corrosion inhibitors such as piper-azine derivatives.16Moreover, there is no report in the literature onthe use of 2-mesityl-1H-imidazo[4,5-f][1,10]phenanthroline ascorrosion inhibitor for mild steel in sulfuric acid.

Quantum chemical calculations have been widely used to studyreactionmechanism. They have also proved to be a very importanttool for studying corrosion inhibition mechanism.16-18 In recenttimes,Density Functional Theory (DFT) has become an attractivetheoretical method because it gives exact basic vital parameters foreven huge complex molecules at low cost.19-21 Furthermore, byusing sophisticated computational tools, we can understandreactivity behavior of hard and soft acid-base (HSAB) theorythat provide a systematic way for the analysis of the inhibitor/surface interaction.22 Thus, the DFT has become a main source ofconnecting some traditional empirical concepts with quantummechanics. Therefore, DFT is a very powerful technique to probethe inhibitor/surface interaction and to analyze experimental data.

In continuation of our quest for developing corrosion inhibitorswith high effectiveness and efficiency, the present paper explores theuse of 2-mesityl-1H-imidazo[4,5-f][1,10]phenanthroline (MEIP)as corrosion inhibitor for mild steel surface in sulfuric acid solutionusing gravimetric method and UV-visible spectrophotometricmethods. The effect of temperature is assessed in order to proposea suitablemechanism for the inhibitory action ofMEIP on themildsteel surface. Quantum chemical calculations have been performedusing DFT, and several quantum chemical indices were calculatedand correlated with the inhibitive effect of MEIP.

2. EXPERIMENTAL SECTION

2.1. General Considerations. All starting materials wereobtained commercially as reagent grade and used without furtherpurification. Phenanthroline-5,6-dione was synthesized accordingto literature procedure.23All air ormoisture sensitivemanipulationswere carried out under an atmosphere of nitrogen using standardSchlenk techniques. All purificationswere doneon silica gel columnto exclude impurities, and TLC glass slides were routinely em-ployed to monitor extents of reactions as well as the progess ofsilica gel column chromatography.2.2. Synthesis of 2-Mesityl-1H-imidazo[4,5-f][1,10]phena-

nthroline. The inhibitor (MEIP) was synthesized as reportedin the literature.23 1,10-Phenanthroline-5,6-dione (0.50 g, 2.37mmol), mesitaldehyde (0.35 g, 2.37 mmol), ammonium acetate(3.65 g, 47.40mmol), and glacial acetic acid (10mL)were heatedunder reflux condition for 2 h followed by cooling, dilution in20 mL of distilled water, and neutralization with concentratedaqueous ammonia solution. The crude product was filtered off asyellow precipitate which was recrystallized from ethanol to obtain2-mesityl-1H-imidazo[4,5-f][1,10]phenanthroline (Scheme 1) asmicrocrystals (0.75 g, Yield: 93%).Analytical Data for Inhibitor. Yield: (0.75 g, 93%).Mp. >300 �C.

Selected IR peaks (KBr disk, cm-1): v 3272 vs, 3061s, 1610 m,1542s, 1454s, 741s, 1H NMR (400 MHz, TMS, CDCl3); δ 8.85(d, 4H); 7.46(br, s, 2H); 6.65(s, 2H); 2.19(s, 3H); 1.93(s, 6H).13CNMR (TMS, CDCl3). δ 151.24, 147.69, 143.87, 139.12,137.83, 130.54, 127.97, 127.56, 123.03, 121.15, 119.90. Anal. Calc.for C22H18N4: C, 70.57; H, 5.92; N, 14.96%. Found: C, 70.74; H,5.74; N, 14.59%.

2.3. Instrumentation. C, H, and N analyses were carried outon a Flash EA 1112 microanalyzer. 1HNMR and 13CNMRspectra were obtained with a Bruker ARX-400 MHz spectrom-eter using CDCl3 as the solvent and TMS as an internalstandard. Fourier transformation infrared (FT-IR) spectra wererecorded in KBr pellets using Shimadzu 8740 FT-IR spectrom-eter as KBr discs in the range of 4000-400 cm-1. The melt-ing point was determined on a digital melting point instrument(Electrothermal model 9200).2.4. Material. Tests were performed on a freshly prepared

sheet of mild steel of the following composition (wt %): 0.13%C,0.18% Si, 0.39% Mn, 0.40% P, 0.04% S, 0.025% Cu, and bal Fe.Specimens used in the weight loss experiment were mechanicallycut into 5.0 cm� 4.0 cm� 0.8 cm dimensions, then abraded withSiC abrasive papers 320, 400, and 600 grit respectively, washed inabsolute ethanol and acetone, dried in room temperature, andstored in a moisture free desiccator before their use in corrosionstudies.24

