# Inhibition of Aluminium Corrosion in 1.0 M HCl by Caffeine: Experimental and DFT Studies

Authors: R. H. B. Beda; P. M. Niamien; E. B. Avo Bilé; A. Trokourey
Journal: Advances in Chemistry
 (2017)

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## Abstract

Aluminium corrosion inhibition in 1.0 M 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 10−2 M for T=303 K. 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.


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## Body

## 1. 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 [3–5] 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 [6–8] that most of the well-known ecofriendly corrosion inhibitors are organic compounds containing nitrogen, oxygen, sulphur, and/or π bonds in their molecular structure. Several heterocyclic N, S, or O containing organic compounds [9–12] 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 [14–16] 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.


## 2. Material and Methods

### 2.1. 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%.


#### 2.1.2. Chemicals

All chemicals were of analytical grade and were used without further purification:(i)
HCl from Merck with purity: 37%.(ii)
Caffeine (C8H10N4O2) was acquired from Sigma Aldrich chemicals and solutions of concentrations range from 0.1 mM to 10 mM were prepared.(iii)
Acetone from Sigma Aldrich with purity: 99.5%.


### 2.2. Methods

#### 2.2.1. 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 (W) in g·cm−2·h−1 was calculated using the following equation:(1)W=m1-m2St,where m1 is the mass of the sample before immersion in the test solution, m2 is the mass after 1 h immersion in the test solution, S is the total surface of the sample, and t is the immersion time. The inhibition efficiency IE (%) was calculated according to the equation below:(2)IE%=W0-WW0×100,where W0 is the corrosion rate in the absence of the tested molecule and W is the corrosion rate in its presence.


#### 2.2.2. 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 Figure1.Figure 1
Optimized structure of caffeine by B3LYP/6-31G (d).DFT [22] has been found to be successful in providing theoretical insights into the chemical reactivity and selectivity, using chemical concepts such as electronegativity (χ), hardness (η), softness (S), electrophilicity index (ω), and local reactivity descriptors including Fukui functions f(r) and the local softness s(r).Density functional theory (DFT) states that changes in electronic energydE[ρr] are related to changes in the number of electrons N and changes in the external potential v(r) felt by the electron distribution (which refers to the nuclear position in chemical systems):(3)dEρr=μPdN+∫ρrdvrdr.According to Parr et al. [23], the chemical potential μP 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:(4)μP=∂E∂Nvr=-χ,where μP is the electronic chemical potential, E is the total energy, N is the number of electrons, and v(r) 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 (η):(5)η=∂2E∂N2vr=∂μP∂Nvr.This quantity [24] measures both the stability and the reactivity of the molecule.According to Koopmans’s theorem [25], the ionization potential (I) and the electron affinity (A) of the inhibitors are calculated using the following equations:(6)I=-EHOMO,A=-ELUMO.The electronegativity (χ) [24] which measures the power of an atom or group of atoms to attract electrons towards itself can then be written as(7)χ=I+A2.The chemical hardness (η) [24] which expresses the resistance of an atom to charge transfer is estimated using the equation below:(8)η=I-A2.The inverse of the hardness, known as softness (S) [24], measures the capacity of an atom or group of atoms to receive electrons; it is estimated by(9)S=1η=2I-A.The fraction of electrons transferred from the inhibitor molecule to the metallic surface was calculated using the following equation [26]:(10)ΔN=χM-χinh2ηM+ηinh=ϕM-χinh2ηinh,where (χM,ηM) and (χinh,ηinh) are, respectively, the electronegativity and hardness of the metal and the inhibitor when ϕM is the work function. In our study, the theoretical values of electronegativity ϕAl=4.28eV [27] and hardness ηAl=0 [26] have been used for aluminium.The global electrophilicity index, introduced by Parr et al. [28], is given by the equation below:(11)ω=μP22η.This index [28] measures the propensity of chemical species to accept electrons. A good nucleophile is characterized by a low value of ω whereas a good electrophile is characterized by a high value of ω.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 ρr with respect to N, the total number of electrons in the system, at constant external potential vr acting on an electron due to all the nuclei in the system:(12)fr=∂μP∂vrN=∂ρr∂Nvr.The condensed Fukui functions are calculated using Yang and Mortier procedure [30] based on a finite difference method:(13)fk+=qkN+1-qkN,fk-=qkN-qkN-1,where qk is the electronic population of atom k in the molecule. The functions fk+ and fk- are, respectively, related to nucleophilic and electrophilic attacks.The local softness [31] is defined as(14)skα=fkαSα=+ou-.The maximum values of relative nucleophilicity index (sk+/sk-) and relative electrophilicity index (sk-/sk+) are used to define, respectively, the probable sites of nucleophilic and electrophilic attacks.


