Experimental and Theoretical Investigation of Inhibition Efficiency of 2

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Experimental and Theoretical Investigation of Inhibition Efficiency of 2-(2-Hydroxyphenyl)-Benzothiazole Using Impedance Spectroscopy, Experimental Design and Quantum Chemical Calculations Marzie Afzalkhah, Saeed Masoum, Mohsen Behpour, Hossein Naeimi, and Adel Reisi-Vanani Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b02030 • Publication Date (Web): 23 Jul 2017 Downloaded from http://pubs.acs.org on July 24, 2017

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Industrial & Engineering Chemistry Research

Experimental and Theoretical Investigation of Inhibition Efficiency of 2-(2Hydroxyphenyl)-Benzothiazole Using Impedance Spectroscopy, Experimental Design and Quantum Chemical Calculations Marzie Afzalkhah †, Saeed Masoum *,†, Mohsen Behpour †, Hossein Naeimi ‡ and Adel ReisiVanani § †

Department of Analytical Chemistry, Faculty of Chemistry, University of Kashan, Kashan, Iran ‡

Department of Organic Chemistry, Faculty of Chemistry, University of Kashan, Kashan, Iran

§

Department of Physical Chemistry, Faculty of Chemistry, University of Kashan, Kashan, Iran

*Corresponding Author Phone: +983155912338. Fax: +983155912397 E-mail: [email protected] (Saeed Masoum)

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ABSTRACT The aim of this research study was to apply experimental design in the optimization of influencing parameters on the corrosion inhibition efficiency of 2-(2-hydroxyphenyl)benzothiazole using electrochemical impedance spectroscopy (EIS). By experimental design, the best measurement for a mixture of 2-(2-hydroxyphenyl)-benzothiazole and ethanol as an impressive corrosion inhibitor for the mild steel in different hydrochloric acid concentration through studying the surface reaction procedure was investigated. Obtained optimum values by response surface methodology (RSM) were 433.7 µM for inhibitor concentration, 3.38 M for hydrochloric acid and 0.36 M for ethanol concentration. Keywords: Inhibitor, Corrosion, Mild steel, Electrochemical impedance spectroscopy, Experimental design, Quantum chemical calculations

1. INTRODUCTION Corrosion is an expensive, potentially disastrous phenomenon and is the damage of materials by reaction with their surrounding environments. The consequences of the corrosion phenomenon have become a serious problem of the world.

1,2

Corrosion inhibitors are materials that coat or

attach to the metal surface and provide protective barrier films, therefore, stop the development of corrosive reactions. In acidic solutions, the corrosion inhibition of metals consists of adsorption of the inhibitor molecule on the metal surface by retarding the corrosion reactions.3-12 Design of experiment that produces valid and reliable data is not easy. Experimental design has been frequently used in the statistical optimization of analytical approaches, because of its advantages such as a reduction on the number of experiments that leads to the considerably less laboratory work and lower reagent consumption and is faster to implement and more cost-


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effective than traditional one-at-a-time approach.13-17 Development of mathematical models that estimate statistical significance of factors is investigated by experimental design. It also facilitates the evaluation of interaction between influencing variables. In the traditional approach (one-at-a-time) that gradually change one factor at a time, a huge number of experiments are appeared with no tool to deal with interactions among factors. In this case, experimental design is used as a powerful tool to identify the relevant factors. Hajeeh attempted to find out the corrosion effect of seawater on pipes made of carbon steel and aluminum–brass using experimental design. Main factors that are utilized in the assessment of seawater corrosion included inhibitor, temperature, oxygen, sulfide, urea and chloride.18 Factorial experimental design was used by Mondel et al. to predict the erosive–abrasive wear rate of the cast Al–Si (LM6) alloy, 10 wt.% SiC particle in a slurry consisting of water that contains hydrochloric acid and sulphuric acid and 40 wt.% sand.19 In this study, inhibitory effect of the 2-(2-hydroxyphenyl)-benzothiazole molecule on corrosion protection of the mild steel in different hydrochloric acid concentration was investigated. The organic compounds that contain the sulfur atoms display excellent inhibition properties even at low concentration. It is notable that in these compounds, the active moiety is the sulfur atom even if nitrogen atoms are present. The sulfur atom can be adsorbed on the surface of metal that describes through 'Hard and Soft Acid Base' principle. It is clear that, the surface of metal (Fe0) is a soft acid and S atom is a soft base. A favorable electrostatic bond occurs between metal as soft acid and organic molecule containing a sulfur atom as a soft base. This interaction can be occurring between metal and hard bases such as O and N centers but is unfavorable and weak. According to Hoar and Holliday, the inhibitory effect of an organic molecule that is adsorbed on the metal surface is related to the induction a partial negative charge at the point of attraction.20