2.5. Solutions. The aggressive solutions, 0.5 M H2SO4 wereprepared by dilution of analytical grade 98%H2SO4 with distilledwater. Stock solution of MEIP was made in a 10:1 water:methanolmixture to ensure solubility.25 This stock solution was used for allexperimental purposes. The concentration range of MEIP pre-pared and used in this study was 2 μM-10 μM.

Scheme 1

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2.6. Gravimetric Measurements. The gravimetric method(weight loss) is probably the most widely used method of inhi-bition assessment.26 The simplicity and reliability of the mea-surement offered by the weight loss method is such that thetechnique forms the baseline method of measurement in manycorrosion monitoring programs.27 Several authors have reportedon comparable agreement between weight loss technique andother well established techniques of corrosion monitoring suchas polarization technique,28 electrochemical impedance spectro-scopy,29 gasometric,30 thermometric,31 and atomic absorptionspectroscopy.32 Recently, weight loss method together with po-tentiodynamic polarization and electrochimical impedance spec-troscopy were used to evaluate the corrosion inhibitive effect ofcigarette butt on N80 steel at 90 �C in hydrochloric acid solu-tion.33 Results obtained for the three independent methods werein good agreement. Recently, there are several reports in interna-tional reputable journals on the use of weight loss alone in cor-rosion inhibition studies.24-27 The weight loss method in com-bination with quantum chemical studies has been found to beadequate in elucidating the mechanism of inhibition.5-8,18,20,22

Thus, weight loss measurements were conducted under totalimmersion using 250 mL capacity beakers containing 200 mL oftest solution at 303-333 K maintained in a thermostatted waterbath. The mild steel coupons were weighed and suspended inthe beaker with the help of rod and hook. The coupons wereretrieved at 2 h intervals progressively for 10 h, washed thor-oughly in 20% NaOH solution containing 200 g/L of zinc dust25

with bristle brush, rinsed severally in deionized water, cleaned,dried in acetone, and reweighed. The weight loss, in grams, wastaken as the difference in the weight of the mild steel couponsbefore and after immersion in different test solutions determinedusing LP 120 digital balance with sensitivity of (0.1 mg. Thenthe tests were repeated at different temperatures. In order to getgood reproducibility, experiments were carried out in triplicate.In this present study, the standard deviation values among paralleltriplicate experiments were found to be smaller than (2%, indi-cating good reproducibility.The corrosion rate (F) in mg cm-2 h-1 was calculated from

the following equation34

F ¼ ΔWAt

ð1Þ

whereW is the average weight loss of three mild steel sheets, A isthe total area of one mild steel specimen, and t is the immersiontime (10 h) . With the calculated corrosion rate, the inhibitionefficiency (%I) was calculated as follows24

%I ¼ F1 -F2F1

� �x100 ð2Þ

where F1 and F2 are the corrosion rates of the mild steel couponsin the absence and presence of inhibitor, respectively.2.7. Spectrophotometric Measurements. UV-visible ab-

sorption spectrophotometric method was carried out on theprepared mild steel samples after immersion in 0.5 M H2SO4

with and without addition of 10 μMofMEIP at 303 K for 3 days.All the spectra measurements were carried out using a Perkin-Elmer UV-visible Lambda 2 spectrophotometer.2.8. Computational Details. B3LYP, a version of the DFT

method that uses Becke’s three parameter functional (B3) andincludes a mixture of HF with DFT exchange terms associatedwith the gradient corrected correlation functional of Lee, Yang

and Parr (LYP), was used in this paper to carry out quantumcalculations. Then, full geometry optimization together with thevibrational analysis of the optimized structures of the inhibitorwas carried out at the (B3LYP/6-31G (d) level of theory usingthe Spartan’06 V112 program package35 in order to determinewhether they correspond to a maximum or a minimum in thepotential energy curve. The quantum chemical parameters werecalculated for molecules in neutral as well as in the protonatedform for comparison. It is well-known that the phenomenon ofelectrochemical corrosion occurs in liquid phase. As a result, itwas necessary to include the effect of a solvent in the computa-tional calculations. In the Spartan ’06 V112 program, SCRFmethods (Self-consistent reaction field) were used to performcalculations in aqueous solution. These methods model the sol-vent as a continuum of uniform dielectric constant, and the soluteis placed in the cavity within it.

