Research on Chemical Intermediates

, Volume 38, Issue 8, pp 1761–1779

Experimental and theoretical investigations of adsorption characteristics of itraconazole as green corrosion inhibitor at a mild steel/hydrochloric acid interface

Authors

    • Department of ChemistryFaculty of Science, University of Uyo
  • E. E. Ebenso
    • Department of ChemistryNorth West University
  • N. O. Obi-Egbedi
    • Department of ChemistryUniversity of Ibadan
  • Ayo S. Afolabi
    • Department of Civil and Chemical EngineeringUniversity of South Africa
  • Zuhair M. Gasem
    • Department of Mechanical EngineeringKing Fahd University of Petroleum and Minerals
Article

DOI: 10.1007/s11164-012-0501-5

Cite this article as:
Obot, I.B., Ebenso, E.E., Obi-Egbedi, N.O. et al. Res Chem Intermed (2012) 38: 1761. doi:10.1007/s11164-012-0501-5

Abstract

The adsorption and inhibition effect of itraconazole (ICZ) on mild steel in 0.5 M HCl at 303–333 K was studied using gravimetric and quantum chemical methods. The adsorption of ICZ has been tested thermodynamically and was found to be mainly a physical adsorption mechanism and weak chemisorption. The activation and thermodynamic functions (such as Ea, \( \Updelta H^{*} ,\,\Updelta G_{\text{ads}}^{\text{o}} \)) of dissolution and adsorption processes have been evaluated and discussed. The analyses of the results obtained showed that ICZ inhibits the corrosion of mild steel effectively at moderate temperatures and adsorbs according to the Temkin adsorption isotherm. An attempt to correlate the molecular structure to quantum chemical indices was made using a semi-empirical (PM3) method. Results of the theoretical study indicate that nitrogen and oxygen atoms (O10, O13, N17, N19, O21, N28, N42, N43, and O45) were the reactive sites.

Keywords

ItraconazoleMild steelCorrosion inhibitorsPM3Hydrochloric acid

Introduction

The corrosion of steel is of fundamental academic and industrial concern and has received a considerable amount of attention [1]. Carbon steels used in structural applications undergo corrosion in humid and acidic environments. Hydrochloric acid is a strong inorganic acid that is used in many industrial processes. The most important areas of applications are iron oxide/rust removal in processes such as acid pickling, acid cleaning, acid descaling, and oil well acidizing [24]. Most inorganic acids enhance the rate of metal dissolution and are indirectly responsible for the failure of the materials. In these conditions, the corrosion of steel can be minimized by the use of organic compounds as inhibitors.

Most of the efficient inhibitors are organic compounds that contain mostly nitrogen, sulfur, or oxygen atoms in their structures. Furthermore, the presence of functional groups such as –N=N–, –CHO–, =NH, R=R, and R–OH, and aromaticity and electron density at donor atoms are found to influence the adsorption of inhibitor molecules on corroding metal surfaces [57]. Though the existing data show that numerous heterocyclic organic compounds have good anticorrosive activity, some of them are highly toxic to both human beings and the environment [8]. Due to the toxicity of widely used corrosion inhibitors and the ever-tightening environmental regulations surrounding their use and disposal, there is great interest in replacing harmful inhibitors with effective non-hazardous alternatives. Over the past two decades, extensive research have led to the development of new classes of corrosion inhibitors, and the importance of the use of several drugs as corrosion inhibitors has grown [9].

A review including an extensive listing of various classes of drugs as corrosion inhibitors has recently been published [10]. The review includes the use of antifungal drugs as corrosion inhibitors for steel/aluminium in acidic media, which was first reported by our research group [1113]. Some of the antifungal drugs previously studied by our research group include: fluconazole [11], ketoconazole [12], and clotrimazole [13]. These antifungal drugs were chosen because of the presence of an azole group in their molecular structure. The azoles, and especially the triazoles, have attracted much attention because of their efficient inhibition abilities to metal [14, 15]. Against this background, the present drug (itraconazole) containing two triazoles and a lot of aromatic rings as part of its molecular structure has been selected for this study. This drug has all the potential characters of an inhibitor, such as the presence of hetero atoms like nitrogen, oxygen, and the aromatic rings. Moreover, itraconazole (ICZ) is expected to be potentially cationic in acidic solution. It has been reported that cationic organic molecules are very effective in HCl against the corrosion of steel [16].

The effect of molecular structure on the chemical reactivity has been the subject of great interest in several fields of chemistry [17, 18]. The quantum chemistry calculations have been widely used to study the reaction mechanisms and to interpret the experimental results, as well as to solve chemical ambiguities. The molecular structure and the electronic parameters that can be obtained through theoretical calculations, such as HOMO (the highest occupied molecular orbital) energy, and LUMO (the lowest unoccupied molecular orbital) energy, the energy gap (ΔE = ELUMO − EHOMO), and atomic charges on the reactive center, are involved in the activity of inhibitors [19, 20].