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In this work, 2-(2-hydroxyphenyl)-benzothiazole as an environmentally nontoxic compound and synthesized from cheap and affordable raw materials was investigated as an impressive corrosion inhibitor even in comparison to other similar benzothiazole derivatives. According to scheme 1, the hydrogen bonding between hydroxyl group and sulfur atom in 2-(2-hydroxyphenyl)benzothiazole, can be increased positive charge on S and the dπ-dπ interaction is more strong and consequently the effect of molecule to decrease the residual negative charge on Fe increased. Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) methods have been applied to study the inhibitory effects of this substance in hydrochloric acid media. These kinds of methods become problematic because many factors have multiple effects and it is hard to find the optimal experimental conditions and the right mathematical system in arrangement of the rules. Objective of the present study is applying rotatable central composite design (RCCD) coupled with response surface methodology (RSM), to find the functional relationship between corrosion efficiency and three operating variables namely inhibitor, hydrochloric acid concentration and ethanol concentration. This relationship can then be utilized to find out the optimal values of effective factors. 2. EXPERIMENTAL PROCEDURES 2.1. Materials and Synthesis of the 2-(2-hydroxyphenyl)-benzothiazole The corrosive media (2.0 to 6.0 M HCl) and ethanol solutions (0.36 to 1.79 M) were prepared by dilution of analytical grade HCl (36.5%) and ethanol (99.9%), respectively. 2-(2hydroxyphenyl)-benzothiazole was synthesized via condensation reaction of 2-aminothiophenol and the corresponding aldehyde catalyzed by nano silica-supported boron trifluoride as a reusable and effective catalyst.21,22 To a mixture of 2-aminothiophenol (1 mmol) and 2hydroxybenzaldehyde (1 mmol) in ethanol (10 ml), nano BF3/SiO2 (0.05 g) was added to a


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beaker and with a glass rod, the reaction mixture was mixed and stirred at ambient temperature for 35 min. Progress of the reaction was monitored by thin layer chromatography (TLC). After the mentioned reaction went to completion, under reduced pressure, the using solvent was removed and the mixture was cooled. 15 ml of dichloromethane was added to the mixture and filtered to remove the catalyst. Under reduced pressure, the filtrate was evaporated to isolate a solid residue, and recrystallized from ethanol to prepare the target product in 87% yield. 2.2. Preparation of the Electrode Composition of the mild steel sample was: 0.03% P, 0.4% Mn, 0.08% C, 0.03% S and balance iron and plate round with a diameter of 10.0 mm. For electrochemical measurements, the working electrode (WE) was a 7.0 cm long stem to provide a surface area of 1.0 cm2. Before each experiment, the mild steel specimens were polished on the SiC paper from 400 to 2500 grade in that order. The mild steel specimen washed with double distilled water ultrasonically and finally dried with air before they were immersed in the acid solution at room temperature. Electrochemical experiments were done in a glass cell. The electrochemical cell was equipped with a three-electrode system: mild steel, a platinum plate and a silver/silver chloride (Ag/AgCl (3 M Cl-)) were used as the working, counter and reference electrode, respectively. All of the experiments were done under unstirred conditions without deaeration at 25 ± 2oC. All the polarization and electrochemical impendence measurements were performed by AUTOLAB PGSTAT100. Polarization studies were carried out in the potential range of (-200) to (+200) mV with the scan rate of 0.5 mVs-1 relative to the corrosion potential. The impedance data were obtained at corrosion potentials, Ecorr vs. Ag/AgCl (3 M Cl-), over a frequency domain from 10 kHz to 100 mHz and NOVA 1.6 software automatically controls the measurements. The impedance data were analyzed using a FRA software and Pentium IV computer.



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2.3. Experimental Design In this work, rotatable central composite design (RCCD) was utilized to investigate the influencing effects of three chemical variables (concentration of inhibitor, hydrochloric acid and ethanol concentration) on the impedance spectroscopy and potentiodynamic polarization responses. The polynomial equations and response surfaces were obtained using the statistical software package, Design-Expert® by Stat-Ease, Inc. The experiments were performed based on fivelevel three-factor RCCD with coded levels of parameters as shown in Table 1. 2.4. Surface Studies The surface study of the mild steel was carried out by a ZEISS scanning electron microscope (SEM), Ara research atomic force microscope (ARA-AFM) and TESCAN Mira3 energy dispersive X-ray (EDX) before and after exposure to 3.38 M HCl in the absence and presence of 2-(2-hydroxyphenyl)-benzothiazole inhibitor. 2.5. Quantum Chemical Calculations To gain better insight to the problem and to obtain some reactivity indices, semi-empirical and DFT calculations were carried out. The geometry of the 2-(2-hydroxyphenyl)-benzothiazole was fully optimized in gas and liquid phases without any symmetry constraints using semi-empirical (AM1, PM3 and PM6) and DFT (B3LYP/6-31+G(d,p)) methods. Vibrational analyses were done for all stationary points at the same level of theory to ensure that they are in true minima on potential energy surface by checking absence of imaginary frequency. Results such as EHOMO, ELUMO, energy gap, dipole moment (μ), absolute hardness (η), absolute electronegativity (χ)


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and electron charge transfer (∆N) were followed for finding the effect of an inhibitor on inhibition of the corrosive process. All of the calculations were done with Gaussian 03 program. 3. RESULTS AND DISCUSSION 3.1. Potentiodynamic Polarization Measurements By evaluation of the polarization parameters, the corrosion behavior can be determined. The potentiodynamic polarization curves of the mild steel in HCl (3.38 M) and ethanol (0.36 M) by adding various concentrations of the inhibitor are depicted in Figure 1. Corrosion parameters from Tafel polarization curves in 3.38 M HCl solution at various concentrations of inhibitors are presented in Table 2. Table 2 indicates in contrary to the inhibitor concentration there is a decrease in the corrosion current density (icorr) that is attributed to the ability of the inhibitor to prevent interaction between corrosive medium and metal. It is predictable that the inhibition efficiency (IEp (%)) is increased with the concentration of inhibitor. Inhibition efficiency percentage is calculated by the help of the following equation: IEp (%) = (i°corr – icorr) / i°corr) × 100

(1)