3. RESULTS AND DISCUSSION

3.1. Weight Loss, Corrosion Rate, and Inhibition Effi-ciency. The mechanism of inhibition and the effect of inhibitorin aggressive acidic environment in the presence of mild steelrequires some knowledge of interaction between the protectivecompound and the metal surface. According to the mechanismfor the dissolution of iron in acidic sulfate solution initially pro-posed by Bockris et al.36 and reported also by us,8 iron electro-dissolution in acidic sulfate solution depends primarily on theadsorbed intermediate as follows

FeþH2OT FeOHads þHþ þ e- ð3Þ

FeOHads f FeOHþ þ e-ðrate determining stepÞ ð4Þ

FeOHþ þHþ T Fe2þ þH2O ð5ÞThe cathodic hydrogen evolution follows the steps

FeþHþ T ðFeHþÞads ð6Þ

ðFeHþÞads þ e- T ðFeHÞads ð7Þ

ðFeHadsÞþHþ þ e- f FeþH2 ð8ÞThe following mechanism involving two adsorbed intermedi-

ates to account for the retardation of Fe anodic dissolution in thepresence of an inhibitor has been reported37

FeþH2OT FeH2Oads ð9Þ

FeH2Oads þMT FeOH-ads þHþ þM ð10Þ

FeH2Oads þMT FeMads þH2O ð11Þ

FeOH-ads f FeOHads þ eðrate determining stepÞ ð12Þ

FeMads T FeMþads þ e ð13Þ

FeOHads þ FeMþads T FeMads þ FeOHþ ð14Þ

FeOHþ þHþ T Fe2þ þH2O ð15Þwhere M represents the inhibitor species.According to the detailed mechanism above, displacement of

some adsorbed water molecules on the metal surface by inhibitor

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species to yield the adsorbed intermediate FeMads (eq 11) re-duces the amount of the species FeOH-

ads available for the ratedetermining steps and consequently retards Fe anodic dissolution.

Figure 1 shows a representative plot of weight loss against timefor mild steel in 0.5 M H2SO4 solution containing no inhibitor(blank) and in the presence of different concentrations of MEIP

Figure 1. Variation of weight loss against time for mild steel corrosion in 0.5 M H2SO4 in the presence of different concentrations of MEIP at 303 K.

Table 1. Calculated Values of Corrosion Rate and Inhibition Efficiency for Mild Steel Corrosion for Mild Steel in 0.5 MH2SO4 inthe Absence and Presence of MEIP at 303-333 K

corrosion rate (mg cm-2 h-1) inhibition efficiency (%I)

system/concentration 303 K 313 K 323 K 333 K 303 K 313 K 323 K 333 K

blank 1.19 1.54 4.00 4.89 - - - -

2 μM 0.57 0.80 2.68 3.36 52 48 33 31

4 μM 0.48 0.76 2.36 2.98 60 51 41 39

6 μM 0.37 0.62 1.92 2.44 69 61 52 50

8 μM 0.29 0.48 1.24 2.03 75 70 69 58

10 μM 0.15 0.27 0.80 1.51 87 83 80 69

Figure 2. The relationship between corrosion rate and temperature for different concentrations of MEIP.

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at 303 K. Similar plots were obtained for other temperatures(313-333 K) (not shown). From the plot, it is evident that theweight loss of mild steel in the different test solutions increaseswith time. The nonuniformity and nonlinearity of the curves of theweight loss plotmay be attributed to the presence ofmill scale on themild steel surface. Itmay also suggest that themild steel corrosion byH2SO4 is a heterogeneous process involving several steps. Similarobservation has been reported recently.38 A further inspection of theplots reveal that the weight loss of mild steel was reduced in thepresence of MEIP compared to the free acid solution, an indicationof inhibiting effect of acid corrosion of mild steel.The calculated values of corrosion rates (F) and inhibition

efficiency (%I) obtained from weight loss measurements for dif-ferent concentrations ofMEIP in 0.5MH2SO4 after 10 h immersionat 303-333 K are listed in Table 1. It is evident from this table andFigure 2 that the corrosion rate decreased with increasing inhibitorconcentration but increased with rise in temperature. Table 1 alsoshows that inhibition efficiency (%I) increased with increasinginhibitor concentration, reaching a maximum of 87.0%. This maybe due to the adsorption of MEIP molecules onto the mild steelsurface through nonbonding electron pairs of the four nitrogen