In continuation of our quest to develop corrosion inhibitors with high effectiveness and efficiency, the present paper explores the use of itraconazole, an antifungal drug, as a corrosion inhibitor for mild steel surfaces in hydrochloric acid solution using a gravimetric method. The effect of temperature on corrosion and inhibition processes is thoroughly assessed and discussed. The thermodynamic parameters governing the adsorption process were also calculated and are discussed. A quantum chemical study using a PM3 semi-empirical method was further employed in an attempt to correlate the inhibitive effect with the molecular structure of itraconazole.

Experimental details

Weight loss tests were carried out on a freshly prepared sheet of mild steel samples of the following composition (wt%): 0.13% C, 0.18% Si, 0.39% Mn, 0.40% P, 0.04% S, 0.025% Cu, and balance Fe. The specimens used in the weight loss measurements were mechanically cut into 5.0 × 4.0 × 0.8 cm dimensions, then abraded with SiC abrasive papers 320, 400, and 600 grit, respectively, washed in absolute ethanol and acetone, dried at room temperature, and stored in a moisture-free dessicator before their use in the corrosion studies [5]. A solution of 0.5 M HCl was prepared by diluting the analytical grade of HCl with distilled water. Itraconazole was obtained commercially from Amela Pharmaceutical, Akwa Ibom State, Nigeria. A stock solution of ICZ was made in 10:1 water:methanol mixture to ensure its solubility [21]. This stock solution was used for all the experiments carried out in this study. The concentration range of ICZ prepared and used in this study was 0.4–1.0 μM. Figure 1 depicts the name and molecular structure of ICZ.
https://static-content.springer.com/image/art%3A10.1007%2Fs11164-012-0501-5/MediaObjects/11164_2012_501_Fig1_HTML.gif
Fig. 1

Name and molecular structure of itraconazole. (2R,4S)-rel-1-(butan-2-yl)-4-{4-[4-(4-{[(2R,4S)-2-(2,4-dichlorophenyl)-2-(1H-1,2,4-triazol-1-ylmethyl)-1,3-dioxolan-4-yl]methoxy}phenyl)piperazin-1-yl]phenyl}-4,5-dihydro-1H-1,2,4-triazol-5-one

The gravimetric method (weight loss) is probably the most widely used method of inhibition assessment [22]. The simplicity and reliability of the measurement offered by this method means that the technique forms the baseline method of measurement in many corrosion monitoring programmes [23]. Several authors have reported comparable agreement between the weight loss technique and other well-established techniques of corrosion monitoring, such as the polarization technique [24], electrochemical impedance spectroscopy [25], and gasometric [26], thermometric [27], and atomic absorption spectroscopy [28]. Recently, the weight loss method together with potentiodynamic polarization and electrochemical impedance spectroscopy were used to evaluate the corrosion inhibitive effect of cigarette butts on N80 steel at 90 °C in hydrochloric acid solution [29]. The results obtained for the three independent methods were in good agreement. There have been several reports on the use of weight loss measurements in evaluating corrosion inhibition [21, 23, 30]. This method in combination with quantum chemical studies has also been found to be adequate in elucidating the mechanism of inhibition [5, 19, 21, 31].

Thus, weight loss measurements were conducted under total immersion using 250-mL capacity beakers containing 200 mL test solution at 303–333 K maintained in a thermostated water bath. The mild steel coupons were weighed and suspended in the beaker with the help of a rod and hook. The coupons were retrieved every 2 h for 10 h at 303 K and 2 h at 313–333 K, washed thoroughly in 20% NaOH solution containing 200 g/L of zinc dust [30] with a bristle brush, rinsed several times in deionized water, cleaned, dried in acetone, and re-weighed. The weight losses, in grams, were taken as the difference in the weight of the mild steel coupons before and after immersion in different test solutions determined using an LP 120 digital balance with a sensitivity of ±0.1 mg. Then, the tests were repeated at different temperatures. In order to get good reproducibility, experiments were carried out in triplicate. In the present study, the standard deviation values among parallel triplicate experiments were found to be smaller than ±2%, indicating good reproducibility.

The corrosion rate (ρ) in g cm−2 h−1 was calculated from the following equation [12]:
$$ \rho = \frac{\Updelta W}{At} $$
(1)
where W is the average weight loss of three mild steel sheets, A is the total area of one mild steel specimen, and t is the immersion time. With the calculated corrosion rate, the inhibition efficiency (%I) was calculated as follows [31]:
$$ \% I = \left( {\frac{{\rho {}_{1} - \rho_{2} }}{{\rho {}_{1}}}} \right)100 $$
(2)
where \( \rho_{1} \) and \( \rho_{2} \) are the corrosion rates of the mild steel coupons in the absence and presence of the inhibitor, respectively.