## 2.1. 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%.


### 2.1.2. Chemicals

All chemicals were of analytical grade and were used without further purification:(i)
HCl from Merck with purity: 37%.(ii)
Caffeine (C8H10N4O2) was acquired from Sigma Aldrich chemicals and solutions of concentrations range from 0.1 mM to 10 mM were prepared.(iii)
Acetone from Sigma Aldrich with purity: 99.5%.


## 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%.


## 2.1.2. Chemicals

All chemicals were of analytical grade and were used without further purification:(i)
HCl from Merck with purity: 37%.(ii)
Caffeine (C8H10N4O2) was acquired from Sigma Aldrich chemicals and solutions of concentrations range from 0.1 mM to 10 mM were prepared.(iii)
Acetone from Sigma Aldrich with purity: 99.5%.


## 2.2. Methods

### 2.2.1. 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 (W) in g·cm−2·h−1 was calculated using the following equation:(1)W=m1-m2St,where m1 is the mass of the sample before immersion in the test solution, m2 is the mass after 1 h immersion in the test solution, S is the total surface of the sample, and t is the immersion time. The inhibition efficiency IE (%) was calculated according to the equation below:(2)IE%=W0-WW0×100,where W0 is the corrosion rate in the absence of the tested molecule and W is the corrosion rate in its presence.


### 2.2.2. 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 Figure1.Figure 1
Optimized structure of caffeine by B3LYP/6-31G (d).DFT [22] has been found to be successful in providing theoretical insights into the chemical reactivity and selectivity, using chemical concepts such as electronegativity (χ), hardness (η), softness (S), electrophilicity index (ω), and local reactivity descriptors including Fukui functions f(r) and the local softness s(r).Density functional theory (DFT) states that changes in electronic energydE[ρr] are related to changes in the number of electrons N and changes in the external potential v(r) felt by the electron distribution (which refers to the nuclear position in chemical systems):(3)dEρr=μPdN+∫ρrdvrdr.According to Parr et al. [23], the chemical potential μP 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:(4)μP=∂E∂Nvr=-χ,where μP is the electronic chemical potential, E is the total energy, N is the number of electrons, and v(r) 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 (η):(5)η=∂2E∂N2vr=∂μP∂Nvr.This quantity [24] measures both the stability and the reactivity of the molecule.According to Koopmans’s theorem [25], the ionization potential (I) and the electron affinity (A) of the inhibitors are calculated using the following equations:(6)I=-EHOMO,A=-ELUMO.The electronegativity (χ) [24] which measures the power of an atom or group of atoms to attract electrons towards itself can then be written as(7)χ=I+A2.The chemical hardness (η) [24] which expresses the resistance of an atom to charge transfer is estimated using the equation below:(8)η=I-A2.The inverse of the hardness, known as softness (S) [24], measures the capacity of an atom or group of atoms to receive electrons; it is estimated by(9)S=1η=2I-A.The fraction of electrons transferred from the inhibitor molecule to the metallic surface was calculated using the following equation [26]:(10)ΔN=χM-χinh2ηM+ηinh=ϕM-χinh2ηinh,where (χM,ηM) and (χinh,ηinh) are, respectively, the electronegativity and hardness of the metal and the inhibitor when ϕM is the work function. In our study, the theoretical values of electronegativity ϕAl=4.28eV [27] and hardness ηAl=0 [26] have been used for aluminium.The global electrophilicity index, introduced by Parr et al. [28], is given by the equation below:(11)ω=μP22η.This index [28] measures the propensity of chemical species to accept electrons. A good nucleophile is characterized by a low value of ω whereas a good electrophile is characterized by a high value of ω.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 ρr with respect to N, the total number of electrons in the system, at constant external potential vr acting on an electron due to all the nuclei in the system:(12)fr=∂μP∂vrN=∂ρr∂Nvr.The condensed Fukui functions are calculated using Yang and Mortier procedure [30] based on a finite difference method:(13)fk+=qkN+1-qkN,fk-=qkN-qkN-1,where qk is the electronic population of atom k in the molecule. The functions fk+ and fk- are, respectively, related to nucleophilic and electrophilic attacks.The local softness [31] is defined as(14)skα=fkαSα=+ou-.The maximum values of relative nucleophilicity index (sk+/sk-) and relative electrophilicity index (sk-/sk+) are used to define, respectively, the probable sites of nucleophilic and electrophilic attacks.


## 2.2.1. 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 (W) in g·cm−2·h−1 was calculated using the following equation:(1)W=m1-m2St,where m1 is the mass of the sample before immersion in the test solution, m2 is the mass after 1 h immersion in the test solution, S is the total surface of the sample, and t is the immersion time. The inhibition efficiency IE (%) was calculated according to the equation below:(2)IE%=W0-WW0×100,where W0 is the corrosion rate in the absence of the tested molecule and W is the corrosion rate in its presence.