atoms as well as the π-electrons of the aromatic rings. Moreover,the high molecular weight of MEIP ensures effective surfacecoverage of the inhibitor on the steel surface. This isolates thesteel from the agressive acid solution thus inhibiting its dissolution.Similar observation has been documented.39 Figure 3 shows thevariation of percentage inhibition efficiency with temperature. It isclear from the figure that percentage inhibition efficiency increaseswith concentration but decreases with temperature. The increasein percentage inhibition efficiency of MEIP with concentrationmay be due to the adsorption of its molecules onto the mild steelsurface. The decrease in inhibition efficiency with increase intemperature may be probably due to decreasing strength ofadsorption (shifting the adsorption-desorption equilibrium to-ward desorption) and roughening of the electrode surface whichresults from enhanced corrosion.10

3.2. Thermodynamic Consideration Using the StatisticalModel. According to statistical physics, the change of free energyof adsorption ΔGads

o can be calculated from eq 10 as follows40

ln1- η

η

� �¼ ΔGo

ads

θ-RTln C

θð16Þ

where C is the concentration of inhibitor particles.The curve fitting of data in Table 1 to the statistical model at

303-333 K is presented in Figure 4. Good correlation coefficient(R2 > 0.96) was obtained. θ andΔGads

o can be calculated from theslope and intercept of eq (16). All the calculated parameters aregiven in Table 2. The negative values of ΔGads

o demonstrates thatthe inhibitor is spontaneously adsorbed onto the metal surface.Generally, values ofΔGads

o up to-20 kJ mol-1 are consistent withphysisorption, while those around -40 kJ mol-1 or higher areassociated with chemisorption as a result of the sharing or transferof electrons from organic molecules to the metal surface to form acoordinate bond.24 In the present study, the calculated values ofΔGads

o obtained for MEIP are more than -20 kJ mol-1 but lessthan -40 kJ mol-1 (Tables 3), indicating that the adsorption ofmechanism of MEIP on mild steel in 0.5 MH2SO4 solution at thestudied temperatures may be a combination of both physisorptionand chemisorption (comprehensive adsorption).41

Figure 3. Variation of inhibition efficiency of MEIP with temperature.

Figure 4. Application of the statistical model to the corrosion protec-tion behavior of MEIP.

Table 2. Some Parameters from Statistical Model for MildSteel in 0.5 M H2SO4 in the Presence of MEIP

temperature (K) R2 θ ΔGoads (kJ mol-1)

303 0.969 2.45� 103 -32.70

313 0.967 2.71� 103 -33.11

323 0.982 2.11� 103 -33.19

333 0.996 2.86� 103 -33.49

Table 3. Activation Parameters of the Dissolution of MildSteel in 0.5MH2SO4 in the Absence and Presence of DifferentConcentrations of MEIP

concentration Ea (kJ mol-1) ΔH*(kJ mol-1) ΔS*(J mol-1K-1)

blank 43.31 203.34 285.52

2 μM 54.26 225.36 339.13

4 μM 55.25 229.57 349.08

6 μM 56.69 232.95 345.83

8 μM 57.73 2.37.84 353.19

10 μM 65.71 270.16 450.95

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The enthalpy of adsorption can also be calculated from theGibbs-Helmholtz equation42

∂ðΔGoads=TÞ∂T

� �p

¼ -ΔHo

ads

T2ð17Þ

Equation 17 can be arranged to give the following equation

ΔGoads

T¼ ΔHo

ads

Tþ k ð18Þ

The variation of ΔGadso /T with 1/T gives a straight line with a

slope which is equal to ΔHadso (Figure5). It can be seen from the

figure that ΔGadso /T decreases with 1/T in a linear fashion. The

obtained value of ΔHadso was -58.00 kJ mol-1.

