An investigation on the inhibitory action of benzazole derivatives as a consequence of sulfur atom induction

Applied Surface Science 317 (2014) 657–665

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An investigation on the inhibitory action of benzazole derivatives as a consequence of sulfur atom induction Z. Moradi, M.M. Attar ∗ Department of Polymer Engineering and Color Technology, Amirkabir University of Technology, Hafez Street, P.O. Box 15875-4413, Tehran, Iran

a r t i c l e

i n f o

Article history: Received 15 June 2014 Received in revised form 11 August 2014 Accepted 13 August 2014 Available online 22 August 2014 Keywords: Benzazole inhibitor Weight loss EIS AFM Contact angle Quantum chemistry

a b s t r a c t The inhibitory action of three benzazole based molecules namely 2-methyl benzimidazole (2-MBI), 2methyl benzothiazole (2-MBT) and 2-mercapto benzthiazole (2-SHBT) in 1 M HCl solution was studied by gravimetric analysis and electrochemical impedance spectroscopy (EIS). Results showed that the inhibitor adsorption on the iron surface was according to Langmuir adsorption isotherm for 2-MBI and 2-MBT and Flory Huggins Isotherm for 2-SHBT. Surface roughness obtained by Atomic Forced Microscopy (AFM) revealed that a good inhibitor decreases the surface roughness significantly which can be related to the formation of more integrated molecular film of inhibitor on steel surface. Based on contact angle (CA) measurements as the efficiency of the inhibitor molecules improve the hydrophobicity increases. These three molecules were chosen to see the effect of introducing sulfur atom into the structure the main effect of which would be on electronic parameters. To better understand this effect, the quantum chemical descriptors including: EHOMO , ELUMO , energy gap (E), dipole moment (), hardness (), softness (), electronegativity index (), fraction of electrons transferred (N), that are most relevant to the potential action of a molecule as corrosion inhibitor, have been calculated in water and vacuum. Electronic parameters of these three inhibitors have been studied using DFT/B3LYP, and HF methods with 6-31G (d,p) basis set. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Organic corrosion inhibitors play the main role in minimizing metallic corrosion in a variety of industrial processes such as cleaning pipeline and oil well acidification. Among different choices hydrochloric acid solutions are widely used spatially in petroleum fields where mild steel is one of the major construction materials that face this corrosive environment [1]. It is generally accepted that organic corrosion inhibitors form a thin layer on the surface of metal to protect it. As reported by several authors [2–10] the effectiveness of organic corrosion inhibitors is related to their chemical composition, molecular electronic structure, surface charge density and their affinity for the metal surface. Specific interactions between heteroatoms like oxygen, nitrogen, sulfur and phosphorus play an important role in inhibition due to the free electron pairs they possess. Recently, theoretical prediction of the efficiency of corrosion inhibitors has become very popular [4–7], although the derived parameters are fundamentally different from experimentally

∗ Corresponding author. Tel.: +98 21 64542404; fax: +98 21 66468243. E-mail address: [email protected] (M.M. Attar). http://dx.doi.org/10.1016/j.apsusc.2014.08.079 0169-4332/© 2014 Elsevier B.V. All rights reserved.

measured quantities but unlike experimental measurements there is no statistical error in quantum chemical calculations. However there is inherent error, associated with the assumptions required to facilitate the calculations. In using quantum chemistry-based parameters with a series of related compounds, the computational error is considered to be approximately constant throughout the series [7]. The aim of this work is to achieve a better understanding on the effect of introducing sulfur atom into benzazole based inhibitor, in this context quantum chemical calculation was applied together with experimental methods such as weight loss, electrochemical impedance spectroscopy and Atomic Force Microscopy in order to characterize the actual behavior of the inhibitor molecule. This work intends to study inhibitory action of 2methylbenzimidazole, 2-methyl benzothiazole and 2-mercapto benzothiazole in hydrochloric acid solution using weight loss, EIS and AFM as experimental techniques to characterize the effect of presence of sulfur atom as a softer atom in inhibitor molecule, the results of which would be compared with DFT predictions. Studying benzimidazole [2–5], and some derivatives of benzoxazole and benzthiazole [6,7] are reported in literature but considering the changes that sulfur atom introduction can cause