## 2.2.2. 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 Figure1.Figure 1
Optimized structure of caffeine by B3LYP/6-31G (d).DFT [22] has been found to be successful in providing theoretical insights into the chemical reactivity and selectivity, using chemical concepts such as electronegativity (χ), hardness (η), softness (S), electrophilicity index (ω), and local reactivity descriptors including Fukui functions f(r) and the local softness s(r).Density functional theory (DFT) states that changes in electronic energydE[ρr] are related to changes in the number of electrons N and changes in the external potential v(r) felt by the electron distribution (which refers to the nuclear position in chemical systems):(3)dEρr=μPdN+∫ρrdvrdr.According to Parr et al. [23], the chemical potential μP 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:(4)μP=∂E∂Nvr=-χ,where μP is the electronic chemical potential, E is the total energy, N is the number of electrons, and v(r) 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 (η):(5)η=∂2E∂N2vr=∂μP∂Nvr.This quantity [24] measures both the stability and the reactivity of the molecule.According to Koopmans’s theorem [25], the ionization potential (I) and the electron affinity (A) of the inhibitors are calculated using the following equations:(6)I=-EHOMO,A=-ELUMO.The electronegativity (χ) [24] which measures the power of an atom or group of atoms to attract electrons towards itself can then be written as(7)χ=I+A2.The chemical hardness (η) [24] which expresses the resistance of an atom to charge transfer is estimated using the equation below:(8)η=I-A2.The inverse of the hardness, known as softness (S) [24], measures the capacity of an atom or group of atoms to receive electrons; it is estimated by(9)S=1η=2I-A.The fraction of electrons transferred from the inhibitor molecule to the metallic surface was calculated using the following equation [26]:(10)ΔN=χM-χinh2ηM+ηinh=ϕM-χinh2ηinh,where (χM,ηM) and (χinh,ηinh) are, respectively, the electronegativity and hardness of the metal and the inhibitor when ϕM is the work function. In our study, the theoretical values of electronegativity ϕAl=4.28eV [27] and hardness ηAl=0 [26] have been used for aluminium.The global electrophilicity index, introduced by Parr et al. [28], is given by the equation below:(11)ω=μP22η.This index [28] measures the propensity of chemical species to accept electrons. A good nucleophile is characterized by a low value of ω whereas a good electrophile is characterized by a high value of ω.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 ρr with respect to N, the total number of electrons in the system, at constant external potential vr acting on an electron due to all the nuclei in the system:(12)fr=∂μP∂vrN=∂ρr∂Nvr.The condensed Fukui functions are calculated using Yang and Mortier procedure [30] based on a finite difference method:(13)fk+=qkN+1-qkN,fk-=qkN-qkN-1,where qk is the electronic population of atom k in the molecule. The functions fk+ and fk- are, respectively, related to nucleophilic and electrophilic attacks.The local softness [31] is defined as(14)skα=fkαSα=+ou-.The maximum values of relative nucleophilicity index (sk+/sk-) and relative electrophilicity index (sk-/sk+) are used to define, respectively, the probable sites of nucleophilic and electrophilic attacks.