The enthalpy and entropy for the adsorption of MEIP on mildsteel were also deduced from the thermodynamic basic equation19

ΔGoads ¼ ΔHo

ads-TΔSoads ð19Þ

where ΔHadso and ΔSads

o are the enthalpy and entropy changes ofadsorption process, respectively. A plot of ΔGads

o versus T waslinear (Figure 6) with the slope equal to -ΔSads

o and intercept ofΔHads

o . The enthalpy of adsorption ΔHadso and the entropy of

adsorptionΔSadso obtained are-58.66 kJmol-1 and 92.00 Jmol-1

K-1, respectively. The enthalpy of adsorptionΔHadso from the two

approaches is in agreement.It has been reported that when the process of adsorption is

exothermic (i.e negative values for ΔHadso ), physisorption can be

distinguished from chemisorption according to the absolutevalue of ΔHads

o . For physisorption processes, this magnitude isusually lower than 40 kJ mol-1 while that for chemisorption ap-proaches 100 kJmol-1.43 In this work, the negative sign ofΔHads

o isan indication that the adsorption of MEIP on steel surface isexothermic while its absolute value (around -50 kJ mol-1) sug-gests that the adsorption of MEIP is not merely physical or chemicalbut a combination of physisorption and chemisorption exists betweenthe inhibitor and themetal surface (comprehensive adsorption).42,43

Figure 5. The variation of ΔGadso /T with 1/T.

Figure 6. The variation of ΔGadso with temperature.

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The positive sign of ΔSadso arises from substitutional process,

which can be attributed to the increase in the solvent entropyand more positive water desorption entropy. This leads toan increase in disorder due to the fact that more water mole-cules can be desorbed from the metal surface by one inhibitormolecule.44

3.3. Effect of Temperature. Temperature has a great effecton the rate of metal electrochemical corrosion. In case of corro-sion in a neutral solution (oxygen depolarization) the increase intemperature has a favorable effect on the overpotential of oxygendepolarization and the rate of oxygen diffusion, but it leads toa decrease of oxygen solubility. In case of corrosion in acidicmedium (hydrogen depolarization), the corrosion rate increasesexponentially with temperature increase because the hydrogenevolution overpotential decreases.38 The relationship betweenthe corrosion rate (F) of mild steel in acidic media and tempera-ture (T) is often expressed by the Arrhenius equation43

log F ¼ log A-Ea

2:303RTð20Þ

where F is the corrosion rate, Ea is the apparent activation energy,R is the molar gas constant (8.314 J K-1 mol-1), T is the absolutetemperature, and A is the frequency factor. The plot of log Fagainst 1/T for mild steel corrosion in 0.5 M H2SO4 in theabsence and presence of different concentrations of MEIP ispresented in Figure 7. All parameters were given in Table 3. Eavalues in the table are higher for inhibited solutions than theunhibited one, indicating a strong inhibitive action of theadditives by increasing the energy barrier for the corrosion pro-cess, emphasizing the electrostatic character of the inhibitor’s ad-sorption on the mild steel surface (physisorption).34 It is logicalto assume that in this case the electrostatic cation adsorption isresponsible for the good protective properties of this compound.However, the adsorption phenomenon of an organic moleculeis not considered only as a physical or as chemical adsorptionphenomenon but a wide spectrum of conditions, ranging from

the dominance of chemisorption or electrostatic effects may arisedue to the complex nature of the corrosion inhibiting process.42

Experimental corrosion rate values obtained from weight lossmeasurements for mild in 0.5 M H2SO4 in the absence andpresence of MEIP was used to further gain insight on the changeof enthalpy (ΔH*) and entropy (ΔS*) of activation for theformation of the activation complex in the transition state usingtransition equation45

F ¼ RTNh

� �exp

ΔS�R

� �exp

-ΔH�RT

� �ð21Þ

where F is the corrosion rate, h is the Plank’s constant (6.626176� 10-34 Js),N is Avogadro’s number (6.02252� 1023 mol-1), Ris the universal gas constant, and T is the absolute temperature.Figure 8 shows the plot of log F/T versus 1/T for mild steelcorrosion in 0.5 M H2SO4 in the absence and presence of dif-ferent concentrations of MEIP. Straight lines were obtained withslope of (ΔH*/2.303R) and an intercept of [log (R/Nh) þ(ΔS*/ 2.303R)] from which the values of ΔH* and ΔS* respec-tively were computed and listed also in Table 3. The positivevalues of ΔH* both in the absence and presence of MEIP reflectthe endothermic nature of steel dissolution process. Results inTable 3 further indicate that the activation enthalpies increaseswith increase in the concentration of MEIP, which vary in thesame manner as the activation energies, supporting the proposedinhibition mechanism. The entropy of activation ΔS* was alsopositive in the absence and presence of MEIP, implying that therate-determining step for the activated complex is dissociation steprather than association. In other words, the adsorption process isaccompanied by an increase in entropy, which is the driving forcefor the adsorption of inhibitor onto the mild steel surface.38

3.4. UV-Visible Spectroscopy. A substantial support for theformation of metal complex is often obtained by UV-visiblespectroscopic investigation. Since there is often a certain quantityof metal cation in the solution that is first dissolved from themetal surface, such procedures were conducted in the presentwork to confirm the possibility of the formation of [MEIP-Fe2þ]

Figure 7. Arrhenius plot for mild steel corrosion in 0.5 M H2SO4 in the absence and presence of different concentrations of MEIP.