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2.3. Electrochemical measurements The corrosion resistance of mounted samples with 1 cm2 exposure area was also studied by an AUTOLAB PGSTAT12 type EIS. The perturbation and frequency range of the measurement was ±10 mV and 10 kHz–10 mHz respectively. The electrochemical measurements were carried out in a conventional three electrode system including platinum electrode (auxiliary electrode), KCl (3 N) Ag/AgCl electrode (reference electrode) and mounted sample (working electrode). The working electrodes were immersed in the test solution for 1 and 24 h to see the effect of immersion time on adsorption process.

Fig. 1. The molecular structure of 2-MIB, 2-MBT and 2-SHBT. Table 1 The elemental composition of the steel panels. Elements

Fe

C

Si

Mn

P

S

Al

Wt%

99.01

0.19

0.34

0.32

0.05

0.05

0.04

in inhibitory action of the molecule is the unique feature of current work. 2. Experimental 2.1. Inhibitors Three benzazole based compounds including 2-methyl benzimidazole (2-MBI); 2-methyl benzothiazole (2-MBT) and 2-mercapto benzthiazole (2-SHBT) were obtained from Merck and used without further purification. The molecular structures of these compounds are shown in Fig. 1. Labatory grade 37% HCl was purchased from Dr. Mojallali Co. to make 1 M HCl solution. 2.2. Weight loss measurements The st-37 type steel panels (4 cm × 5 cm × 0.15 cm) were prepared from Foolad Mobarake Co. The elemental composition of the steel is presented in Table 1. Weight loss measurements were carried out in the glass vessel containing 250 mL of 1 M HCl solution without and with different concentrations of the inhibitor as illustrated in Table 2. The plates were polished with emery paper of grades 320–800, washed twice in deionized water, degreased with acetone and dried with compressed air flow. After immersion in acidic solution for 24 h at 293 K, the specimens were withdrawn and carefully washed with bi-distilled water and acetone, then dried and weighed. Triplicate experiments were performed in each case and the average value of the weight loss was calculated. Table 2 The corrosion rate (CR) and inhibition efficiency (IE) obtained from weight loss measurements of mild steel in 1 M HCl containing various concentrations of 2-MBT. Inhibitor

C (ppm)

CR (mpy)

IE%

2-MBI

Blank 50 100 200 400 800 1200 1400

245.94 223.20 206.89 180.48 145.98 90.43 83.91 71.76

± ± ± ± ± ± ± ±

14 4.5 6.5 20 8.8 2 0.5 1

0 9.24 15.87 26.61 40.64 63.23 65.88 70.82

2-MBT

50 100 200 400

141.48 81.52 33.82 24.07

± ± ± ±

1.5 10.1 1.7 2.3

42.47 66.85 86.24 90.21

2-SHBT

10 25 50 100

54.54 20.83 13.97 4.99

± ± ± ±

3.8 1.8 2.1 0.5

77.82 91.53 94.32 97.97

2.4. Atomic forced microscopy (AFM) The specimen of size 4.0 cm × 5.0 cm × 0.15 cm were polished with emery paper of grades 320–800, washed twice in deionized water (Millipore), degreased with acetone and dried with compressed air flow. After immersion in 1.0 M HCl without and with addition of inhibitor for 24 h, the specimen was cleaned with deionized water, dried with a cold air blaster, and then used for AFM examinations by means of an Ambios Technologies USPM.

2.5. Contact angle measurement Contact angles of the metal surfaces were determined with a homemade instrument. Before fulfilling the contact angle Measurements, the metal coupons were immersed in 1 M HCl without and with the inhibitor in its ultimate concentration obtained from Weight loss measurements. Then the samples were removed from solution dried with a cold air blaster and kept in a desiccator. Contact angle of the surfaces was measured with Sessile Drop method by dropping 1 water drop to the metal surface from a syringe. The volume of the water that filled to the syringe was 4 ␮L. Three separate photos were taken from different parts of surfaces and contact angle values were measured for each drop. Measured contact angle values were obtained as the mean value of left and right contact angle.