Where i°corr and icorr are the densities of corrosion current in the absence and presence of the inhibitor, respectively. 23–25 The cathodic slopes polarization curves were similar in inhibited and uninhibited solution thus adding of the inhibitor to the violent solution does not modify the proton reduction mechanism. Evidence shows that some of the surface remains inhibited, therefor the cathodic reaction in the negative sweep is lower than in solution without the inhibitor. Anodic Tafel lines of closely equal slopes were achieved, indicating that the mild steel dissolution mechanism did not change by addition of the 2-(2-hydroxyphenyl)-benzothiazole. The increasing current density at higher over potentials could be attributed to the significant dissolution of the mild steel, because of desorption of inhibitor film from the metal surface. As 7 ACS Paragon Plus Environment


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shown in Figure 1, the addition of the 2-(2-hydroxyphenyl)-benzothiazole produced a negative shift of Ecorr as the concentration of the inhibitor increased. Also, the presence of the inhibitor had an effective influence both on the cathodic behavior and the anodic behavior. It is also clear that there is a shift towards a cathodic region in the values of corrosion potential (Ecorr vs. Ag/AgCl (3 M Cl-)). It is according to the literature if the shift in Ecorr (inh) vs. Ag/AgCl (3 M Cl-) is bigger than 85 mV from Ecorr vs. Ag/AgCl (3 M Cl-), the inhibitor can be calssified as a cathodic or anodic type; and if the shift in Ecorr (inh) vs. Ag/AgCl (3 M Cl-) is less than 85 mV, the inhibitor can be classified as mixed type. In this study shift in Ecorr vs. Ag/AgCl (3 M Cl-) is inclined toward the cathodic region, which designates that 2-(2-hydroxyphenyl)-benzothiazole is a cathodic type inhibitor. The Ecorr value shifts towards more negative potentials as the inhibitor concentration increased. However, it is more dominated on the cathodic reaction. The results show that by increasing the concentration of inhibitor, its efficiency will be increased. 3.2. Electrochemical Impedance Measurements As a film is formed on the mild steel surface by the inhibitor, the film resistance should be occurred as well as double layer resistance (Cdl) and other resistance at metal/solution interface that are totally designated as polarization resistant (Rp). 23-28 The Nyquist diagrams along the real impedance axis (Zreal), indicate that the corrosion of mild steel in 3.38 M HCl is controlled by a polarization resistant (Rp) process. The polarization resistance values (Rp) were obtained from the Zreal. The Nyquist diagrams of the mild steel immersed in 3.38 M HCl and 0.36 M ethanol without and with various concentrations of the inhibitor (200 µM to 1000 µM) are depicted in Figure 2. The polarization resistance (Rp) that is obtained from the extrapolation of the impedance spectra to the low frequency limit increased with inhibitor concentration. A considerable increase in the inhibition efficiency could be observed as the inhibitor concentration


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increased from 200 µM to 400 µM while increasing concentration from 400 µM to 1000 µM resulted in no significant improvement of inhibition efficiency. By fitting to the equivalent circuit model, the impedance spectra are analyzed (Figure 3). The impedance spectra of the mild steel electrode modified with the inhibitor were fitted by solution resistance (Rs), charge transfer resistance (Rct) and double layer capacitance (Cdl). The inhibition efficiency percentage was calculated based on the following equation: °

IE (%) = (Rp – Rp ) / Rp) ×100

(2)

°

Where Rp and Rp are the polarization resistance in the absence and presence of the inhibitor, respectively.23-25 Some electrochemical parameters such as Rp, Rs and proportionality coefficient (Q), double layer capacitance (Cdl) were extracted from the equivalent electrical circuit in Figure 3. In Table 3, the values of Q, Rp, n (degree of surface inhomogeneity was shown by n as a phase shift) and Cdl are shown. As shown in Table 3, by adding the inhibitor to HCl solution an increase in polarization resistance and a decrease in Cdl were occurred. Because of the water molecules replacement with 2-(2-hydroxyphenyl)-benzothiazole inhibitor on the mild steel surface, the decrease in Cdl can be seen. This observation indicates that increasing in the concentration of inhibitor on the surface of mild still leads to reduction of metal dissolution rate. The Bode and phase angle plots for the mild steel in 3.38 M HCl without and with various amounts of the inhibitor molecule are shown in Figure 4. As shown in the Bode plot, by increasing the concentration of inhibitor, the increase of absolute impedance has occurred at low frequencies and it confirms the protection of the mild steel surface in 3.38 M HCl, due to the adsorption of inhibitor molecule on it (Figure 4a). Figure 4b shows more negative values of phase angle by increasing the concentration of inhibitor, which indicates the excellent inhibitive 9 ACS Paragon Plus Environment


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behavior of 2-(2-Hydroxyphenyl)-Benzothiazole due to more adsorption of it on the mild steel surface.29 3.3. Adsorption Isotherms Various types of adsorption isotherms such as Langmuir, Flory–Huggins, Temkin and Frumkin were used to fit the obtained data for finding the most suitable adsorption isotherm for adsorption of 2-(2-hydroxyphenyl)-benzothiazole on the mild steel surface. For various concentrations of the inhibitor in 3.38 M HCl solution, the following equation is used to acquire the degrees of surface coverage (θ): θ = (I0corr - Icorr )/I0corr

(3)

Where Icorr and I0corr are the current density values of the mild steel in the absence and presence of the inhibitor molecule, respectively.29 As shown in Figure 5, for 2-(2-hydroxyphenyl)benzothiazole, the plots of C/θ vs. C yield straight line with slope and correlation coefficient close to 1 that indicates the adsorption of 2-(2-hydroxyphenyl)-benzothiazole on the mild steel surface is fitted to the Langmuir adsorption isotherm: C/θ=1/Kads+ C

(4)