Results and discussion

Weight loss, corrosion rates and inhibition efficiency

The anodic dissolution of iron in acidic media and the corresponding cathodic reaction has been reported to proceed as follows [32]:
$$ {\text{Fe}} \to {\text{Fe}}^{ 2+ } + {\text{ 2e}}^{ - } $$
(3)
$$ 2 {\text{H}}^{ + } + {\text{ 2e}}^{ - } \to 2 {\text{H}}_{\text{ads}} \to {\text{H}}_{ 2} $$
(4)
As a result of these reactions, including the high solubility of the corrosion products, the metal loses weight in the solution. Figure 2 shows the plot of weight loss versus time for mild steel in 0.5 M HCl without and with different concentrations of ICZ at 303 K. From the plot, it is clear that the weight loss of mild steel in the different test solutions increases linearly with time. It is also observed from the plots that weight loss of mild steel decreases on introduction of ICZ into the corrodent, indicating that ICZ functioned as an inhibitor isolating the metal from attack by the anions present in the solution.
https://static-content.springer.com/image/art%3A10.1007%2Fs11164-012-0501-5/MediaObjects/11164_2012_501_Fig2_HTML.gif
Fig. 2

Variation of weight loss against time for mild steel corrosion in 0.5 M HCl in the presence of different concentrations of ICZ at 303 K

The various parameters derived from weight loss measurements regarding the corrosion of mild steel in 0.5 M HCl solution without and with various concentrations of ICZ are summarized in Table 1. A comparison of the corrosion rate of mild steel in 0.5 M HCl in the absence and presence of different concentrations of ICZ reveals that the corrosion rate was reduced in the presence of different concentrations of ICZ, with the lowest value obtained at the highest concentration (1.0 μM) of ICZ used at all the temperatures studied. A closer look at the table reveals that the corrosion rate of mild steel in the presence of ICZ increases with an increase in temperature.
Table 1

Calculated values of corrosion rate and inhibition efficiency for corrosion of mild steel in 0.5 M HCl in the absence and presence of ICZ at 303–333 K

System/concentration

Corrosion rate (mg cm−2 h−1)

Inhibition efficiency (%I)

303 K

313 K

323 K

333 K

303 K

313 K

323 K

333 K

Blank

5.23

10.98

17.31

24.33

0.4 µM

2.97

7.13

12.74

20.66

43.1

35.1

26.4

15.1

0.6 µM

1.79

6.02

10.51

17.03

65.6

45.2

39.3

30.0

0.8 µM

1.32

5.13

9.97

16.28

74.7

53.3

42.4

33.1

1.0 µM

1.04

4.29

8.27

14.27

80.0

60.9

52.2

41.3

The results presented in Table 1 show that the corrosion rate decreases in the presence of ICZ which corresponds to an increase in inhibition efficiency. It can also be seen from the table that the inhibition efficiency increases with an increase in ICZ concentration. This could be attributed to the adsorption of ICZ onto the mild steel surface leading to a corrosion inhibition phenomenon. This phenomenon can be explained by the fact that the corrosion inhibition was initiated by the displacement of adsorbed water molecules by the inhibitor species, which led to specific adsorption of the inhibitor on the metal surface [26]. The inhibition efficiency was also found to decrease with an increase in temperature.

Effect of immersion time

In order to assess the stability of inhibitive behavior of ICZ on a time scale, weight loss measurements were performed in 0.5 M HCl in the absence and presence of the ICZ at different concentrations for 2–10 h immersion time at 303 K. The corrosion rates and inhibition efficiencies were plotted against the immersion period as shown in Figs. 3 and 4, respectively. Figure 3 shows that the corrosion rate decreased with an increase in immersion time at all the concentrations studied, while Fig. 4 shows that the inhibition efficiency of ICZ remains fairly constant with immersion time from 2 to 10 h, due to the strong adsorption of ICZ on the mild steel surface. This resulted in a more protective layer being formed at the mild steel/hydrochloric acid solution interface. Thus, ICZ effectively inhibits the mild steel corrosion in a hydrochloric acid solution, which is similar to an already reported findings [33].
https://static-content.springer.com/image/art%3A10.1007%2Fs11164-012-0501-5/MediaObjects/11164_2012_501_Fig3_HTML.gif
Fig. 3

The relationship between corrosion rate and concentration at different immersion time at 303 K

https://static-content.springer.com/image/art%3A10.1007%2Fs11164-012-0501-5/MediaObjects/11164_2012_501_Fig4_HTML.gif
Fig. 4

Variation of inhibition efficiency of ICZ with time at 303 K

Effect of temperature

To evaluate the stability of adsorbed layer/film of inhibitor on mild steel surface as well as activation parameters of the corrosion process of steel in acidic media, weight loss measurements were carried out in the range of temperature 303–333 K, in the absence and presence of different concentrations of ICZ during 2 h immersion. The results obtained are shown in Table 1. It is evident from this table that inhibition efficiency decreases with increasing temperature. This is due to an increased rate of the dissolution process of mild steel and partial desorption of the inhibitor from the metal surface with temperature [34]. The relationship between the corrosion rate (ρ) of mild steel in acidic media and temperature (T) is often expressed by the Arrhenius equation [25]:
$$ \log \rho = \log A - \frac{{E_{\text{a}} }}{2.303RT} $$
(5)
where ρ 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 absolute temperature, and A is the frequency factor. The plot of log ρ against 1/T for mild steel corrosion in 0.5 M HCl in the absence and presence of different concentrations of ICZ is presented in Fig. 5. All parameters are shown in Table 2.
https://static-content.springer.com/image/art%3A10.1007%2Fs11164-012-0501-5/MediaObjects/11164_2012_501_Fig5_HTML.gif
Fig. 5