2.6. Computational details Electronic parameters of these three inhibitors have been studied using DFT/B3LYP, and HF methods with 6-31G (d,p) basis set. B3LYP, a version of the DFT method that uses Becke’s three parameter functional (B3) and includes a mixture of HF with DFT exchange terms associated with the gradient corrected correlation functional of Lee, Yang and Parr (LYP) [5,11], was used in this paper to carry out quantum calculations using Spartan’08V1.2.0 program package. The theoretical parameters were calculated in vacuum, but to include the effect of 1 M HCl solution the parameters were also calculated in water. Quantum chemical parameters including energy of the highest occupied molecular orbital (EHOMO ) and lowest unoccupied molecular orbital (ELUMO ), dipole moment and charge on possible active atoms on the azole molecules was calculated. According to DFTKoopmans’ theorem [12] the ionization potential, I, and electron affinity, A, can be approximated as the negative of EHOMO and ELUMO respectively (1) and (2): I = EHOMO

(1)

A = −ELUMO

(2)

Z. Moradi, M.M. Attar / Applied Surface Science 317 (2014) 657–665 blank

100

400 Z"(Ohm.cm2)

2-MBI Z"(Ohm.cm2)

2-MBT 2-SHBT

50

659

Series2 Series3 Series4

200

Series1

0

0 0

50

100

150

200

250

0

300

200

600

800

1000

Z'(Ohm.cm2)

Z'(Ohm.cm2)

Fig. 2. Nyquist plots for mild steel in 1.0 M HCl without and with inhibitors in their ultimate concentrations after 1 h immersion at 293 K.

400

Fig. 3. Nyquist plots for mild steel in 1.0 M HCl without and with inhibitors in their ultimate concentrations after 24 h immersion at 293 K.

Chemical reactivity parameters such as electronegativity, , the global hardness, , and the global softness, , are calculated from Eqs. (3)–(5) [8]:  = − =

I+A 2

(3)

=

I−A 2

(4)

=

1 

(5)

Fig. 4. Equivalent electrochemical circuit for EIS data.

Benzazole compound having the lower electronegativity is likely to donate electrons to the higher electronegativity iron surface as an act of potential equalization. According to the Pearson the fraction of electron transferred, N, can be estimated using Eq. (6) [13]: N =

m − i m − i = 2i 2(m + i )

(6)

where the indices m and i refer to iron atom and benzazole derivative, respectively. As for the metal surface, the work-function, Ф, is taken as its electronegativity (Fe = 7 eV), whereas the chemical hardness is neglected because  of bulk metals is related to the inverse of their density of states at the Fermi level—an exceedingly small number [14]. 3. Results and discussion 3.1. Gravimetric measurements The corrosion rate (CR) and the values of inhibition efficiency (IE %) obtained from gravimetric measurements of steel panels in the absence and presence of various concentrations of 2-MBI, 2MBT and 2-SH-BT after 24 h of immersion are shown in Table 2. The values of CR and IE% were calculated using Eqs. (7) and (8): CR =

3.45 × 106 × W At

CRa − CRp IE% = × 100 CRa

(7)

When the steel surface is immersed in acidic solution, as a consequence of being under its isoelectric point [15], positive charge is accumulated on the surface, the negatively charged ions in solution try to neutralize this excess charge and an outer Helmholtz Plane is formed 1–10 nm from the surface [16]. The formed electrical double layer (dl) acts as a non-ideal capacito. In this point of view, the resistance between the metal/OHP must be equal to the Rct (charge transfer resistance). In Fig. 2, for 2-SHBT an elongation can be seen in low frequencies. Özcan et al. described this observed elongation in Nyquist diagrams, which looks like a smaller merged semicircle, attributed to all kinds of accumulated species. The related electrochemical equivalent circuit used to model the mild steel/acidic solution interface for 2-SHBT for 1 and 24 h is shown in Fig. 4b, and other Nyquist plots can be fitted to Fig. 4a (Randels Cell) where Rs represents the solution resistance, Rct is the charge transfer resistance, Rf represents the film resistance, CPEdl and CPEf represent constant phase elements to replace a double layer capacitance (Cdl ) and film capacitance (Cf ), respectively, and n shows the phase shift which can be explained as the degree of surface inhomogeneity [17]. The difference in real impedance at lower and higher frequencies is considered as polarization resistance. The impedance parameters obtained by fitting the EIS data to the equivalent circuit are listed in Table 2. The values of fitted elements are represented in Table 3 where Y0 is a proportional factor and n is related to the phase shift. The IE% is calculated using polarization resistance as Eq. (9): IE% =