Where C is the inhibitor concentration, θ is the surface coverage and Kads is the adsorption equilibrium constant. From the intercept of the straight line Kads value can be determined and is related to the standard free energy of adsorption (∆G0ads), by the following equation: Kads =1/55.5(exp (-∆G0ads/RT)

(5)

Where, 55.5 (mol dm-3) is the molar concentration of water in solution, R (J mol-1 K-1) is the gas constant and T (K) is the temperature.29 The calculated value of Kads for 2-(2-hydroxyphenyl)benzothiazole is 2.75 × 104 M-1 , while the value of ∆G0ads is -35.27 kJ mol-1. The obtained value 10 ACS Paragon Plus Environment

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of |∆G0ads| indicates that 2-(2-hydroxyphenyl)-benzothiazole molecule adsorbs on the surface of mild steel by recombination of both chemisorption and physisorption.30-32 3.4. Weight loss measurements Table 4 shows the weight loss measurements result for the corrosion of the mild steel in 3.38 M HCl in the absence and presence of 433.7 µM inhibitor in various immersion times (2 - 24 h) at room temperature. The percentage of inhibition efficiency was obtained by the following equation: IE (%) = (w0 – w) / w0) × 100

(6)

where, w0 and w are the weight loss of mild steel for same immersion time in the HCl solution without and with the inhibitor molecule, respectively.25 The obtained results demonstrate the stability of the 2-(2-hydroxyphenyl)-benzothiazole with time. The mild steel corrosion rate (Wcorr) was reduced upon the addition of 2-(2hydroxyphenyl)-benzothiazole for different immersion times, that demonstrates the inhibitory effect of the 2-(2-hydroxyphenyl)-benzothiazole towards mild steel corrosion in HCl solution. 3.5. Analysis of Variance (ANOVA) and Response Optimization Analysis of variance (ANOVA) and response optimization are interesting in corrosion involve relationships between independent response variables and one or more dependent variables of interest. Corrosion is complex and typically, many potential variables, both those measured and included in an analysis and those not measured, may influence the variable of interest. In order to obtain a perspective on the effect of each variables, the interaction between variables and how they influence the response, the response surface plots based on the quadratic model must be obtained. Table 5 shows a series of experiments that was obtained by considering a rotatable central composite design. 11 ACS Paragon Plus Environment


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Table 6 shows the analysis of variance for the reduced quadratic model. Analysis of variance for different kinds of models, based on higher F and R-squares values and lower lack of fit (LOF), and also predicted residual sum of squares (PRESS), indicated that the reduced quadratic polynomial model adequately explains the changes in a current by varying the influencing parameters. As shown in Table 6, the F-value of 43.58 implies the model is significant. The pvalues obtained from the ANOVA are shown in Table 6. The P-values are important to find out the pattern of interactions between the influencing variables and are used to determine the significant parameter. Any variable or interaction of variables with P < 0.05 is considered to be significant, and insignificant terms can be removed from the model. In this case x1, x2, x1x2, x1x3, x2x3, x12 and x22 are significant model terms so the model was reconstructed without x32 term (Eq. 7). y = 23.96 - 6.49x1 + 33.38x2 + 3.91x3 - 9.64x1x2 - 9.33x1x3 + 6.91x2x3 + 9.2x12 + 24.49x22 (7) Furthermore, the P-value obtained from lack of fit (LOF) was 0.0777, which indicated that the model provides the true shape of the response surface. As can be seen in Table 6, the predicted values match the experimental values reasonably well with R-squared of 0.9776 and R-squared (adj) of 0.9551 for current. The response plots relationship between two variables and response y (current) at the center level of other variables are shown in Figure 6. Figure 6a shows the effect of concentration of inhibitor (x1) and concentration of HCl (x2) on current (y) at the center level of volume of ethanol (x3). The effect of concentration of inhibitor (x1) and volume of ethanol (x3) on current (y), while keeping concentration of HCl (x2) at the center level, is also shown in Figure 6b. Figure 6c shows the effect of concentration of HCl (x2) and volume of ethanol (x3) on current (y) at the center level of concentration of the inhibitor (x1). Curvature plot of Figures 4a, b, c, indicate that there was nonlinear relation between current (y) and influencing variables. The


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result demonstrates that all the response surfaces have the minimum points. Therefore, response surface optimization could be found depending on concentration of inhibitor (x1) and concentration of HCl (x2) and volume of ethanol (x3). Response optimization results of the reduced quadratic model indicate that the response y presented the minimum result 7.07 µA at the optimal parameters of x1 (-0.73), x2 (-0.54), x3 (-1.68), i.e. current showed result of 7.07 µA at the condition of concentration of inhibitor 433.7 µM, concentration of HCl 3.38 M and concentration of ethanol 0.36 M. 3.6. Surface Analysis Valuable information could be obtained from the chemical composition and morphology of a coating and the mild steel surface using energy dispersive X-ray analysis (EDXA) and scanning electron microscopy (SEM). Figure 7 shows an SEM micrograph of mild steel immersed for 1.5 h in 3.38 M HCl in the absence (Figure 7a) and the presence of 433.7 µM 2-(2-hydroxyphenyl)-benzothiazole (Figure 7b), at room temperature. Figure 7a shows the uniform corrosion of mild steel with a rough surface in HCl, but a smooth surface can be obtained in the presence of the 2-(2-hydroxyphenyl)benzothiazole (Figure 7b) that indicates the mild steel surface was protected by the inhibitor. The EDX spectra of the mild steel sample in the absence and presence of inhibitor were shown in Figure 8. As shown in Fig. 8b, when the mild steel was immersed in the inhibitor, the strength of the iron band decreased and presence of the nitrogen and oxygen atoms from the inhibitor were confirmed. This data indicates the formation of a protective film of 2-(2-hydroxyphenyl)benzothiazole adsorbed on the mild steel surface that inhibits the corrosion of it. The AFM morphology for the mild steel surface and the coated one with the inhibitor after immersion in 3.38 M HCl are shown in Figure 9. 13 ACS Paragon Plus Environment


