Experimental and Theoretical Investigations on Copper Corrosion Inhibition by Cefixime Drug in 1M HNO3 Solution

Cefixime, a third-generation semi-synthetic cephalosporin antibiotic was used as a copper corrosion inhibitor in 1M HNO3 solution. The study was conducted through the weight loss technique at 298 318 K and theoretical studies based on quantum chemistry. The studied drug inhibited the corrosion of copper in 1M HNO3 over the cefixime concentration range (0.02 2 mM). The inhibition efficiency increased with an increase in the inhibitor concentration to reach 91.07% at 2 mM, but decreased with an increase in temperature. The thermodynamic functions related to the adsorption of cefixime on the copper surface and that of the metal dissolution were computed and analyzed. The results point out spontaneous adsorption, mainly through a physisorption mechanism following Langmuir adsorption isotherm model and an endothermic dissolution process. Quantum chemical calculations were also performed at B3LYP level with 6-31G (d, p) basis set and lead to molecular descriptors such as EHOMO (energy of the highest occupied molecular orbital), ELUMO (energy of the lowest unoccupied molecular orbital), ΔE (energy gap) and μ (dipole moment). The global reactivity descriptors such as χ (electronegativity), χ (global hardness), S (global softness), and ω (electrophilicity index) were derived using Koopman’s theorem and analyzed. The local reactivity parameters, including Fukui functions and dual descriptors were determined and discussed. Experimental and theoretical results were found to be in good agreement.


Introduction
Copper [1] [2] is one of the most used metals for several engineering and industrial applications including electricity and electronics, communications, pipelines for domestic and industrial water utilities, fabrication of heat exchanger tubes, and cooling water systems due to its excellent properties. Thus, copper and its alloys are deployed in various environments containing acids, alkalis, and salt solutions where they undergo corrosive attacks although the copper is relatively a noble metal [3]. Some ways of corrosion prevention are the use of more resistant alloys, protective coating, and the addition of inhibitors to the corrosive environment [4] [5]. Among numerous ways to protect the copper from acid corrosion, the most effective and practical method [6] [7] to slow down corrosion processes, is the addition of organic corrosion inhibitors to the metal's environment. It is reported [8] [9] that the adsorption of these compounds is influenced by the electronic structure of inhibiting molecules, the steric factor, aromaticity, electron density at the donor site, the functional group, and also the polarizability of the group. Previous studies revealed that heterocyclic compounds [10] are employed as corrosion inhibitors because of the presence of various adsorption centers (O, N, S, P, and π electrons) which can help to form complexes with metal ions. Unfortunately, the use of some organic chemical inhibitors is limited by diverse reasons especially their costly synthesis, poor biodegradability, toxic and hazardous for human beings and the environment as well. For this purpose, natural products of plant origin or drugs are a better choice due to the fact that they are environmentally benign and contain incredibly rich sources of naturally synthesized organic compounds among which most of them are known to have inhibitive action. Plant extracts are often insoluble in aqueous media and extraction efficiencies are mainly low. The choice of some drugs used as corrosion inhibitors is based on the following justifications: 1) drugs are mainly soluble in aqueous media; 2) drug molecules contain oxygen, nitrogen and sulphur as active centers; 3) drugs are reportedly environmentally friendly and important in biological reactions; and 4) drugs including β-lactam antibiotics, quinolones, antifungal, and cephalosporins can be easily produced and purified [11] [12] [13] [14]. The aim of the present paper is to highlight the relationship between the calculated quantum chemical parameters and the experimentally determined inhibition efficiency of cefixime drug (Scheme 1) against copper corrosion in 1M HNO 3 medium. This can be achieved by computing the most relevant electronic properties of the studied compound including E HOMO , E LUMO , energy Gap (ΔE), dipole moment (μ), electronegativity (χ), global hardness (η), fraction of electrons transferred (ΔN) and charges (δ) on atoms.

Copper Samples
The copper specimens were in form of rod measuring 10 mm in length and 2.2 mm of diameter. They were cut in commercial copper of purity 95%.

Solutions
Cefixime of analytical grade was acquired from Sigma-Aldrich. Analytical grade 65% nitric acid solution from Sigma-Aldrich Chemicals was used to prepare the corrosive aqueous solution. The solution was prepared by dilution of the commercial nitric acid solution using double distilled water. The blank was a 1M HNO 3 solution. Solutions of cefixime with concentrations in the range of 0.02 mM to 2 mM were prepared. Acetone of purity 99.5% was also purchased from Sigma-Aldrich Chemicals.