(8)

where W is the weight loss, A is the surface area of the specimen (cm2 ), t is immersion time (h),  is the density of metal and CR is expressed in mpy. 3.2. Electrochemical impedance spectroscopy (EIS) The corrosion behavior of mild Steel in 1 M HCl in the absence of inhibitor and in its ultimate concentration according to weight loss measurements was investigated by impedance technique at 293 K. The experiments were carried out in 1 and 24 h. The results are shown in Figs. 2 and 3 for 1 and 24 h respectively. Generally, the results are analyzed in terms of electrochemical equivalent circuit.

(1/(Rp )a ) − (1/(Rp )p ) 1/(Rp )a

× 100

(9)

where (Rp )a and (Rp )p are polarization resistances in the absence and presence of inhibitor, respectively [13]. Eq. (10) converts admittance of the constant phase element to double layer capacitance. Cx =

(Rx Qx )1/n Rx

(10)

where C, Q, R and n represent double layer capacitance, magnitude of impedance of the Constant Phase Element, resistance and capacitive factor, respectively. The Nyquist plots show a depressed semi-circle, with the center being below the real X-axis, which is the characteristic for solid electrodes and is referred to frequency dispersion which occurs because of roughness or other inhomogeneity of the surface. As

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Table 3 Impedance data for st-37 in 1 M HCl without and with ultimate concentration of 2-MBI, 2-MBT and 2-SHBT at 293 K after 1 and 24 h. Inhibitor

Conc. (ppm)

Rp (ohm)

Y0

Rs (ohm)

n

Cdl (F/cm2 ) × 106

EI%

1h

Blank 2-MBI 2-MBT 2-SHBT

0 1400 400 100

57.3 59 213.1 199.2

1.40E−04 3.30E−04 1.16E−04 1.12E−04

1.61 0.98 1.557 1.417

0.90 0.84 0.89 0.82

82.23 155.84 75.57 50.55

– 2.88 73.11 71.23

24 h

Blank 2-MBI 2-MBT 2-SHBT

0 1400 400 100

60.8 213.8 347 870

7.38E−04 1.61E−04 2.23E−04 5.00E−05

1.707 1.761 1.283 1.5

0.85 0.88 0.76 0.89

441.96 103.91 100.41 33.93

– 71.56 82.47 93.01

y = 59479x R² = 0.960

40

2-MBI 2-MBT

30

2-SHBT

20

y = 3217.x R² = 0.942

10

Linear (2-MBI)

y = 264.1x R² = 0.993

Linear (2-MBT) Linear (2-SHBT)

0 0.002

0

0.004

0.006

0.008

0.01

Conc. (M)

4.8 4.6 4.4 4.2 y = 0.887x + 4.585 R² = 0.976

log(θ/c)

In order to obtain the adsorption isotherm, the surface coverage ( ) of the inhibitor as a function of the concentration (C) was fitted to various models including Temkin, Frumkin, Freundlich, Langmuir and Florry Huggins. Langmuir adsorption isotherm, Eq. (11), is properly fitted on data obtained from gravimetric measurements for three inhibitors, but for 2-SHBT Florry Huggins isotherm [2], Eq. (12), shows a slightly better fit and the results obtained for EIS can be better explained with the latter.

