Inhibition of Aluminium Corrosion in 1 . 0 M HCl by Caffeine : Experimental and DFT Studies

Aluminium corrosion inhibition in 1.0M hydrochloric acid solution by caffeine has been studied using mass loss technique and quantum chemical calculations based on DFT. The inhibition efficiency was found to increase with increasing concentration of caffeine but decreases with a rise in temperature. The molecule shows the highest inhibition efficiency of 74% at 10M for T = 303K. The experimental data were used to fit isotherms including Langmuir, Temkin, Freundlich, and El-Awady. The best fits were obtained with the Langmuir model and the kinetic-thermodynamic adsorption model of El-Awady. However, it was found that the adsorption parameters suit well with the isotherm of El-Awady which was chosen as the appropriate isotherm. To distinguish between physisorption and chemisorption, the Dubinin-Radushkevich adsorption model was used. The thermodynamic parameters governing the adsorption of caffeine onto aluminium and that of the metal dissolution were calculated and discussed. DFT study gave further insight into the mechanism of the inhibiting action of caffeine.


Introduction
Aluminium and its alloys [1,2] are used in many applications in industries (aerospace, household industries, electronic devices, food industry, etc.) due to their low price, high electrical capacity, and their high energy density.Though aluminium has the ability to form a stable thin oxide film that protects it from the corrosion phenomenon, it undergoes corrosion [3][4][5] when being in contact with aggressive media such as hydrochloric acid.
Corrosion control can be achieved by many methods; however, the use of corrosion inhibitors is actually the most practical method used in industries and academic studies.A survey of the literature reveals [6][7][8] that most of the wellknown ecofriendly corrosion inhibitors are organic compounds containing nitrogen, oxygen, sulphur, and/or  bonds in their molecular structure.Several heterocyclic N, S, or O containing organic compounds [9][10][11][12] have been used to protect aluminium from hydrochloric acid corrosion.
Generally, localized corrosion [4] can be avoided by the action of adsorption inhibitors, which prevent the adsorption of aggressive anions or by the formation of a more resistant oxide film on the metal surface.
The adsorption of a given organic compound onto a metal surface is influenced by its chemical structure.Therefore, the use of quantum chemical calculations will lead to structural parameters of the molecule.Recently, density functional theory (DFT) has emerged as a reliable and inexpensive method [13] for predicting the properties of chemical systems.Several papers [14][15][16] have documented the use of DFT to get insight into the corrosion inhibition mechanism by organic molecules.
The aim of this paper is to study the behaviour of caffeine against the hydrochloric acid corrosion of aluminium, using mass loss method and density functional theory calculations.

Material
2.1.1.Aluminium Specimens.The aluminium specimens were in the form of rod measuring 10 mm in length and 2 mm in diameter; they were cut in commercial aluminium of purity 99.5%.(iii) Acetone from Sigma Aldrich with purity: 99.5%.

Mass Loss Method.
The mass loss method [17,18] is probably the most widely used method of inhibition assessment.The simplicity and reliability of the measurement offered by mass loss method [19] make the technique form the baseline method in many corrosion monitoring programs.
Prior to the immersion of the specimens in the test solution (50 mL), their surface was abraded with different grade of emery papers (120-180-240-600-1200) to mirror finish, rinsed with acetone, washed with bidistilled water, and finally dried in a desiccator.The mass loss experiments were performed under total immersion in the acidic solution of 1.0 M (with or without caffeine), opened to the air.The temperature was controlled by a water thermostat.After one-hour immersion, the specimens were retrieved from the solution, washed with a bristle brush under running water in order to remove the corrosion product, dried in a desiccator, and weighed accurately.The study was performed in the temperature range of 303 K to 323 K and the concentration range of 0.1 mM to 10 mM.The tests were repeated three times for each solution and at the same temperature.The mean value was then recorded.
The corrosion rate () in g⋅cm −2 ⋅h −1 was calculated using the following equation: where  1 is the mass of the sample before immersion in the test solution,  2 is the mass after 1 h immersion in the test solution,  is the total surface of the sample, and  is the immersion time.The inhibition efficiency IE (%) was calculated according to the equation below: where  0 is the corrosion rate in the absence of the tested molecule and  is the corrosion rate in its presence.