Arrhenius plot for mild steel corrosion in 0.5 M HCl in the absence and presence of different concentrations of itraconazole (ICZ)

Table 2

Activation parameters of the dissolution of mild steel in 0.5 M HCl in the absence and presence of different concentrations of ICZ

Concentration

A (g cm−2 h−1)

Ea (kJ mol−1)

ΔH* (kJ mol−1)

ΔS* (J mol−1 K−1)

Blank

1.15 × 105

42.38

41.12

−156.43

0.4 µM

4.89 × 106

53.25

50.75

−125.25

0.6 µM

8.65 × 107

61.54

58.32

−103.65

0.8 µM

1.06 × 109

68.54

66.07

−80.49

1.0 µM

2.61 × 109

71.43

68.93

−73.03

The activation energy increased in the presence of ICZ, which indicated physical adsorption (electrostatic) in the first stage [1]. Nevertheless, the adsorption of an organic molecule is not only a physical or chemical adsorption phenomenon. A wide spectrum of conditions ranging from the dominance of chemisorption or electrostatic effects can be seen in other adsorption experimental data [35]. The activation energy rose with increasing inhibitor concentration, suggesting strong adsorption of inhibitor molecules at the metal surface. The increase in the activation energy was due to the corrosion reaction mechanism in which charge transfer was blocked by the adsorption of ICZ molecules on the mild steel surface [36]. It also revealed that the whole process was controlled by the surface reaction since the energy of the activation corrosion process in both the absence and presence of ICZ was greater than 20 kJ mol−1 [37].

The experimental corrosion rate values obtained from weight loss measurements for mild steel in 0.5 M HCl in the absence and presence of ICZ was used to further gain insight into the change of enthalpy (\( \Updelta H^{*} \)) and entropy (\( \Updelta S^{*} \)) of activation for the formation of the activation complex in the transition state using the transition equation [12]:
$$ \rho = \left( \frac{RT}{Nh} \right)\exp \left( {\frac{{\Updelta S^{*} }}{R}} \right)\exp \left( {\frac{{ - \Updelta H^{*} }}{RT}} \right) $$
(6)
where ρ is the corrosion rate, h is the Plank’s constant (6.626176 × 10−34 Js), N is the Avogadro’s number (6.02252 × 1023 mol−1), R is the universal gas constant, and T is the absolute temperature. Figure 6 shows the plot of log ρ/T versus 1/T for mild steel corrosion in 0.5 M HCl in the absence and presence of different concentrations of ICZ. Straight lines were obtained with slope of \( (\Updelta H^{*} / 2. 30 3R) \) and an intercept of \( [\log \left( {R/Nh} \right) \, + \, (\Updelta S^{*} / \, 2.303R)] \) from which the values of \( \Updelta H^{*} \) and \( \Updelta S^{*} \), respectively, were computed and are also listed in Table 2.
https://static-content.springer.com/image/art%3A10.1007%2Fs11164-012-0501-5/MediaObjects/11164_2012_501_Fig6_HTML.gif
Fig. 6

Transition state plot for mild steel corrosion in 0.5 M HCl in the absence and presence of different concentrations of itraconazole (ICZ)

The positive values of \( \Updelta H^{*} \) both in the absence and presence of ICZ reflect the endothermic nature of steel dissolution process [35]. The results in Table 3 further indicate that the activation enthalpies increased with an increase in the concentration of ICZ, which varies in the same manner as the activation energies, hence supporting the proposed inhibition mechanism.
Table 3

Some parameters from the Temkin isotherm model for mild steel in 0.5 M HCl

Temperature (K)

f

Kads (M−1)

(R2)

ΔG°ads (kJ mol−1)

303

2.48

77,260

0.966

−38.47

313

3.60

87,030

0.994

−40.05

323

3.80

89,420

0.963

−41.40

333

3.69

45,470

0.964

40.81

The entropy of activation, \( \Updelta S^{*} , \) was negative both in the absence and presence of inhibitor, implying that the activated complex represented the rate-determining step with respect to the association rather than the dissociation step. This implies that a decrease in disorder occurred when proceeding from the reactants to the activated complex [38]. In addition, the less negative values of \( \Updelta S^{*} \) in the presence of inhibitor imply that the presence of inhibitor created a near-equilibrium corrosion system state [37].