2-SHBT

4

Linear (2-SHBT)

3.8 3.6 3.4

(11)

3.2 3

x(1 − )

x

= Kads C

(12)

In which Kads is adsorption equilibrium constant. Langmuir isotherm shows that the surface of steel has a limited number of adsorption sites each of which would hold at most one molecule. And there is no attraction or repulsion forces between the inhibitor molecules. Here the fraction of the surface covered by inhibitor molecules ( ) is in equilibrium with the number of available molecules (concentration) and the remained empty sites (1 − ) and Kads is the constant for this equilibrium [18]. The parameter ‘x’ in Eq. (12) is the number of water molecules replaced by an inhibitor molecule on metal surface. The value of x obtained here is 0.887 which is smaller than 1 and means that more than when a water molecule is removed from an active site more than 1 inhibitor can is adsorbed on that site, taking into account much larger volume of the inhibitor this probably means that 2-SHBT can be adsorbed on already occupied sites. The plot of /(1 − ) versus C is given in Fig. 5 for three inhibitors. Fig. 6 is log(1 − ) versus log( /C) and shows the Flory Huggins adsorption isotherm. In Eqs. (11) and (12) Kads is related to standard Gibb’s free energy of adsorption, Eq. (13): Kads =

50

Fig. 5. Langmuir adsorption plots for st-37 in 1 M HCl solution for 2-MBI, 2-MBT and 2-SHBT at 293 K.

3.3. Adsorption and isotherm

= Kads C 1−

60

θ/(1-θ θ)

can be seen from Fig. 2 after 1 h of immersion no significant change is seen in 2-MBI’s charge transfer resistance, the loop seen in low frequencies could be aroused from (FeCl− Inh+ )ads according to the mechanism proposed by Solmaz et al. [14]. For 2-MBT and 2-SHBT the inhibition is obvious but as the immersion time increases to 24 h, Fig. 3, the inhibitory action of the three molecules increases and approaches to the weight loss results. The observed decrease in Cdl values with addition of inhibitor, Table 3, is due to the decrease in local dielectric constant or an increase in the thickness of the electrical double layer, indicating that inhibitor molecules act by adsorption at the solution/interface [17]. In the present case, it can be assumed that water molecules are replaced by organic compounds adsorbed on the surface of steel.

1 exp 55.5

 −G

ads

RT

 (13)

where R is the universal gas constant, T is the absolute temperature and 55.5 is concentration of water in solution (mol/L). The

-2

-1.5

-1 log(1-θ)-0.5

0

0.5

Fig. 6. Flory Huggins adsorption isotherm for st-37 in 1 M HCl solution for 2-SHBT at 293 K.

isotherm assumes that the solid surface contains a fixed number of adsorption sites and each site holds one adsorbed species. Different organic inhibitors may adsorb on the metal surface via different mechanisms involving chemisorption of neutral molecules by sharing electron, interactions between ␲ electrons of the aromatic ring in organic molecule and the metal surface, also adsorption can occur via electrostatic interaction between a negatively charged surface, which is provided with a specifically adsorbed anion on iron and the positive charge of the inhibitor [17]. As can be seen in Table 4, the negative sign of Gads suggests the spontaneous phenomenon of adsorption. −Gads values less than 20 kJ/mol is consistent with electrostatic attraction between Table 4 Thermodynamic parameters for adsorption of 2-MBI, 2-MBT and 2-SHBT on st-37 in 1 M HCl solution. Inhibitor

R2

−Gads (kJ/mol)

2-MBI 2-MBT 2-SHBT

0.993 0.942 0.977

23.368 29.457 35.972

Kads (L/mol) 264 3217.3 43,343

Adsorption isotherm Langmuir Langmuir Flory–Huggins

Z. Moradi, M.M. Attar / Applied Surface Science 317 (2014) 657–665

the charged surface of the metal and charged organic molecules (physisorption) and values greater than 40 kJ/mol involve charge sharing or transfer from the inhibitor molecule to the metal surface and formation of chemical bond (chemisorption) [19]. 3.4. Atomic forced microscopy The atomic force microscope provides a powerful means of characterizing the microstructure of the surface [20]. Fig. 7 shows the sequence of AFM 3D topography images obtained from the steel surface (40 ␮m × 40 ␮m area). Fig. 7a shows the image of untreated steel substrate before contacting the solution. Fig. 7b is the topography after 24 h of immersion in uninhibited 1 M HCl solution. It