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complexes as described in literature.46-48 Furthermore, Abboudet al.47 has reported that change in position of the absorbancemaximum and change in the value of absorbance indicate theformation of a complex between two species in solution. In orderto confirm the possibility of the formation of MEIP-Fe complex,UV-visible absorption spectra obtained from 0.5 M H2SO4

solution containing 10 μMMEIP before and after 3 days of mildsteel immersion is shown in Figure 9. The electronic absorptionspectra of MEIP before the steel immersion display two bands inthe visible region (550 and 680 nm). The absorption bands atthis longer wavelengths are as a result of the presence of aromaticsystems in MEIP which are highly conjugated. These bands mayalso be assigned to π-π* transition involving the whole electro-nic structure system of the compound with a considerable chargetransfer character.34 After 3 days of steel immersion (Figure 10),it is evident that there is an increase in the absorbance of thisband. However, there was no significant difference in the shape of

the spectra before and after the immersion of MEIP showinga possibility of weak interaction between MEIP and mild steel(physisorption). These experimental findings give strong evi-dence for the possibility of the formation of a complex betweenFe2þ cation and MEIP in H2SO4.3.5. Quantum Chemical Studies Using Density Functional

Theory (DFT). There is no doubt that the recent progress inDFT has provided a very useful tool for understanding molecularproperties and for describing the behavior of atoms in molecules.DFT methods have become very popular in the past decade dueto their accuracy and less computational time. The mechanism of theinhibition action can be elucidatedwith the help of quantum chemi-cal calculation which is widely reported in the literature.21,22,24,25

However, further studies using electrochemical methods likepolarization and impedance spectroscopy coupled with somesurface analytical and spectroscopic techniques like SEM,FT-IR, EDAX, and AFM will soon be carried out on the

Figure 8. Transition state plot for mild steel corrosion in 0.5 M H2SO4 in the absence and presence of different concentrations of MEIP.

Figure 9. UV-visible spectra of the solution containing 0.5 M H2SO4 (10 μM) MEIP before (blue) and after 3 days of mild steel immersion (red).

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inhibitor to further characterize the mechanism of inhibitionaction.Figures 10-13 show the optimized geometry, the HOMO

density distribution, the LUMO density distribution, and theMulliken charge population analysis plots for MEIP moleculeobtained with DFT at the B3LYP/6-31G (d) level of theory inboth the neutral and protonated form in aqueous phase. Thetheoretical parameters which provide information about the reac-tive behavior of MEIP are presented in Table 4. These theoretical

parameters were calculated in the neutral as well as in theprotonated form of MEIP in the aqueous phase.Frontier orbital theory is useful in predicting adsorption

centers of the inhibitor molecules responsible for the interactionwith surface metal atoms. Terms involving the frontier molecularorbitals (MO) could provide dominative contribution, becauseof the inverse dependence of stabilization energy on orbitalenergy difference.49 EHOMO is often associated with the electrondonating ability of a molecule; high values of EHOMO are likely toindicate the tendency of the molecule to donate electrons toappropriate acceptor molecules with lower energy MO. ELUMO,on the other hand, indicates the ability of the molecule to acceptelectrons.50 The binding ability of the inhibitor to the metalsurface increases with increasing HOMO and decreasing LUMOenergy values. Thus, the lower the value of ELUMO, the mostprobable it is that the molecule would accept electrons. More-over, the gap between the HOMO and LUMO energy levels ofthe molecule is an important parameter that determines thereactivity of the inhibitor molecule toward the adsorption on themetallic surface. As ΔE decreases (most especially for thecationic species), the reactivity of the molecule increases leadingto increase in the inhibition efficiency of the molecule.49

From Figures 11 and 12, it could be seen that MEIP havedifferent HOMO and LUMO distributions in the neutral and inthe protonated forms. The HOMO densities were concentratedon both the phenanthroline and the imidazole rings in the neutralform, while the protonated form has the HOMO mainly in thephenanthroline ring. For the LUMO distributions, the reverse isthe case. Thus, unoccupied d orbitals of an Fe atom can accept

Figure 10. Optimized structure of MEIP (ball and stick model): (a)neutral molecule and (b) protonated at N4.