Adsorption isotherm and thermodynamic studies

A direct relationship between inhibition efficiency (%I) and the degree of surface coverage (θ) (%I = 100 × θ) can be assumed for the different concentrations of the inhibitor. The degree of surface coverage (θ) for the different concentrations of ICZ has been evaluated from the weight loss measurements in 2 M HCl at 303–333 K for a period of 2 h of immersion. The data were tested graphically by fitting to various adsorption isotherms including the Freundlich, Temkin, Flory–Huggins, Bockris–Swinkles, Langmuir, and Frumkin isotherms. The correlation coefficient (R2) was used to determine the best fit isotherm which was obtained for Temkin. According to this isotherm, θ is related to the inhibitor concentration by the following equation [35]:
$$ \exp ( - 2\alpha \theta ) = K{}_{\text{ads}}C $$
(7)
where the molecular interaction parameter α can have both positive and negative values. The positive values of α indicate attraction forces between the adsorbed molecules, while negative values indicate repulsive forces between the adsorbed molecules [39]. Equation 7 can be rearranged to obtain Eq. 8.
$$ \theta = [1/( - 2\alpha )]\ln (K_{\text{ads}} C) $$
(8)
If the parameter f is defined as:
$$ f = - 2\alpha $$
(9)
where f is the heterogeneous factor of the metal surface describing the molecular interactions in the adsorption layer and the heterogeneity of the metal surface. Equation 9 shows that the sign between f and α is reversed, that is, if α < 0, then f > 0; if α > 0, then f < 0. Accordingly, if f > 0, mutual repulsion of molecules occurs and f < 0 attraction takes place. If Eq. 9 is substituted into Eq. 8, then the Temkin isotherm equation has the following form:
$$ \theta = (1/f)\ln (K_{\text{ads}} C) $$
(10)
Eq. 10 can be transformed into:
$$ \theta = (1/f)\ln K_{\text{ads}} + (1/f)\ln C $$
(11)
Eq. 11 is a different form of the Temkin isotherm. The plot of θ versus ln C gives a straight line graph with a slope of (1/f) and an intercept of [(1/f)ln Kads]. It is shown that f can be calculated from the slope, with the calculated f, the value of Kads can be obtained from the intercept. All the parameters are listed in Table 3. Figure 7 shows the relationship between θ and ln C at different temperatures. These results show that all the linear correlation coefficients (R2) are close to 1, which indicates that the adsorption of ICZ onto the steel surface obeys the Temkin adsorption isotherm. Furthermore, it can be deduced that there is a repulsion force in the adsorption layer due to f > 0. It has been reported that the value of Kads > 100 M−1 could be attributed to a stronger and more adsorbed layer formation on the metal surface [35]. Therefore, the large values of Kads obtained in this work (Table 3) mean better inhibition efficiency of the inhibitor, i.e., strong electrical interaction between the double-layer existing at the phase boundary and the adsorbing inhibitor molecules [35].
https://static-content.springer.com/image/art%3A10.1007%2Fs11164-012-0501-5/MediaObjects/11164_2012_501_Fig7_HTML.gif
Fig. 7

The relationship between θ and ln C at different temperatures

Kads is related to the free energy of adsorption \( \Updelta G_{\text{ads}}^{\text{o}} \) by the equation [40]:
$$ \log K_{\text{ads}} = - \log C_{{{\text{H}}_{2} {\text{O}}}} - \frac{{\Updelta G_{\text{ads}}^{\text{o}} }}{2.303RT} $$
(12)
where \( {\text{C}}_{{{\text{H}}_{ 2} {\text{O}}}} \) is the concentration of water expressed in mol/L (the same as that of the inhibitor concentration), R is the molar gas constant (J mol−1 K−1), and T is the absolute temperature (K). The values of \( \Updelta G_{\text{ads}}^{\text{o}} \) are also listed in Table 3. The negative values of \( \Updelta G_{\text{ads}}^{\text{o}} \) suggest that the adsorption of ICZ is a spontaneous process. Generally speaking, the values of \( \Updelta G_{\text{ads}}^{\text{o}} \) up to −20 kJ mol−1 indicate the electrostatic attraction between the charged metal surface and charged organic molecules in the bulk of the solution (physisorption). Those around −40 kJ mol−1 or smaller involve charge sharing or charge transfer between the metal and the organic molecules (chemisorption) [31]. In this case, the calculated values of \( \Updelta G_{\text{ads}}^{\text{o}} \) for ICZ ranged from −38.47 to −40.81 kJ mol−1. Previous findings have shown that the values of \( \Updelta G_{\text{ads}}^{\text{o}} \) indicate that adsorption of ICZ on the mild steel surface may involve complex interactions: predominantly physisorption and weak chemisorptions [41]. An inference can also be confirmed from analyzing the molecular structure of ICZ. Generally speaking, when a mild steel sample was immersed in the HCl solution, the chloride ions were adsorbed to the metal surface, forming interconnecting bridges between the metal atoms and the protonated molecule of ICZ. Therefore, the inhibitor molecules were first adsorbed on the metal surface by electrostatic attractions (physisorption). Afterwards, the relative strong coordinate covalent bonds can be formed by transference of electrons from active sites of the pre-adsorbed ICZ molecule, such as lone sp2 electron pairs present on N and O atoms, π-electrons of aromatic rings, and the double bonds to the vacant d orbitals of iron surface atoms (chemisorption).
According to statistical physics, the change of free energy of adsorption \( \Updelta G_{\text{ads}}^{\text{o}} \)can be calculated from Eq. 13 as follows [42]:
$$ \ln \left( {\frac{1 - \eta }{\eta }} \right) = \frac{{\Updelta G_{\text{ads}}^{\text{o}} }}{\theta } - \frac{RT\ln C}{\theta } $$
(13)
where C is the concentration of inhibitor particles, θ is the distribution modulus, η is surface coverage, R is the molar gas constant, and T is the absolute temperature. The curve fitting of data in Table 1 to the statistical model at 303–333 K is presented in Fig. 8. It can be seen that a good correlation coefficient (R2 > 0.96) was obtained, while θ and \( \Updelta G_{\text{ads}}^{\text{o}} \) can be calculated from the slope and intercept of Eq. 13. All the calculated parameters are given in Table 4. The values of \( \Updelta G_{\text{ads}}^{\text{o}} \) ranged from −29.79 to −40.80 kJ mol−1, which shows the validity of this approach. The results of \( \Updelta G_{\text{ads}}^{\text{o}} \) obtained from Temkin adsorption isotherm and the statistical physics approach are also in good agreement.
https://static-content.springer.com/image/art%3A10.1007%2Fs11164-012-0501-5/MediaObjects/11164_2012_501_Fig8_HTML.gif
Fig. 8