661

can be seen that although steel surface shows local rough texture in comparison with bare metal, corrosion in 1 M HCl is almost uniform. Fig. 7c–f shows samples immersed in 400 ppm 2-MBI, 1400 ppm 2MBI, 400 ppm 2-MBT and 100 ppm 2-SHBT with values of 0.4, 0.71, 0.9 and 0.98 respectively. AFM topography results shown in Table 5, declares that for metals immersed in inhibited solutions as value increases mean roughness of surface decreases. 3.5. Contact angle measurements Typically the chemical composition of the metal surface changes after the adsorption of inhibitor molecules which also changes the surface wettability [21]. Water contact angle measurements is a

Fig. 7. Comparative AFM topography of the surface (a) bare polished metal, (b) immersed in uninhibited 1 M HCl, (c) 400 ppm 2-MBI, (d) 1400 ppm 2-MBI, (e) 400 ppm 2-MBT and (f) 100 ppm 2-SHBT.

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Fig. 8. Contact angle measurements for (a) bare polished metal, (b) immersed in uninhibited 1 M HCl, (c) 100 ppm 2-SHBT, (d) 400 ppm 2-MBT and (e) 1400 ppm 2-MBI.

Table 5 Comparative AFM topography results and inhibition efficiency (IE%)of the surface of (a) bare polished metal, (b) immersed in uninhibited 1 M HCl, (c) 400 ppm 2-MBI, (d) 1400 ppm 2-MBI, (e) 400 ppm 2-MBT and (f) 100 ppm 2-SHBT. Sample

IE%

Mean roughness (nm)

Max peak value (nm)

Max valley depth (nm)

Bare polished metal Uninhibited 1 M HCl 400 ppm 2-MBI 1400 ppm 2-MBI 400 ppm 2-MBT 100 ppm 2-SHBT

– – 40 71 90 98

206 1000 2000 1600 1000 373

203 2552 2014 2285 2473 786

205 1000 1992 1572 1036 372

means to characterize the metal surface. Fig. 8a, represents the surface of polished metal before immersion in corrosive medium. Fig. 8b is related to the metal surface after 24 h of immersion in blank 1 M HCl which shows complete wetting of metal surface. The contact angle of plates immersed in inhibited solution can be seen in Fig. 8c–e for 100 ppm concentration of 2-SHBT, 400 ppm concentration of 2-MBT and 1400 ppm concentration of 2-MBI respectively. Table 6 represents the relative data, as can be clearly seen the better adsorption of inhibitor molecule on the surface for 2-SHBT and formation of an integrated molecular film caused a remarkable increase in contact angle. For 2-MBT weaker adsorption on the surface results in smaller contact angle, however its

hydrophobicity is more than 2-MBI which is the weakest inhibitor among these three molecules. For the three chosen inhibitors the surface shows a partial wetting ( < 90) and the work of adhesion, a can be calculated from Eq. (14) (Young’s equation) [22]: WSL a a −GSL = WSL = LV (1 + cos )

(14)

a WSL = 2( LV SV )1/2 exp[−ˇ( LV − SV )2 ]

(15)

where LV and SV are surface tension of water and surface free energy of the steel sample, respectively. is water contact angle and a was calculated from Eq. ˇ is 0.0001247 ± 0.000010 (mJ/m2 )−2 . WSL (14) and SV was then calculated from Eq. (15) [23]. Contact angle and surface free energy values obtained for different samples are shown in Table 6. For blank 1 M HCl solution formation of corrosion products and at the same time increase in surface roughness resulted in a completely hydrophilic surface ( = 0◦ ) with highest surface free energy. As the effectiveness (coverage) of inhibitor increases the contact angle increases which can be related first to the change in nature of the steel surface as a result of the inhibitor molecule adsorption and second to the change in roughness of surface reported in Table 5.

Table 6 Contact angles, work of adhesion and surface free energy of bare polished metal, metal immersed in uninhibited 1 M HCl, 1400 ppm 2-MBI, 400 ppm 2-MBT and 100 ppm 2-SHBT.