Table 4. Some Molecular Properties of MEIP CalculatedUsing DFT at the B3LYP/6-31G (d) Basis Set in AqueousPhase

quantum chemical properties neutral form protonated form

total energy (au) -1068.25 -1068.63

EHOMO (eV) -5.72 -9.50

ELUMO (eV) -1.32 -5.86

ΔE (eV) 4.40 3.64

dipole moment (D) 5.64 3.89

ionization potential (I) (eV) 5.72 9.50

electron affinity (A) (eV) 1.32 5.86

electronegativity (χ) 3.02 7.68

hardness (η) 2.20 1.82

softness (σ) 0.45 0.55

fraction of electrons transferred (ΔN) 0.90 -0.186

neucleophilicity (ω) 2.07 16.20

Figure 11. The highest occupiedmolecular orbital (HOMO) density ofMEIP using DFT at the B3LYP/6-31G (d) basis set level: (a) neutralmolecule and (b) protonated molecule.

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electrons from inhibitor molecule mainly in the neutral form toform a coordinate bond. Also the inhibitor molecule can acceptelectrons from an Fe atom with its antibonding orbitals to formback-donating bond mainly in the protonated form. Thesedonation and back-donation processes strengthen the adsorptionof MEIP onto the mild steel surface.22

Figure 13 shows the Mulliken atomic charges calculated forMEIP. It has been reported that the more negative the atomiccharges of the adsorbed center, the more easily the atom donatesits electron to the unoccupied orbital of the metal.51 It is clearfrom Figure 13, that all the nitrogen atoms as well as somecarbons atoms carries negative charge centers which could offerelectrons to the mild steel surface to form a coordinate bond. Itshould be noted that there are more negative charge centers inthe neutral form of MEIP than in the protonated form. Thenitrogen atoms (N1, N2, N3, and N4) are the probable reactivesites for the adsorption of iron. Thus, the neutral form of MEIPdonates more negative charge to the d-orbitals of Fe than theprotonated form. The protonation of the inhibitor molecule tookplace at N4 because of the following reasons: (1) It does notpossess any attached proton like the N3, which although has thehighest negative charge cannot form another bond again (hasthree bonds already); (2) It has the next higher negative chargeand so protonation was most favorable on this nitrogen and itgives the least total energy after protonation.It is evident from Table 4 that MEIP has the highest EHOMO in

the neutral form and a lower EHOMO in the protonated form.This means that the electron donating ability of MEIP is weakerin the protonated form. It is clear from Table 4 that the

protonated form of MEIP exhibits the lowest ELUMO, makingthe protonated form the most likely form for the interaction ofmild steel with MEIP molecule. The calculations in Table 4further show that MEIP in the protonated form (MEIPHþ) hasthe smallest ΔE value (3.64 eV) indicating that MEIPHþ is themost reactive inhibitor that can easily adsorb on themetal surfacecausing higher protection. This agrees with the experimentalresults that MEIP could have better inhibitive performance onmild steel surface in the protonated form i.e. through electrostaticinteraction between the cation form of MEIP and the vacantd-orbital of mild steel (physisorption). Moreover, the adsorptionof MEIP on the steel surface using the neutral form also plays apart in the overall inhibiting process. This also agrees well withthe value of ΔGads

o and ΔHadso obtained experimentally. The

dipole moment, which is defined as the first derivative of theenergy with respect to an applied electric field, is mainly used tostudy the intermolecular interactions involving the van derWaalstype dipole-dipole forces etc., because the larger the dipolemoment the stronger will be the intermolecular attraction.38

There is lack of agreement in the literature on the correlationbetween dipole moment and inhibition efficiency.52 In this study,it could be noted that the protonated compound exhibit lowervalue of dipole moment than the neutral form of MEIP.Absolute hardness, η, and softness, σ, are important properties

to measure themolecular stability and reactivity. A hardmoleculehas a large energy gap and a soft molecule has a small energygap. Soft molecules are more reactive than hard ones becausethey could easily offer electrons to an acceptor. For the simplesttransfer of electrons, adsorption could occur at the part of the

Figure 12. The lowest unoccupied molecular orbital (LUMO) densityof MEIP using DFT at the B3LYP/6-31G (d) basis set level: (a) neutralmolecule and (b) protonated molecule.