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3.7. Molecular Parameters from Quantum Chemical Calculations In this work, mild steel and inhibitor are Lewis acid and Lewis base, respectively. According to the frontier molecular orbital theory, if frontier molecular orbitals (the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO)) have suitable overlap, their interaction is considerable and a transition state is formed. Therefore, some molecular reactivity indicators were calculated by quantum chemical calculations to explain the electronic structure of the inhibitor and foretell adsorption characteristics of it to the surface of mild steel. The structure of the inhibitor was optimized in gas and liquid phases without any symmetry constraints by semi-empirical methods (AM1, PM3 and PM6) and by DFT method at B3LYP/631+G(d,p) level of theory. Optimized structure of inhibitor, Mulliken charges, HOMO, HOMO1, HOMO-2, LUMO, LUMO+1 and LUMO+2 of the inhibitor at B3LYP/6-31+G(d,p) level of theory in gas phase are brought in Figure 10. Some structural and electrical parameters and molecular reactivity indicators related to the inhibitor have been shown in Table 7. EHOMO concerns to electron donating ability of a molecule (inhibitor) to an electron acceptor (3d orbital of Fe in mild steel) to form a coordination bond.33, 34

A good inhibitor must have higher EHOMO value to give more electron and form stronger bond.

On the other hand, ability of a molecule to receive electron is concerned to ELUMO. A good inhibitor must have lower ELUMO to accept more electron with its anti-bonding orbitals from metal and forms a back-donating bond with metal.35 So, having a high EHOMO and low ELUMO means a small energy gap is a good character for an inhibitor. High values of EHOMO that determine electron donating from the inhibitor to vacant d orbitals within the Fe atom in mild steel and low values of energy gap (Eg), reflected that the required 14 ACS Paragon Plus Environment

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energy for transfer electron from HOMO to LUMO will be low and resulting in improved inhibition efficiencies. HOMO and LUMO images show contribution of each atom in frontier molecular orbitals. HOMO shows pronounced contributions from atoms which represents suitable site for electrophilic attack and LUMO shows sites of molecule that are ready for nucleophilic attack. The HOMO and LUMO energy levels are corresponding to the ionization potential (I) and the electronaffinity (A), means that:36 ‫ = ܫ‬−‫ܧ‬ுைெை

(8)

‫ = ܣ‬−‫ܧ‬௅௎ெை

The values of absolute electronegativity (χ) and absolute hardness (η) for an inhibitor are connected to A and I and can be specified by the following equations:37 ߯= ߟ=

ூା஺

(9)

ଶ ூି஺

(10)

ଶ

The HOMO–LUMO gap, is an important stability index so that a large Eg value implies high stability for the molecule in chemical reactions. As Eg decreases, the reactivity of molecule increases that leads to improve inhibition efficiencyof the inhibitor. Calculated quantum chemical parameters were exhibited that 2-(2-hydroxyphenyl)-benzothiazole as inhibitor has high value of HOMO and low value of LUMO with low Eg. An important parameter for measuring inhibition process is electron charge transfer (∆N) that determine electron charge transfer from an inhibitor molecule (Lewis base) to a metal surface (Lewis acid). ∆N is given by following equation: ఞ ିఞ

߂ܰ = ଶ(ఎ೘ ାఎ೔ ) ೘

(11)

೔



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that χm and χi are the absolute electronegativity and ηm and ηi are absolute hardness of metal and inhibitor molecule, respectively. Calculated values of ∆N in Table 7 were computed using theoretical values of χm and ηm equal to 7 eV/mol and 0 eV/mol, respectively.38 It is clear that stronger adsorption energy for metal−inhibitor interactions belong to larger ∆N values.39 High and positive values of ∆N in Table 7 confirm a strong metal-inhibitor interaction, therefore the formation of a good inhibition adsorption layer against corrosion was occurred. The Mulliken charge distributions of the inhibitor are brought in Figure 10. Oxygen, nitrogen and the some carbons atoms have higher charge densities. Therefore, these areas are usually the best positions for electrophilic attack and these sites are the active center with the strongest bonding to the mild steel surface. Also, HOMO was principally distributed on the domain including these atoms.

4. Conclusion A novel organic molecule was used as a corrosion inhibitor for mild steel alloy. This compound was synthesized by nano silica-supported boron trifluoride as an efficient and reusable catalyst. Response surface methodology (RSM) in conjunction with rotatable central composite design (RCCD) was utilized to modeling and optimizing corrosion current. CCRD was applied to design an experimental program for modeling the effects of inhibitor concentration, hydrochloric acid and ethanol concentration on the corrosion current. Quantum chemical calculations confirm that the 2-(2-hydroxyphenyl)-benzothiazole has a strong interaction with mild steel and formed a good inhibition adsorption layer against corrosion.