Quantum Chemistry Calculations.
The calculations were performed using the hybrid functional B3LYP, a version of DFT functional that uses Becke's three parameter functional (B3) with a mixture of HF and DFT exchange terms associated with the gradient corrected correlation functional of Lee et al. [20].The full geometry optimization was carried out at B3LYP/6-31G (d) level of theory, using Gaussian 03W [21].The calculations were carried out in gas phase to show the relationship between the molecular descriptors of caffeine and its inhibition efficiency.The optimized minimum energy geometrical configuration of caffeine is given in Figure 1.
DFT [22] has been found to be successful in providing theoretical insights into the chemical reactivity and selectivity, using chemical concepts such as electronegativity (), hardness (), softness (), electrophilicity index (), and local reactivity descriptors including Fukui functions () and the local softness ().
Density functional theory (DFT) states that changes in electronic energy [()] are related to changes in the number of electrons  and changes in the external potential V() felt by the electron distribution (which refers to the nuclear position in chemical systems): According to Parr et al. [23], the chemical potential   is linked with the first derivative of the energy with respect to the number of electrons and therefore with the negative of the electronegativity by the following equation: where   is the electronic chemical potential,  is the total energy,  is the number of electrons, and V() is the external potential of the system.The second partial derivative of the energy with respect to the number of electrons has been defined as hardness (): This quantity [24] measures both the stability and the reactivity of the molecule.According to Koopmans's theorem [25], the ionization potential () and the electron affinity () of the inhibitors are calculated using the following equations: The electronegativity () [24] which measures the power of an atom or group of atoms to attract electrons towards itself can then be written as The chemical hardness () [24] which expresses the resistance of an atom to charge transfer is estimated using the equation below: The inverse of the hardness, known as softness () [24], measures the capacity of an atom or group of atoms to receive electrons; it is estimated by The fraction of electrons transferred from the inhibitor molecule to the metallic surface was calculated using the following equation [26]: where (  ,   ) and ( inh ,  inh ) are, respectively, the electronegativity and hardness of the metal and the inhibitor when   is the work function.In our study, the theoretical values of electronegativity  Al = 4.28 eV [27] and hardness  Al = 0 [26] have been used for aluminium.The global electrophilicity index, introduced by Parr et al. [28], is given by the equation below: This index [28] measures the propensity of chemical species to accept electrons.A good nucleophile is characterized by a low value of  whereas a good electrophile is characterized by a high value of .
Fukui function [29] is one of the widely used local density functional descriptors to model chemical reactivity and site selectivity; it is defined as the derivative of the electron density () with respect to , the total number of electrons in the system, at constant external potential V() acting on an electron due to all the nuclei in the system: The condensed Fukui functions are calculated using Yang and Mortier procedure [30] based on a finite difference method: where   is the electronic population of atom  in the molecule.The functions  +  and  −  are, respectively, related to nucleophilic and electrophilic attacks.
The local softness [31] is defined as The maximum values of relative nucleophilicity index ( +  / −  ) and relative electrophilicity index ( −  / +  ) are used to define, respectively, the probable sites of nucleophilic and electrophilic attacks.