Application of the statistical model to the corrosion protection behavior of itraconazole

Table 4

Some parameters from statistical model for mild steel in 0.5 M HCl

Temperature (K)

R2

θ

ΔG°ads (kJ mol−1)

303

0.999

3.27 × 103

−29.79

313

0.970

7.97 × 103

−36.04

323

0.960

10.65 × 103

−39.52

333

0.973

12.07 × 103

−40.80

Thermodynamically, \( \Updelta G_{\text{ads}}^{\text{o}} \) is related to the enthalpy and entropy of the adsorption process, \( \Updelta H_{\text{ads}}^{\text{o}} \) and \( \Updelta S_{\text{ads}}^{\text{o}} , \) via Eq. 14:
$$ \Updelta G_{\text{ads}}^{\text{o}} = \Updelta H_{\text{ads}}^{\text{o}} - T\Updelta S_{\text{ads}}^{\text{o}} $$
(14)
The plot of A plot of \( \Updelta G_{\text{ads}}^{\text{o}} \) versus T was linear (Fig. 9), with the slope equal to −\( \Updelta S_{\text{ads}}^{\text{o}} \) and intercept of \( \Updelta H_{\text{ads}}^{\text{o}} . \) The enthalpy of adsorption \( \Updelta H_{\text{ads}}^{\text{o}} , \) and the entropy of adsorption \( \Updelta S_{\text{ads}}^{\text{o}} , \) obtained are −13.56 kJ mol−1 and 83.7 J mol−1 K−1, respectively. The negative sign of \( \Updelta H_{\text{ads}}^{\text{o}} \) indicates that the adsorption of ICZ molecules is an exothermic process. In an exothermic process, physisorption is distinguished from chemisorption by considering the absolute value of \( \Updelta H_{\text{ads}}^{\text{o}} . \) For the physisorption process, the enthalpy of adsorption is lower than 40 kJ mol−1, while that for chemisorption approaches 100 kJ mol−1 [43]. In this study, \( \Updelta H_{\text{ads}}^{\text{o}} \) is lower than 40 kJ mol−1, which indicates that physical adsorption is the major mode of inhibition mechanism. Since \( \Updelta H_{\text{ads}}^{\text{o}} \) is negative, it should have been accompanied by a decrease in entropy, i.e. \( \Updelta S_{\text{ads}}^{\text{o}} < \, 0. \) However, the result obtained in this study seems to contrast with that normally accepted in the adsorption phenomena. In aqueous solution, the adsorption of organic molecules is generally accompanied by the desorption of water molecules. The adsorption of an organic adsorbate at the metal/solution interface is considered a “substitutional adsorption” phenomenon [44]. Therefore, the positive value of \( \Updelta S_{\text{ads}}^{\text{o}} \) related to “substitutional adsorption” can be attributed to the increase in the solvent entropy. A similar observation has been reported [45]
https://static-content.springer.com/image/art%3A10.1007%2Fs11164-012-0501-5/MediaObjects/11164_2012_501_Fig9_HTML.gif
Fig. 9