Contact angle a (mJ/m2 ) WSL

SV (mN/m)

Polished metal

Blank 1 M HCl

2-MBI-1400 ppm

2-MBT-400 ppm

2-SHBT-100 ppm

63.5 ± 0.5 105.28 45.558

0 145.6 72.8

6±1 145.2 72.4

52.5 ± 0.5 117.12 52.3

82.5 ± 0.5 82.3 33.91

Z. Moradi, M.M. Attar / Applied Surface Science 317 (2014) 657–665

663

Table 7 Quantum chemical parameters for 2-MBI, 2-MBIH+ , 2-MBT, 2-SHBT extracted by RHF/6-31G (d,p) and DFT/B3LYP/G-31* in water and vacuum. Method (phase)

Inhibitor

EHOMO (eV)

E (eV)

 (Debay)

X



N

RHF,6-31G (d,p) (vacuum)

2-MBI 2-MBT 2-SHBT

−8.02 −8.59 −8.09

ELUMO (eV) 3.69 3.1 2.41

−3.69 −3.1 −2.41

3.6 0.79 6.26

2.16 2.74 2.84

5.85 5.84 5.25

0.412 0.363 0.396

RHF, 6-31G (d,p) (water)

2-MBI 2-MBT 2-SHBT

−8.28 −6.34 −5.84

3.45 −0.84 −1.32

−3.45 0.84 1.32

3.5 1.26 8.77

2.41 3.59 3.58

5.86 2.75 2.26

0.390 0.620 0.756

RHF,6-31G (d,p) (Pro. in water)

2-MBI

−8.98

2.96

−2.96

6.89

3.01

5.97

0.334

DFT, B3LYP G-31* (vacuum)

2-MBI 2-MBT 2-SHBT

−5.89 −6.34 −5.84

−0.22 −0.84 −1.32

0.22 0.84 1.32

3.5 0.76 5.27

3.05 3.59 3.58

2.83 2.75 2.26

0.695 0.621 0.756

DFT B3LYP G-31*, PM3 (water)

2-MBI 2-MBT 2-SHBT

−6.15 −6.51 −5.94

−0.45 −1.01 −1.29

0.45 1.01 1.29

4.92 1.21 7.92

3.30 3.76 3.61

2.85 2.75 2.32

0.649 0.589 0.727

DFT B3LYP G-31*, Pro. in water

2-MBI

−6.85

−1.12

1.12

6.58

3.98

2.86

0.526

Fig. 9. population of HOMO for (a) 2-MBI, (b) protonated form of 2-MBI, (c) 2-MBT and (d) 2-SHBT (N blue, C gray, S yellow and H white).

Fig. 10. population of LUMO for (a) 2-MBI, (b) protonated form of 2-MBI, (c) 2-MBT and (d) 2-SHBT (N blue, C gray, S yellow and H white).

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3.6. Quantum chemical calculations As can be seen from Fig. 1, the three molecules are not very different in shape, size or volume. The varying factor is the substitution of S atom with C and N which would result in their different electronic structure. As presented in Table 7, EHOMO , ELUMO , E, ,  and N using DFT/B3LYP, and HF methods with 6-31G (d,p) basis set in vacuum and water phase are calculated. EHOMO is often associated with the electron-donating ability of a molecule, high values of EHOMO are likely to indicate a tendency of the molecule to donate electrons to appropriate acceptor molecules with low-energy, empty molecular orbital [24]. Conversely, ELUMO indicates the ability of the molecule to accept electrons, and the lower the value of ELUMO , the more probable it is that the molecule would accept electrons [6]. According to this theory and experimental results obtained from weight loss measurements and EIS technique the expected order of EHOMO values would be 2-MBI < 2MBT < 2-SHBT. And this order is satisfied in RHF, 6-31G*(D), PM3 results in water. The distribution of HOMO and LUMO orbitals of 2-MBI, 2-MBIH+ , 2-MBT and SHBT are given in Figs. 9 and 10 respectively. For 2-MBI in neutral form, the HOMO is distributed on pyridine-type nitrogen but not on pyrrole-type nitrogen. But in acidic media it can take the protonated form, the positive charge is delocalized between two nitrogen and both are equally potent for electrophilic attack. HOMO is distributed through the whole, implying that the preferred sites