Mass Loss Technique. Figures 2 and 3 illustrate the evolution of the inhibition efficiency of caffeine against aluminium corrosion in 1.0 M HCl after 1-hour immersion, respectively, for different concentrations and temperatures.
It is clear from these figures that the inhibition efficiency increases with increasing concentration of caffeine but decreases with a rise in temperature.The increase in inhibition efficiency with concentration may be due to the adsorption of caffeine onto the aluminium surface through nonbonding electron pairs of nitrogen and oxygen atoms as well as the -electrons of the aromatic rings.The surface of the metal is therefore covered by a protective layer film, which separates it from its environment.Similar observation [32,33] has been reported in the literature.
Though the effect of temperature on the inhibited acidmetal reaction is complex [34] due to many changes on the metal surface (rapid etching, desorption of molecules, etc.), the decrease in inhibition efficiency with a rise in temperature may probably due to increased rate of desorption of the inhibitor.3.1.1.Adsorption Isotherm.It is admitted [35] that the first step in corrosion inhibition of metals by organic compounds is their adsorption onto the metal surface.This phenomenon is regarded [35] as a quasi-substitution process between organic compounds in the aqueous phase Org (sol) and water molecules at the metal surface H 2 O (ads) : where  is the size ratio, the number of water molecules replaced by one inhibitor.Information on the interaction between the inhibitor and the metal surface can be obtained using adsorption isotherms.The isotherms are in general form: where (, ) is the configurational factor, subject to the physical model and assumptions involved in the derivation of the isotherm,  is a molecular interaction parameter,  inh is the inhibitor concentration, and  ads is the adsorption constant.
Attempts were made to fit experimental data ( and  inh ) to some classical isotherms including Langmuir, El-Awady, Freundlich, and Frumkin.By far the best fits were obtained with the isotherm of Langmuir ( 2 > 0.99) and that of El-Awady ( 2 > 0.98).In Langmuir isotherm,  and  inh are related by the following equation: Figure 4 depicts the plots of  inh / versus  inh .The slopes of the straight lines obtained are higher than unity for all the temperatures.The considerable deviation from unity observed may be due to the interactions among the adsorbed species on the metal surface.It is therefore pertinent to say that the adsorption can be more appropriately represented by a modified Langmuir equation: the isotherm of Villamil et al. [36] or that of El-Awady.The parameters of these isotherms are collected in Table 1.The adsorption constants  ads have been calculated using the equations in Table 1.The changes in standard adsorption free enthalpy have been calculated using the following equation: where  is the gas constant,  is the absolute temperature, and 55.5 is the concentration of water (in mol L −1 ) in the solution.In Villamil equation "" is a constant introduced to consider all the factors, not taken into account in the derivation of Langmuir isotherm.
The constant  in the isotherm of El-Awady is the number of active sites on the material surface; 1/ less than one implies a multilayer adsorption, while 1/ greater than one suggests that a given inhibitor molecule occupies more than one active site.
It is clear from Table 1 that only the kinetic-thermodynamic adsorption isotherm of El-Awady supports well the trend of decrease in inhibition efficiency (decrease of the adsorption constant with rise of temperature); therefore the appropriate isotherm is that of El-Awady.
In order to determine change in standard adsorption enthalpy Δ 0 ads and change in standard adsorption entropy Δ 0 ads , we used the basic equation: Plotting Δ 0 ads versus temperature gives the two adsorption parameters (Figure 5).The negative values of Δ 0 ads suggest that the adsorption of caffeine onto aluminium is spontaneous.These values range from −28.9 kJmol −1 to −19 kJmol −1 , indicating [37] both physisorption and chemisorption processes.
Change in standard adsorption enthalpy (Δ 0 ads = −168 kJmol −1 ) is negative, showing an exothermic adsorption process, and its absolute value is higher than 100 kJmol −1 , which is according to the literature [38] not a typical chemisorption when referring to the values of Δ 0 ads .It may be indicative of both physisorption and chemisorption processes.
Change in standard adsorption entropy (Δ 0 ads = −462 Jmol −1 K −1 ) is negative; it may be explained by desorption of the inhibitor species when the temperature increases.Moreover, according to the thermodynamic principles, since the adsorption is an exothermic process, it may be accompanied by a decrease in entropy.
In order to distinguish between physisorption and chemisorption, the isotherm of Dubinin-Radushkevich has been used.This isotherm is characterized [39] by the equation below: where  max is the maximum surface coverage and  is the Polanyi potential which is given by In this equation,  is the perfect gas constant,  is the absolute temperature, and  inh is the concentration of the inhibitor expressed in g L −1 .Figure 6 gives the plots of ln  versus  2 .The parameters of this model are in Table 2.
The value of the parameter  in (20) leads to the mean adsorption energy   for the related temperature.This energy is the transfer energy of 1 mol of adsorbate from infinity (bulk solution) to the surface of the adsorbent.  is defined as In order to determine the range of temperatures for physisorption and chemisorption, we plot   versus temperature (Figure 7).According to the literature [39] the magnitude of   gives information about the adsorption:   values less than   each type of adsorption is predominant (( < 309.1 K for physisorption) and ( > 309.1 K for chemisorption)).The decrease in  max values confirms that the adsorption decreases with increasing temperatures.