Figure 13. Mulliken charges population analysis of MEIP using DFT atthe B3LYP/6-31G (d) basis set level: (a) neutral molecule and (b)protonated molecule.

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molecule where σ has the highest value and η the lowest value.19

The result from Table 4 shows thatMEIP in the protonated formhas the lowest energy gap, lowest hardness, and the highestsoftness; this agrees with the experimental results that MEIPcould have better inhibitive performance on mild steel surface inthe protonated form i.e. through electrostatic interaction be-tween the cation form of MEIP and the vacant d-orbital of mildsteel (physisorption). Futhermore, the calculations show that theneutral form of MEIP have positive ΔN value which becomesnegative value upon protonation. In all cases, the value of ΔN <3.6, agrees with the study initially proposed by Lukovit andreported elsewhere that inhibition efficiency increased withincreasing electron donating ability at the metal surface.53 Thusin the present study MEIP in both the neutral and protonatedforms were donor of electrons and the mild steel surface was theacceptor of electrons. The electrophilicity index, ω, whichmeasures the electrophilic power of a molecule was calculatedfor both neutral and protonated forms of MEIP. It has beenreported that the higher the value ofω, the higher the capacity ofthe molecule to accept electrons.38 In this study, the protonatedform of MEIP has the highest value of ω and by extension thehighest capacity to accept electrons from the metal. This processincreases the adsorption capacity of MEIP on the steel surface. Ina corroding system, it is important to note that the inhibitor actsas a Lewis base while the metal acts as a Lewis acid.The calculations from Table 4 indicate that η > 0 andΔET < 0

in both neutral and protonated form ofMEIP. This result impliesthat the charge transfer to MEIP molecule followed by back-donation from the molecule is energetically favorable. Similarobservation has been reported.20 However, it is important tonote that ΔET values obtained does not predict that a back-donation process is going to occur; it only establishes that if bothprocesses occur (charge transfer to the molecule and back-donation from the molecule), the energy change is directlyproportional to the hardness of the molecule. The calculatedFukui indices for the charged species (Nþ1 and N-1) as well asthe neutral specie (N) are presented in Table 5. For simplicity,only the charges and Fukui functions over the Nitrogen (N)atoms is presented. For a finite system such as an inhibitormolecule, when the molecule is accepting electrons one has fk

þ,the index for nucleophilic attack; when the molecule is donatingelectrons, one has fk

-, the index for electrophilic attack. It ispossible to observe from Table 5 that Nitrogen atoms (N1, N2)are the most susceptible sites for electrophilic attacks. These sitespresent the highest values of fk

-which are 0.052 for N1 and 0.039for N2, respectively. In the same vein, N1 and N2 are the mostsusceptible sites for nucleophilic attacks. These sites have thehighest values of fk

þ which are 0.014 for N1 and 0.014 for N2,respectively. Similar conclusions have been reported by Liu et al.,54

on the molecular modeling study on inhibition performance ofimidazolines for mild steel in CO2 corrosion.

4. CONCLUSIONS

The following conclusions may be drawn from the study:1. 2-Mesityl-1H-imidazo[4,5-f][1,10]phenanthroline (MEIP)

acts as an inhibitor for the corrosion of mild steel in 0.5 MH2SO4. Inhibition efficiency values increase with the inhib-itor concentration but decrease with rise in temperaturesuggesting physical adsorption mechanism.

2. The Gibbs free energy for the adsorption process calculatedusing Statistical Physics is negative indicating that theprocess is spontaneous.

3. UV-visible spectrophotometric studies clearly reveal theformation of Fe-MEIP complex which may be responsiblefor the observed inhibition.

4. Data obtained from quantum chemical calculations usingDFT at the B3LYP/6-31G (d,p) level of theory werecorrelated to the inhibitive effect of MEIP. Both experi-mental and theoretical calculations are in agreement.

’ASSOCIATED CONTENT

bS Supporting Information. Derivation of thermodynamicparameters using statistical model and some global and localreactivity parameters based on the density functional theory.This material is available free of charge via the Internet at http://pubs.acs.org.

’AUTHOR INFORMATION

Corresponding Author*Phone: þ234 8067476065. E-mail: proffoime@yahoo.com.

’ACKNOWLEDGMENT

The authors wish to acknowledge the Department of Chem-istry, University of Uyo, Nigeria, for providing the facilities forthe work. One of the authors Dr. A. O. Eseola is also acknowl-edged for providing the newly synthesized inhibitor used in thisresearch.

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