The relationship between \( \Updelta G_{\text{ads}}^{\text{o}} \) and T

The Langmuir adsorption isotherm, however, can be expressed by Eq. 15 [46]:
$$ \log \left( {\frac{\theta }{1 - \theta }} \right) = \log A + \log C - \frac{{Q_{\text{ads}} }}{2.303RT} $$
(15)
where A is a constant, and Qads is the heat of adsorption, equal to the enthalpy of adsorption \( \left( {\Updelta H_{\text{ads}}^{\text{o}} } \right) \) as a good approximation at constant pressure. If log [θ/(1 − θ)] is plotted against 1,000/T at various ICZ concentrations (Fig. 10), the slopes of the linear part of these curves give \( \Updelta H_{\text{ads}}^{\text{o}} / 2. 30 3R. \) The average value of \( \Updelta H_{\text{ads}}^{\text{o}} \) calculated gives −14.2 kJ mol−1. The values of \( \Updelta H_{\text{ads}}^{\text{o}} \) obtained by the two approaches are in good agreement.
https://static-content.springer.com/image/art%3A10.1007%2Fs11164-012-0501-5/MediaObjects/11164_2012_501_Fig10_HTML.gif
Fig. 10

Plot of Langmuir adsorption isotherm for ICZ

Quantum chemical studies

In the past few decades, quantum chemistry has become an effective way to study the correlation of the molecular structure and inhibition performance. Theoretical investigations based on quantum chemical calculations have been proposed as a powerful tool for predicting a number of molecular parameters directly related to the corrosion inhibiting property of any chemical compound [47, 48]. The major driving force of quantum chemical research is to understand and explain the inhibitory effects of these compounds in molecular terms. Among quantum chemical methods for evaluation of corrosion inhibitors, semi-empirical PM3 method has shown significant promise and appears to be adequate for pointing out the changes in electronic structures responsible for inhibitory action [49, 50].

The optimized molecular structure and the Mulliken charge distribution of itraconazole molecule, using a semi-empirical PM3 method implemented using Gaussian 03 [51], are shown in Fig. 11 and Table 5, respectively. The charges on heteroatoms only are given for simplicity. It has been reported that the more negative the atomic charges of the adsorbed centre, the easier the atom donates its electron to the unoccupied orbital of the metal [52]. The calculated quantum chemical indices EHOMO, ELUMO, ΔE, and dipole moment (D) are given in Table 6. Table 5 shows that, in itraconazole molecule, O10, O13, N17, N19, O21, N28, N42, N43, and O45 have more negative charges, while chlorine atoms Cl7 and Cl8, N15, N31, and N40 have more positive charges. This means that O10, O13, N17, N19, O21, N28, N42, N43, and O45 are the negative charge centers, which can offer electrons to the Fe atoms to form a coordinate bond, and Cl7, Cl8, N15, N31, and N40 are the positive charge centers, which can accept electrons from the orbital of Fe atoms to form a feedback bond. The optimized structure is in accordance with the fact that excellent corrosion inhibitors can not only offer electrons to unoccupied orbital of the metal but also accept free electrons from the metal. Therefore, it can be inferred that, in itraconazole, nitrogen, oxygen, and chlorine atoms are the possible active adsorption sites.
https://static-content.springer.com/image/art%3A10.1007%2Fs11164-012-0501-5/MediaObjects/11164_2012_501_Fig11_HTML.gif
Fig. 11

The optimized structure of itraconazole

Table 5

Mulliken atomic charges of itraconazole with PM3 method

Atom

Charges

Cl7

0.082071

Cl8

0.087121

O10

−0.266888

O13

−0.278067

N15

0.232964

N17

−0.195781

N19

−0.258736

O21

−0.193690

N28

−0.001778

N31

0.000084

N40

0.072041

N42

−0.155058

N43

−0.009388

O45

−0.367530

Table 6

Quantum chemical indices for itraconazole using PM3 semiempirical method

Calculated parameters for itraconazole

Value

EHOMO (eV)

−0.31156

ELUMO (eV)

−0.02556

ΔE (ELUMO − EHOMO) (eV)

0.286

Dipole moment (D)

2.515

ΔN

23.00

η

0.143

ΔET

−0.0357

According to the description of frontier orbital theory, HOMO is often associated with the electron-donating ability of an inhibitor molecule. High EHOMO values indicate that the molecule has a tendency to donate electrons to the metal with unoccupied molecule orbitals. ELUMO indicates the ability of the molecules to accept electrons [1]. The lower value of ELUMO is the easier acceptance of electrons from the metal surface [17]. The gap between the LUMO and HOMO energy levels of the inhibitor molecules is another important index; the low absolute values of the energy band gap (ΔE = ELUMO − EHOMO) means good inhibition efficiency [18]. The data listed in Table 6 verified that itraconazole has a high value of EHOMO and a low value of ELUMO with a low energy band gap. The negative sign of the EHOMO value obtained and other thermodynamic parameters indicate that the data obtained support the physical adsorption mechanism [1].