Acknowledgments The authors are grateful to the University of Kashan for supporting this work by grant no. 573586/2. 16 ACS Paragon Plus Environment

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References (1) Roberge, P. R. Handbook of Corrosion Engineering; McGraw-Hill: New York, 2000. (2) Cramer, S. D.; Covino, B. S. ASM handbook, Corrosion: Materials; ASM International, 2005. (3) Sheng , X.; Ting, Y. P.; Pehkonen, S. O. Evaluation of an organic corrosion inhibitor on abiotic corrosion and microbiologically influenced corrosion of mild steel. Ind. Eng. Chem. Res. 2007, 46, 7117. (4) Kuznetsov, Yu. I. Progress in the science of corrosion inhibitors. Int. J. Corros. Scale Inhib. 2015, 4, 15. (5) Satapathy, A. K.; Gunasekaran, G.; Sahoo, S. C.; Amit, K.; Rodrigues, P.V. Corrosion inhibition by Justicia gendarussa plant extract in hydrochloric acid solution. Corros. Sci. 2009, 51,

2848. (6) Ebadi, M.; Basirun, W. J.; Khaledi, H.; Ali, H. M. Corrosion inhibition properties of pyrazolylindolenine compounds on copper surface in acidic media. Chem. Cent. J. 2012, 6, 163. (7) Ashassi-Sorkhabi, H.; Majidi M. R.; Seyyedi, K. Investigation of inhibition effect of some amino acids against steel corrosion in HCl solution. Appl. Surf. Sci. 2004, 225, 176. (8) Behpour, M.; Ghoreishi, S. M.; Soltani, N.; Salavati-Niasari M. The inhibitive effect of some bis-N,S-bidentate Schiff bases on corrosion behaviour of 304 stainless steel in hydrochloric acid solution, Corros. Sci. 2009, 51, 1073. (9) Verma. C. B.; Quraishi, M. A. Schiff’s bases of glutamic acid and aldehydes as green corrosion inhibitor for mild steel: weight-loss, electrochemical and surface analysis, IJIRSET, 2014, 3, 7.



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(10) M. Hosseini, S.F.L. Mertens, M. Ghorbani, M.R. Arshadi, Asymmetrical Schiff bases as inhibitors of mild steel corrosion in sulphuric acid media. Mater. Chem. Phys. 2003, 78, 800. (11) Yurt, A.; Balaban, A.; Kandemir, S. U.; Erk, B. Investigation on some Schiff bases as HCl corrosion inhibitors for carbon steel. Mater. Chem. Phys. 2004, 85, 420. (12) Agrawal, Y. K.; Talati, J. D.; Shah, M. D.; Desai, M. N.; Shah, N.K. Schiff bases of ethylenediamine as corrosion inhibitors of zinc in sulphuric acid. Corros. Sci. 2004, 46, 633.

(13) Garcia-Manrique, P.; Matos, M.; Gutierrez, G.; Estupinan, O. R.; Blanco-Lopez, M. C.; Pazos, C. Using factorial experimental design to prepare size-tuned nanovesicles. Ind. Eng. Chem. Res. 2016, 55, 9164. (14) Ghoreishi, S. M.; Saeidinejad, F.; Behpour, M.; Masoum, S. Application of multivariate optimization to electrochemical determination of methyldopa drug in the presence of diclofenac at a nanostructured electrochemical sensor, Sensor Actuat B-Chem. 2015, 221, 576. (15) Yang, J.; Qiu, K. Experimental design to optimize the preparation of activated carbons from herb residues by vacuum and traditional zncl2 chemical activation. Ind. Eng. Chem. Res. 2011, 50, 4057. (16) Brereton, R. G. Chemometrics : data analysis for the laboratory and chemical plant. John Wiley & Sons: Chichester, 2003. (17) Miller, J. N.; Miller. J. C. Statistics and chemometrics for analytical chemistry. Pearson Education Limited: Harlow, 2010. (18) Hajeeh, M. Estimating corrosion: a statistical approach. Mater. Design. 2003, 24, 509. (19) Mondel, D. P.; Das, S.; Prasad, B. K. Study of erosive–corrosive wear characteristics of an aluminium alloy composite through factorial design of experiments, Wear 1998, 217, 1.


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(20) Hoar, T. P.; Holliday, R. D. The inhibition by quinolines and thioureas of the acid dissolution of mild steel, J. Appl. Chem. 1953, 3, 502. (21) Ben-Alloum, A.; Bakkas, S.; Soufiaoui, M. Novel synthesis of 2-arylbenzothia-zolesmicrowave-activated electron transfer, Tetrahedron Lett. 1997, 38, 6395. (22) Naeimi, H.; Heidarnezhad, A. Synthesis of 2-arylbenzothiazoles using nano BF3/SiO2 as a reusable and efficient heterogeneous catalyst under mild conditions J. Sulfur Chem. 2014, 35, 493. (23) Saha; S. Kr., Dutta; A., Ghosh; P., Sukul; D., Banerjee; P. Novel Schiff base molecules as efficient corrosion inhibitors for mild steel surface in 1 M HCl medium: Experimental and theoretical approach, Phys. Chem. Chem. Phys. 2016, 18, 17898. (24) Solmaz; R., Kardas; G., Çulha M., Yazıcı; B., Erbil; M. Investigation of adsorption and inhibitive effect of 2-mercaptothiazoline on corrosion of mild steel in hydrochloric acid media, Electrochim. Acta 2008, 53, 5941. (25) Dutta; A., Saha; S. Kr., Banerjee; P., Patra; A. K., Sukul; D. Evaluating corrosion inhibition property of some Schiff bases for mild steel in 1M HCl: competitive effect of heteroatom and stereochemical conformation of the molecule, RSC Adv. 2016, 6, 74833. (26) Özcan, M.; Dehri, I.; Erbil, M. Organic sulphur-containing compounds as corrosion inhibitors for mild steel in acidic media: correlation between inhibition efficiency and chemical structure, Appl. Surf. Sci. 2004, 236, 155. (27) Solmaz; R. Investigation of the inhibition effect of 5-((E)-4-phenylbuta-1,3dienylideneamino)-1,3,4-thiadiazole-2-thiol Schiff base on mild steel corrosion in hydrochloric acid, Corros. Sci. 2010, 52, 3321.