Weight Loss Technique
Prior to all measurements, the copper samples were mechanically abraded with different grade emery papers (1/0, 2/0, 3/0, 4/0, 5/0, and 6/0). The specimens were washed thoroughly with double distilled water, degreased and rinsed with acetone and dried in a moisture-free desiccator. Weight loss measurements were carried out in a beaker of 100 mL capacity containing 50 mL of the test solution.
The immersion time for weight loss was 1 h at a given temperature. In order to get good reproducible data, parallel triplicate experiments were performed accurately and the mean value of the weight loss was used to assess the corrosion rate (CR), the degree of surface coverage (θ) and the inhibition efficiency (IE) using Equations (1)-(3) respectively: where CR 0 and CR (expressed in mg·h −1 ·cm −2 ) are respectively the corrosion rate without and with cefixime, ∆m is the weight loss, S is the total surface of the copper specimen and t is the immersion time.

Quantum Chemical Approach
The present quantum chemistry calculations were performed with Gaussian 09  [15]. For this purpose, we used Becke's three parameter exchange functional along with the Lee-Yang-Parr non-local correlation functional (B3LYP) [16] using 6-31G (d, p) basis Set. Figure 1 shows the optimized chemical structure of cefixime.
In order to explore the theoretical-experimental consistency, quantum chemical calculations were performed and the electronic properties resulting from this study were analyzed through global reactivity parameters and local reactivity parameters.

Global Reactivity Parameters
Recently, DFT has become a very powerful technique to probe the inhibitor/ surface interaction and to analyze experimental data by providing insights into the chemical reactivity and selectivity [17] [18]. So, the molecular descriptors namely the highest occupied molecular orbital energy (E HOMO ), the lowest unoccupied molecular orbital energy (E LUMO ), the energy gap (ΔΕ = Ε LUMO − Ε HOMO ) and the dipole moment (μ) were calculated. The reactivity descriptors, including ionization energy (I), electron affinity (A), electronegativity (χ), hardness (η), softness (S), the fraction (ΔN) of electrons transferred and the electrophilicity index (ω) were also calculated. According to Koopman's theorem [19], the ionization energy (I) can be approximated as the negative of the highest occupied molecular orbital (HOMO) energy (Equation (4)): The negative of the lowest unoccupied molecular orbital (LUMO) energy is related to the electron affinity A and is obtained from Equation (5): The electronegativity [20] is obtained using the ionization energy I and the electron affinity A by Equation (6): The hardness which is the reciprocal of the electronegativity was obtained by Equation (7): when the organic molecule is in contact with the metal, electrons flow from the system with lower electronegativity to that of higher electronegativity until the chemical potential becomes equal. The fraction of electrons transferred, ΔN, was estimated according to Equation (8) [21]: The values of experimental work function Cu 4.98 eV Φ = [22] and hardness Cu 0 η = [23] (since for bulk metallic atoms I = A) were considered to calculate ΔN.
The global electrophilicity index, introduced by Parr [24] is given by Equation

Local Reactivity Parameters
The local selectivity of a corrosion inhibitor [25] is generally assessed using Fukui functions which enable us to distinguish each part of the studied compound on the basis of its chemical behavior due to different substituent functional groups. The Fukui function is defined as the derivative of the electronic density ρ(r) with respect to the number N of electrons: The condensed Fukui functions provide information about atoms in a molecule that have a tendency to either donate (nucleophilic character) or accept (electrophilic character) an electron or a pair of electrons [26]. The nucleophilic and electrophilic Fukui function for an atom k [27] can be computed using a finite difference approximation as seen in Equations (11)-(12) respectively: where ( ) Recently, it has been reported [28] that a new descriptor has been introduced [29] [30] which allows the determination of individual sites within the molecule with particular behaviors. A mathematical analysis reveals that dual descriptor is a more accurate tool than nucleophilic and electrophilic Fukui functions [31].
This descriptor is defined through Equation (13): The condensed form [29] of the dual descriptor is given by Equation (14):   The inhibition efficiency reaches a value of 91.07% for the concentration of 2 mM at 298 K ( Figure 3). The inhibitive action of the studied drug may be due mainly to the presence of heteroatoms such as oxygen, nitrogen, sulfur and aromatic rings with π-bonds in cefixime structure and its ability with these heteroatoms to create a protective film on the metal corroding surface [34].