Effect of Temperature.
To determine the activation parameters of the corrosion process, the Arrhenius and the transition state equations were used: where   is the apparent activation energy,  is the perfect gas constant,  is the frequency factor, ℎ is Planck's constant, ℵ is the Avogadro number, Δ *  is the change in activation enthalpy, and Δ *  is the change in activation entropy.Values of apparent activation energy of corrosion (  ) for aluminium in HCl in the absence and presence of various concentrations of caffeine were determined from the slope of log  versus 1/ plots (Figure 8).
The values of change in enthalpy (Δ *  ) and change in entropy (Δ *  ) were obtained, respectively, from the slopes and intercepts of the plots of log(/) versus 1/ (Figure 9).All these parameters are collected in Table 3.  From Table 3, one can notice that   and Δ *  vary in the same way.This result permitted verifying the known thermodynamic relation between the two activation parameters: The activation energies   are all positive and those of the inhibited solutions are higher than those of the blank (uninhibited solution), suggesting [40] a physisorption process or a mix process.The positive sign of change in activation enthalpy Δ *  reflects the endothermic nature of the aluminium dissolution process.The values of change in activation enthalpy increase with increasing concentration of caffeine, showing that the dissolution of the aluminium becomes more and more difficult and slow.
The change in activation entropy Δ *  increases with increasing concentration in caffeine, indicating that an increase in disordering takes place on going from the reactants to the activated complex.This situation could explain the decrease in the rate of surface coverage.

Quantum Chemical Approach.
The quantum chemical parameters of caffeine obtained from the calculations include the energy of the highest occupied molecular orbital ( HOMO ), the energy of the lowest unoccupied molecular orbital ( LUMO ), the energy gap (Δ =  LUMO −  HOMO ), the dipole moment,  and the total energy (TE).Based on frontier molecular orbital (FMO), the reactivity parameters such as the ionization energy (), the electronic affinity (), the global electronegativity (), the global hardness (), the global softness (), the fraction of electrons transferred (Δ), and the electrophilicity index () were also calculated.All these parameters are listed in Table 4.
The HOMO energy [41] is directly related to the ionization energy and characterizes the tendency of the molecule to donate electrons to the unoccupied orbitals of metals.Organic molecules [42] with less negative HOMO values are expected to have high donation ability and therefore high inhibition efficiency.The LUMO energy is another significant reactivity parameter which is related to the electron affinity and characterizes the capacity of a molecule to gain electron from a metal.The lower the value of the LUMO energy, the stronger the electron accepting ability of the molecule [43].In our case, caffeine has a high value of HOMO energy ( HOMO = −5.959eV) and a low value of  LUMO ( LUMO = −0.819eV) when compared with values in the literature [15,44].So, the incomplete filled 3p of aluminium (electronic structure: 1s 2 2s 2 2p 6 3s 2 3p 1 ) could bond with the HOMO of caffeine while the filled 3s orbital could interact with its LUMO.
The energy gap (Δ =  LUMO −  HOMO ) is an important parameter related to the reactivity of an inhibitor towards the adsorption onto a metallic surface.Lower values of Δ [45] suggest better adsorption and then better inhibition efficiency.In our case (Δ = 5.140 eV) can be considered [46][47][48] as a low value when compared with other values in the literature.
The dipole moment  is widely used as a reactivity parameter; it results from the nonuniform distribution of charges on atoms in the molecule.Though many authors state that low values of dipole moment [49] favour accumulation of the inhibitor molecule in the surface layer and therefore higher inhibition efficiency, the survey of literature [50,51] reveals several irregularities in case of correlation of dipole moment with inhibitor efficiency.So, in general [52], there is no significant relationship between dipole moment values and inhibition efficiencies.
The global hardness () and softness () are important parameters which measure the reactivity and the molecular stability.A hard molecule has a large hardness value and vice versa [53].In our study, the hardness of caffeine ( = 2.570 eV) which can be considered as a low value [54] could explain the inhibiting properties of caffeine.
The number (Δ) of electrons transferred is a parameter which indicates the tendency of a molecule to donate electrons.The higher the value of Δ, the greater the tendency of the molecule to donate electrons to the metal.In our case (Δ = 0.173), the positive sign shows that the molecule could donate electrons to the metal.
The electrophilicity index () is another important parameter [55] which 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 indicates a good nucleophile.In our study  = 2.234 eV shows that caffeine has a good capacity to accept electrons from the metal.
The HOMO and LUMO orbital densities distributions are given in Figure 10.
In order to ascertain the role of individual atoms in the molecule, its local parameters including Mulliken charges  +1 ,   , and  −1 , Fukui functions  +  and  −  , and local softness  +  and  −  were calculated.All these parameters are listed in Table 5.
The analysis of Table 5 shows that carbon C (10) is the probable site for nucleophilic attacks, whereas carbon C ( 14) is the probable site for electrophilic attacks.However, according to the literature [56] Figure 10 allows verifying the belonging of each atom  to the HOMO or LUMO densities regions in the molecule.From all these results, one can deduce (Figure 11) a pictorial presentation of forces acting between caffeine and aluminium surface.
At low temperatures, physical interactions exist between the protonated form of caffeine and chloride ions adsorbed on aluminium surface: physisorption is predominant.The rise in temperature leads to desorption of the protonated form of caffeine; only the neutral form, which is bonded to the metal, allows the inhibition of aluminium corrosion by chemisorption.