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(28) Saha; S. Kr., Dutta; A., Ghosh; P., Sukul; D., Banerjee; P. Adsorption and corrosion inhibition effect of Schiff base molecules on the mild steel surface in 1 M HCl medium: A combined experimental and theoretical approach, Phys. Chem. Chem. Phys. 2015, 17, 5679. (29) Xu; B., Yang; W., Liu; Y., Yin; X., Gong; W., Chen; Y. Experimental and theoretical evaluation of two pyridinecarboxaldehyde thiosemicarbazone compounds as corrosion inhibitors for mild steel inhydrochloric acid solution, Corros. Sci. 2014, 78, 260. (30) Yurt; A., Bereket; G., Kivrak; A., Balaban; A., Erk; B. Effect of schiff bases containing pyridyl group as corrosion inhibitors for low carbon steel in 0.1 M HCl, J. Appl. Electrochem. 2005, 35, 1025. (31) Fekry; A. M., Mohamed; R. R. Acetyl thiourea chitosan as an eco-friendly inhibitor for mild steel in sulphuric acid medium, Electrochim. Acta 2010, 55, 1933. (32) Saliyan; V. R., Adhikari; A.V. Quinolin-5-ylmethylene-3-{[8-(trifluoromethyl)quinolin-4yl]thio}propanohydrazide as an effective inhibitor of mild steel corrosion in HCl solution, Corros. Sci. 2008, 50, 55. (33) Ahamad; I., Prasad; R., Quraishi; M. A. Adsorption and inhibitive properties of some new Mannich bases of isatin derivatives on corrosion of mild steel in acidic media, Corros. Sci. 2010, 52, 1472. (34) Fang; J., Li; J. Quantum chemistry study on the relationship between molecular structure and corrosion inhibition efficiency of amides, J. Mol. Struct. 2002, 593, 179. (35) Zhao; P., Liang; Q., Li; Y. Electrochemical, SEM/EDS and quantum chemical study of phthalocyanines as corrosion inhibitors for mild steel in 1 mol/L HCl, Appl. Surf. Sci. 2005, 252, 1596.


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(36) Dewar; M. J. S., Thiel; W. Ground states of molecules. 39. MNDO results for molecules containing hydrogen, carbon, nitrogen, and oxygen, J. Am. Chem. Soc. 1977, 99, 4899. (37) Pearson; R. G. Absolute electronegativity and hardness: application to inorganic chemistry, Inorg. Chem. 1988, 27, 734 (38) Ju; H., Kai; Z., Li; Y. Aminic nitrogen-bearing polydentate Schiff base compounds as corrosion inhibitors for iron in acidic media: A quantum chemical calculation, Corros. Sci. 2008, 50, 865. (39) Kovacevic; N., Kokalj; A. DFT study of interaction of azoles with Cu(111) and Al(111) surfaces: role of azole nitrogen atoms and dipole–dipole interactions, J. Phys. Chem. C 2011, 115, 24189.



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Scheme caption Scheme 1. Schematic representation of the 2-(2-hydroxyphenyl)-benzothiazole molecule (a), the hydrogen bonding between hydroxyl group and sulfur atom in 2-(2-hydroxyphenyl)benzothiazole molecule (b)


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Figure captions Figure 1. The potentiodynamic polarization curves of mild steel in 3.38 M HCl with the addition of different concentrations of the inhibitor (ethanol concentration, 0.36 M). Figure 2. The Nyquist plots of the working electrode in acid solution in the absence and presence of inhibitor (in HCl 3.38 M and ethanol 0.36 M). Figure 3. The equivalent circuit model that is used to fit the obtained data from EIS. Figure 4. The Bode (a) and phase angle plots (b) for the mild steel in 3.38 M HCl solution without and with different concentrations of the inhibitor. Figure 5. Langmuir adsorption plot for the mild steel in 3.38 M HCl with 2-(2-hydroxyphenyl)benzothiazole. Figure 6. Response surface plots of current against different influencing parameters. Figure 7. SEM micrographs of (a) bare mild steel immersed in 3.38 M HCl; (b) mild steel coated with the inhibitor after immersion in 3.38 M HCl.

Figure 8. EDX spectra of (a) mild steel, (b) mild steel coated with 2-(2-hydroxyphenyl)benzothiazole after immersion in 3.38 M HCl solution. Figure 9. AFM images of the mild steel surface (a) and the mild steel surface coated with the inhibitor (b) after immersion in 3.38 M HCl.

Figure 10. Optimized structure of the inhibitor at B3LYP/6-31+G(d,p) level of theory, Mulliken charges, HOMO, HOMO-1, HOMO-2, LUMO, LUMO+1 and LUMO+2 with their energy levels.