Results and
As shown in Figure 3, inhibition efficiency decreases when the temperature rises. This occurs due to the desorption effect of the inhibitor molecules [35].   Moreover, decrease in inhibition efficiency with temperature rise can also be attributed to increased solubility of the protective film and/or any reaction product precipitated on the surface of the metal [36]. As a result, the metal surface becomes more accessible to corrosive attack.

Adsorption Isotherms and Thermodynamic Functions
The basic information on the interaction between the inhibitor and the metal can be provided by the adsorption isotherm. The adsorption isotherms tested in this work are the models of Langmuir, Temkin, Freundlich, El-Awady and Flory Huggins. By fitting the degree of surface coverage (θ) and the inhibitor concentration (Figure 4), the best adsorption isotherm obtained graphically is Langmuir adsorption isotherm with a strong correlation (R 2 > 0.999) and the slopes of the straight lines are close to unity.
The obtained Langmuir adsorption parameters for different temperatures are presented in Table 1.
Assumptions of Langmuir relate the concentration of the adsorbate in the bulk of the electrolyte (C inh ) to the degree of surface coverage (θ) as Equation: 1 inh inh ads where C inh is cefixime concentration and K ads is the equilibrium constant of the adsorption process.
In order to evaluate the strength of the interactions between the inhibitor mo-    In the above equation [38], 55.5 is the concentration of water in mol·L −1 , T is the absolute temperature while R is the universal gas constant. The values of 0 ads G ∆ and the other adsorption thermodynamic functions are gathered in Table 2.
The negative values of 0 ads G ∆ indicate that the adsorption process is spontaneous and the adsorbed layer on the aluminum surface is stable [9]. Generally, The literature [41] stated that an exothermic process means either physisorption

Effect of the Temperature and Activation Parameters
The activation energy ( a E ) can be obtained by using Arrhenius equation (Equation (18)): where CR is the corrosion rate and A is the Arrhenius pre-exponential constant.
A plot of log CR versus 1 T yields a straight line ( Figure 6) (Table 3).
From Table 3, it seems that a E and

Global Reactivity
The values of selected quantum chemical parameters computed for the studied cefixime molecule by using DFT methods are listed in Table 4.
According to the frontier molecular orbital (FMO) theory, of chemical reactivity,   Increasing values of the E HOMO facilitate adsorption (and therefore inhibition) by influencing the transport process through the adsorbed layer [49]. On the other hand, the LUMO energy [48], indicates the ability of the molecule to accept electrons. The lower the value of E HOMO , the more probable it is that the molecule accepts electrons. In this work, the higher value of E HOMO (−5.8680 eV) and the lower value of E LUMO (−2.0510 eV) could involve charge sharing or charge transfer from the inhibiting molecule to the metal surface [50]. That could explain good inhibition efficiency of cefixime. In the same way, low values of the energy gap (ΔΕ = Ε LUMO − Ε HOMO ) lead to high inhibition efficiencies because [36] the energy to remove an electron from the last occupied orbital will be low. Literature [51] revealed that excellent corrosion inhibitors are organic compounds which not only offer electrons to unoccupied orbital of metal but also accept free electrons from the metal. A molecule with a low energy gap [36] is more polarizable and is generally associated with high chemical reactivity and is consi- The electronegativity (χ) indicates the capacity of a system to attract electrons. In our work the low value of the electronegativity of the studied molecule The electrophilicity index measures the propensity of chemical species to accept electrons; a high value of electrophilicity index describes a good electrophile while a small value of electrophilicity index describes a good nucleophile. In this work the obtained value (ω = 4.1073 eV) shows the good capacity of cefixime to accept electrons. The HOMO and LUMO diagrams of the studied drug are presented in Figure 8.
As seen in Figure 8, the density HOMO is distributed around the five-membered ring containing heteroatoms whereas, the LUMO density is distributed almost homogeneously throughout the molecule. So, these regions are probably the most active areas where transfer of electrons could be achieved (from the inhibitor molecule to copper or vice-versa).

Local Reactivity
Local reactivity descriptors including atomic charges, condensed Fukui functionsand dual descriptor are collected in Table 5.
It can be clearly observed from shaded rows in Table 5

Conclusion
Cefixime drug shows good inhibition properties for copper corrosion in 1M HNO 3 . The inhibition efficiency increases with increasing concentration in the studied inhibitor but decreases with a rise in temperature. Adsorption of cefixime follows the Langmuir adsorption isotherm. The adsorption and activation thermodynamic functions reveal a spontaneous adsorption process of the studied molecule onto copper and both physisorption and chemisorption mechanisms with the predominance of physisorption. Quantum chemical calculations data from B3LYP/6-31G (d, p) fit well the experimental results.