Conclusion
Caffeine was found to act as an effective corrosion inhibitor for aluminium in 1.0 M HCl.The efficiency depends on the concentration and the temperature.The inhibition efficiency increases with increasing concentration of the inhibitor but decreases with rise in temperature.The adsorption of caffeine onto aluminium obeys the kinetic-thermodynamic adsorption isotherm of El-Awady.The negative sign of Δ 0 ads suggests a spontaneous adsorption process.The values of  change in adsorption free enthalpy Δ 0 ads and that of the activation energy   indicate the presence of both physisorption and chemisorption.The quantum chemical calculations are in good agreement with experimental results.

2
Advances in Chemistry2.1.2.Chemicals.All chemicals were of analytical grade and were used without further purification: (i) HCl from Merck with purity: 37%.(ii) Caffeine (C 8 H 10 N 4 O 2 ) was acquired from Sigma Aldrich chemicals and solutions of concentrations range from 0.1 mM to 10 mM were prepared.

Figure 2 :
Figure 2: Inhibition efficiency of caffeine against aluminium corrosion in 1.0 M HCl versus concentration for different temperatures.

Figure 3 :
Figure 3: Inhibition efficiency of caffeine against aluminium corrosion in 1.0 M HCl versus temperature for different concentrations.

Figure 4 :
Figure 4: Langmuir adsorption plots for aluminium in presence of caffeine.

Figure 5 :
Figure 5: Δ 0 ads versus temperature for aluminium in presence of caffeine.

Figure 8 :
Figure 8: Arrhenius plots for aluminium in 1.0 M HCl at different concentrations of caffeine.

Figure 9 :
Figure 9: Transition state plots of aluminium in 1.0 M HCl at different concentrations of caffeine.

Table 1 :
Parameters of the modified Langmuir adsorption isotherms.

Table 3 :
Dissolution parameters of the aluminium in 1.0 M HCl.
, the local parameters  +  ,  −  ,  +  , and  −  are influenced by the basis sets.So, it is judicious to use relative indexes such as the relative nucleophilicity ( +

Table 5 :
Local parameters of Caffeine.