for electrophilic attack of the inhibitors would be heteroatoms of the azole rings as well as the benzene ring. For 2-MBT HOMO is not only distributed on pyrrole-type nitrogen but also it is weakly distributed on sulfur atom, this molecule is probably adsorbed in a vertical form unlike 2-SHBT which shows a relatively high population of HOMO on both sulfur atoms which probably has a horizontal orientation on metal surface. On the other hand, the LUMO orbitals, which can accept electrons from the metal using ␲* orbitals to form ␲-back bonds are important in more effective interaction of molecule-metal and low values of ELUMO are favored. Fig. 10 represents the LUMO orbitals, it can be easily recognized that these orbitals are saturated around the rings and seemingly delocalized. For 2-SHBT as can be seen from Fig. 10d the population of LUMO is relatively lower than two other molecules. High values of HOMO and low values of LUMO, in other words can be referred to as softness of a molecule [14]. According to Hard Soft Acid Base theory (HSAB), soft interacts better with soft. As bulk metals are chemically the softest materials, this would imply that the smaller is the HOMO–LUMO gap of inhibitor molecules the stronger is their interaction with metal surfaces. On this basis the obtained results from all four states of calculation indicates the correct order of inhibition potential. Electrostatic potential map of the inhibitor is illustrated in Fig. 11. 2-MBI as shown in Fig. 11a has a high tendency to protonation Fig. 11b. A comparison between the electrostatic potential of 10b–d declares that for 2-MBIH+ having two electrophille

Fig. 11. population of ionization potential for (a) 2-MBI, (b) protonated form of 2-MBI, (c) 2-MBT and (d) 2-SHBT (N blue, C gray, S yellow and H white).

Z. Moradi, M.M. Attar / Applied Surface Science 317 (2014) 657–665

nitrogen the interaction would be with [FeCLOH]− ads [25] for 2-MBT we have nucleophillic areas which are more likely to have electrostatic interaction with positive surface of metal and for 2-SHBT sepration of charge is stroger and existance of positive and negative charge centers facilitates a better donation and back-donation which makes 2-SHBT the most effective inhibitor between these three molecules. Other computational factor matching with inhibition potential that is widely used in literature [26] is the fraction of transferred electron, N. Again like EHOMO the predicted order according to the experimental data is observed for RHF, 6-31G (d,p), PM3 results in water. Although some authors suggested the lower values of  favorable [4] and some others having the opposite idea [24], no direct relation between dipole moment and inhibition potential was observed for three molecules investigated here. 4. Conclusion The effect of introducing S atom to the benzazole based inhibitor molecules was investigated in 1 M HCl solution. Although the three molecules follow the same adsorption isotherm, Weight loss and EIS technique revealed a marked difference between their potential inhibitory actions. To better characterize changes in the behavior of surface, AFM and contact angle measurements were performed. AFM results showed a decrease in roughness as a result of introducing sulfur which was related to the ability of the molecule to better adsorb and formation of more integrated inhibitor film on metal surface. In the same regard contact angle measurements showed the direct relation between hydrophobicity and inhibitor Efficiency which also confirmed our statement on AFM results. In order to better determine the natural ability of these molecules to adsorb on metal surface studying the electronic structure by means of quantum chemical descriptors was performed in vacuum and water media using DFT/B3LYP, and HF methods. HF method with 6-31G (d,p) basis set in water medium gave the most consonant results. References [1] H. Keles¸, M. Keles¸¸, I. Dehri, O. Serinda˘g, The inhibitive effect of 6-amino-mcresol and its Schiff base on the corrosion of mild steel in 0.5 M HCI medium, Mater. Chem. Phys. 112 (2008) 173–179. [2] A. Ghanbari, M.M. Attar, M. Mahdavian, Corrosion inhibition performance of three imidazole derivatives on mild steel in 1 M phosphoric acid, Mater. Chem. Phys. 124 (2010) 1205–1209. [3] K.F. Khaled, The inhibition of benzimidazole derivatives on corrosion of iron in 1 M HCl solutions, Electrochim. Acta 48 (2003) 2493–2503. [4] N. Kovaˇcevic´ı, A. Kokalj, Analysis of molecular electronic structure of imidazoleand benzimidazole-based inhibitors: a simple recipe for qualitative estimation of chemical hardness, Corros. Sci. 53 (2011) 909–921.

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