The fraction of electrons transferred from the inhibitor molecule to the metallic atom (ΔN) was also calculated in this study [17]. The idea behind this is that, in the reaction of two systems with different electronegativities (as a metallic surface and an inhibitor molecule), the following mechanism will take place: the electron flow will happen from the molecule with the low electronegativity towards that of a higher value, until the chemical potentials reach the same level. In order to calculate the fraction of electrons transferred, a theoretical value for the electronegativity of bulk iron was used \( \chi_{\text{Fe}} = 7 {\text{e}}\;{\text{V}}, \) and a global hardness of \( \eta_{\text{Fe}} = \, 0, \) by assuming that, for a metallic bulk, I = A because they are softer than the neutral metallic atoms. For the calculation, the following formula was used [1].
$$ \Updelta N = \frac{{\chi_{\text{Fe}} - \chi_{\text{inb}} }}{{2(\eta_{\text{Fe}} + \eta_{\text{inh}} )}} $$
(16)
These quantities are related to electron affinity (A) and ionization potential (I) which are useful in their ability to help predict chemical behavior [12].
$$ \chi = \frac{I + A}{2} $$
(17)
$$ \eta = \frac{I - A}{2} $$
(18)
I and A are related in turn to EHOMO and ELUMO as follows:
$$ I = - E_{\text{HOMO}} $$
(19)
$$ A = - E_{\text{LUMO}} $$
(20)
The fraction of electrons transferred from inhibitor to the mild steel surface ΔN was calculated and is listed in Table 6. According to Lukovits [53], if ΔN < 3.6, the inhibition efficiency increased with increasing electron-donating ability at the metal surface. In this study, the value of ΔN for ICZ was greater than 3.6, which shows that the increase in inhibition efficiency was not due solely to the electron-donating ability of ICZ.
Thus, the simple charge transfer model for donation and back-donation of charges, proposed recently by Gomez et al., [54], can be applied to the present study. According to this model, when the molecule receives a certain amount of charge, \( \Updelta N^{ + } ; \)
$$ \Updelta E^{ + } = \mu^{ + } \Updelta N^{ + } + \frac{1}{2}\eta (\Updelta N^{ + } )^{2} $$
(21)
while when the molecule back-donates a certain amount of charge, \( \Updelta N^{ - } , \) then:
$$ \Updelta E^{ - } = \mu^{ - } \Updelta N^{ - } + \frac{1}{2}\eta (\Updelta N^{ - } )^{2} $$
(22)
If the total energy change is approximated by the sum of the contributions of Eqs. 21 and 22, and assuming that the amount of charge back-donation is equal to the amount of charge received, \( \Updelta N^{ - } = - \Updelta N^{ + } , \) then;
$$ \Updelta E_{T} = \Updelta E^{ + } + \Updelta E^{ - } = (\mu^{ + } - \mu^{ - } )\Updelta N^{ + } + \eta (\Updelta N^{ + } )^{2} $$
(23)
The most favorable situation corresponds to the case when the total energy change becomes a minimum with respect to \( \Updelta N^{ + } , \) which implies that \( \Updelta N^{ + } = - (\mu^{ + } - \mu^{ - } )/ 2\eta \) and that;
$$ \Updelta E_{\text{T}} = - (\mu^{ + } - \mu^{ - } )^{2} /4\eta = - \eta /4 $$
(24)
The calculations from Table 6 indicate that η > 0 (η = hardness) and ΔET < 0. This result implies that the charge transfer to a molecule followed by back-donation from the molecule is energetically favorable and is responsible for the high inhibitive effect of ICZ. This is similar to the observation reported by Obot et al. [1] and Obi-Egbedi et al. [17].

Conclusions

  1. 1.

    The results obtained from the experimental data show that itraconazole acted as an effective inhibitor for the corrosion of mild steel in 0.5 M HCl.

     
  2. 2.

    The inhibition efficiency increased with an increase in the concentration of the inhibitor studied, but decreased with an increase in temperature.

     
  3. 3.

    The adsorption of ICZ on the mild steel surface from 0.5 M HCl obeyed the Tenkin adsorption isotherm and statistical physics model.

     
  4. 4.

    The mechanism of inhibition action of ICN on the mild steel surface was mainly physical adsorption from the values of Ea, \( \Updelta H_{\text{ads}}^{\text{o}} \) obtained, although weak chemisorption seemed to play a part judging from the values of \( \Updelta G_{\text{ads}}^{\text{o}} . \)

     
  5. 5.

    The adsorption of ICZ onto the mild steel surface was a spontaneous process.

     
  6. 6.

    The quantum chemical calculations showed that, apart from ICZ molecules adsorbing as cationic species on the mild steel surface, it could also be adsorbed as molecular species using oxygen, nitrogen, and the p electrons of the aromatic ring as its active centers.

     
  7. 7.

    The relationship between the inhibition efficiency of ICZ on mild steel in 0.5 M HCl and the EHOMO, ELUMO, ELUMO − EHOMO gap, ΔN, and ET of ICZ were calculated by a PM3 semi-empirical method.

     

Acknowledgements

The authors are grateful to Dr. S.A. Umoren who is the head of the Department of Chemistry, University of Uyo, Nigeria, for providing the facilities including the Gaussian 03 software for the quantum chemical calculations.

Copyright information

© Springer Science+Business Media B.V. 2012