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Table 1. Levels of independent variables based on five-level three-factor RCCD -1.68

-1

0

1

1.68

x1: Concentration of inhibitor (µM)

200

400

600

800

1000

x2: Concentration of HCl (M)

2.0

3.0

4.0

5.0

6.0

x3: Concentration of ethanol (M)

0.36

0.72

1.07

1.43

1.79


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Table 2. Corrosion characteristics from Tafel polarization curves obtained in 3.38 M HCl solution at different concentrations of inhibitor. Corrosion Ecorr vs. Ag/AgCl Cinh (M)

(3 M Cl-) (mV)

bc (mV dec-1)

ba (mV

Icorr (µA

dec-1)

cm-2)

IE rate (%) (mm/year)

Blank ̶

-438.20

88.535

54.522

405.01

4.7062

-

Inhibitor

0.0002

-476.22

88.185

57.142

134.08

1.558

66.89

Inhibitor

0.0006

-525.62

85.884

59.967

94.75

1.101

76.61

Inhibitor

0.0010

-541.97

87.092

59.263

37.64

0.437

90.17



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Table 3. Obtained electrochemical parameters from electrochemical impedance spectroscopy for the mild steel sample in the presence of inhibitor at different concentrations in 3.38 M HCl solution Rs ( mΩ cm2 )

Rp ( Ω cm2)

Cdl (µF cm-2)

Q Y0 (µF cm-2 )

n

IE (%)

Blank

227

48.24

99.62

381

0.752

-

0.0002 M Inhibitor

222

179.29

60.42

194

0.739

73.09

0.0006 M Inhibitor

117

228.52

53.89

160

0.743

78.89

0.001 M Inhibitor

634

397.45

51.60

120

0.789

87.86


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Table 4. Mild steel weight loss results in 3.38 M HCl in the absence and presence of inhibitor for the various

Immersion time Inhibitor (M) Without inhibitor 0.0002 0.0006 0.001 immersion times

2h Wcorr (mg cm-2 h-1) 6.37 1.34 1.34 0.06

IE (%) 78.96 79.69 99.06

4h Wcorr (mg cm-2 h-1) 3.44 0.81 0.75 0.10

IE (%) 76.45 78.20 97.09

8h Wcorr (mg cm-2 h-1) 1.72 0.45 0.44 0.06


IE (%) 73.84 74.42 96.51

24 h Wcorr (mg cm-2 h-1) 0.78 0.25 0.21 0.05

IE (%) 67.95 73.08 93.59


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Table 5. Rotatable central composite design and obtained response from each experiment Run order

x1

x2

x3

Response (Current, µA)

1

1.00

1.00

1.00

70.09

2

1.00

1.00

-1.00

68.28

3

1.00

-1.00

1.00

12.75

4

1.00

-1.00

-1.00

29.80

5

-1.00

1.00

1.00

126.87

6

-1.00

1.00

-1.00

78.97

7

-1.00

-1.00

1.00

22.185

8

-1.00

-1.00

-1.00

10.70

9

1.68

0.00

0.00

48.20

10

-1.68

0.00

0.00

66.49

11

0.00

1.68

0.00

156.03

12

0.00

-1.68

0.00

44.886

13

0.00

0.00

1.68

28.319

14

0.00

0.00

1.68

22.86

15

0.00

0.00

0.00

24.25

16

0.00

0.00

0.00

29.80

17

0.00

0.00

0.00

26.74


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Table 6. Analysis of variance for the reduced quadratic model (coded units) Source

Sum of Squares

df

Mean Square

F-value

P-value

Model

25363.40

8

3170.43

43.58

< 0.0001

Signifi cant x1

574.43

1

574.43

7.90

0.023

x2

15205.61

1

15205.61

208.00

< 0.0001

x3

208.36

1

208.36

2.86

0.129

x1x2

743.73

1

743.73

10.22

0.013

x1x3

696.11

1

696.11

9.57

0.015

x2x3

381.92

1

381.92

5.25

0.051

x12

1044.62

1

1044.62

14.36

0.005

x22

7380.93

1

7380.93

101.45

< 0.0001

Residual

582.04

8

72.75

Lack of Fit

566.56

6

94.43

12.20

0.078

Not significant Pure Error

15.48

2

Total

25945.44

16

R-squared

7.74

0.9776

R-squared 0.8481 (Pred) Radj-squared

0.9551



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Table 7. Calculated values of some reactivity indices of inhibitor in gas and liquid phases using various semiempirical and DFT methods. µ Method

Phase

EHOMO (eV)

ELUMO (eV)

Eg (eV)

I

A

χ

η

∆N* (Debye)

GAS

-6.09

-1.77

4.33

6.09

1.77

3.93

2.16

0.71

2.62

SLN

-6.26

-1.93

4.32

6.26

1.94

4.10

2.16

0.67

3.69

GAS

-8.78

-0.92

7.86

8.78

0.92

4.85

3.93

0.27

2.56

SLN

-9.24

-1.27

7.97

9.24

1.27

5.25

3.98

0.22

3.66

GAS

-8.90

-0.95

7.95

8.90

0.95

4.92

3.97

0.26

0.83

SLN

-9.15

-1.23

7.92

9.15

1.23

5.19

3.96

0.23

1.00

GAS

-8.44

-0.81

7.62

8.44

0.81

4.63

3.81

0.31

1.67

SLN

-8.85

-1.05

7.81

8.85

1.05

4.95

3.90

0.26

2.40

B3LYP

PM6

PM3

AM1


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b

a

Scheme 1



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Fig. 1


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Fig. 2



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Fig. 3


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a

b

Fig. 4



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Fig. 5


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Fig. 6



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

a

b

Fig. 7


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b

a

Fig. 8



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

a

b

Fig. 9


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Optimized structure of the inhibitor

Mulliken charges of the inhibitor

HOMO (-6.09 eV)

LUMO (-1.77 eV)

HOMO-1 (-6.43 eV)

LUMO+1 (-0.47 eV)

HOMO-2 (-6.81 eV)

LUMO+2 (-0.41 eV)

Fig. 10



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Table of Contents (TOC) Graphic


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Experimental and Theoretical Investigation of Inhibition Efficiency of 2

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