## 3. Results and Discussion

### 3.1. Mass Loss Technique

Figures2 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.Figure 2
Inhibition efficiency of caffeine against aluminium corrosion in 1.0 M HCl versus concentration for different temperatures.Figure 3
Inhibition efficiency of caffeine against aluminium corrosion in 1.0 M HCl versus temperature for different concentrations.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 theπ-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 acid–metal 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 Orgsol and water molecules at the metal surface H2O(ads):(15)Orgsol+xH2Oads⇄Orgads+xH2Osolwhere x 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:(16)KadsCinh=fθ,xexp⁡-αθ,where fθ,x is the configurational factor, subject to the physical model and assumptions involved in the derivation of the isotherm, α is a molecular interaction parameter, Cinh is the inhibitor concentration, and Kads is the adsorption constant.Attempts were made to fit experimental data (θ and Cinh) to some classical isotherms including Langmuir, El-Awady, Freundlich, and Frumkin. By far the best fits were obtained with the isotherm of Langmuir (R2>0.99) and that of El-Awady (R2>0.98). In Langmuir isotherm, θ and Cinh are related by the following equation:(17)Cinhθ=1Kads+Cinh.Figure 4 depicts the plots of Cinh/θ versus Cinh.Figure 4
Langmuir adsorption plots for aluminium in presence of caffeine.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.Table 1
Parameters of the modified Langmuir adsorption isotherms.
Isotherm
Equation
T (K)
R
2
Slope
Intercept
K
a
d
s (×103 M)
Δ
G
a
d
s
0 (kJmol−1)
Villamil et al.
C
i
n
h
θ
=
n
K
a
d
s
+
n
C
i
n
h
303
0.994
1.3542
0.0007
2.768
-
30.1
308
0.999
1.6943
0.0005
5.648
-
32.4
313
0.998
1.8394
0.0007
4.598
-
32.3
318
0.994
1.9882
0.0012
3.314
-
32.0
323
0.995
2.3771
0.0014
3.962
-
33.0
El-Awady
log
⁡
θ
1
-
θ
=
log
⁡
K
′
+
y
log
⁡
C
i
n
h Kads=K′1/y
303
0.999
0.3421
1.1085
1.738
-
28.9
308
0.996
0.2442
0.6466
0.444
-
25.9
313
0.978
0.2413
0.5539
0.197
-
24.2
318
0.968
0.2435
0.4809
0.094
-
22.6
323
0.992
0.2226
0.2950
0.021
-
19.0The adsorption constantsKads have been calculated using the equations in Table 1. The changes in standard adsorption free enthalpy have been calculated using the following equation:(18)ΔGads0=-RTln⁡55.5×Kads,where R is the gas constant, T is the absolute temperature, and 55.5 is the concentration of water (in mol L−1) in the solution.In Villamil equation “n” is a constant introduced to consider all the factors, not taken into account in the derivation of Langmuir isotherm.The constanty in the isotherm of El-Awady is the number of active sites on the material surface; 1/y less than one implies a multilayer adsorption, while 1/y greater than one suggests that a given inhibitor molecule occupies more than one active site.It is clear from Table1 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 enthalpyΔHads0 and change in standard adsorption entropy ΔSads0, we used the basic equation:(19)ΔGads0=ΔHads0-TΔSads0.Plotting ΔGads0 versus temperature gives the two adsorption parameters (Figure 5). The negative values of ΔGads0 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.Figure 5
Δ
G
a
d
s
0 versus temperature for aluminium in presence of caffeine.Change in standard adsorption enthalpy (ΔHads0=-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 ΔGads0. It may be indicative of both physisorption and chemisorption processes.Change in standard adsorption entropy (ΔSads0=-462 Jmol−1K−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:(20)ln⁡θ=ln⁡θmax-aδ2,where θmax is the maximum surface coverage and δ is the Polanyi potential which is given by(21)δ=RTln⁡1+1Cinh.In this equation, R is the perfect gas constant, T is the absolute temperature, and Cinh is the concentration of the inhibitor expressed in g L−1. Figure 6 gives the plots of ln⁡θ versus δ2. The parameters of this model are in Table 2.Table 2
Parameters of the Dubinin-Radushkevich model.
T (K)
R
2
a (kJ−2 mol2)
θ
m
a
x
E
m (kJ mol−1)
303
0.991
0.0121
0.696
6.4
308
0.995
0.0081
0.582
7.9
313
0.984
0.0063
0.517
8.9
318
0.965
0.0063
0.466
8.9
323
0.974
0.0061
0.401
9.0Figure 6
Dubinin-Radushkevich isotherm in presence of caffeine.The value of the parametera in (20) leads to the mean adsorption energy Em 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. Em is defined as(22)Em=12a.In order to determine the range of temperatures for physisorption and chemisorption, we plot Em versus temperature (Figure 7).Figure 7
Adsorption energy versus temperature for caffeine onto aluminium.According to the literature [39] the magnitude of Em gives information about the adsorption: Em values less than 8 kJ mol−1 indicate physical adsorption, while that higher than 8 kJ mol−1 suggest chemisorption. Using the equation of the straight line in Figure 7, we derived the domains where each type of adsorption is predominant ((T<309.1 K for physisorption) and (T>309.1 K for chemisorption)). The decrease in θmax values confirms that the adsorption decreases with increasing temperatures.


#### 3.1.2. Effect of Temperature

To determine the activation parameters of the corrosion process, the Arrhenius and the transition state equations were used:(23)log⁡W=log⁡A-Ea2,303RT,log⁡WT=log⁡Rℵh+ΔSa∗2,303R-ΔHa∗2.303RT,where Ea is the apparent activation energy, R is the perfect gas constant, A is the frequency factor, h is Planck’s constant, ℵ is the Avogadro number, ΔHa∗ is the change in activation enthalpy, and ΔSa∗ is the change in activation entropy.Values of apparent activation energy of corrosion (Ea) for aluminium in HCl in the absence and presence of various concentrations of caffeine were determined from the slope of log⁡W versus 1/T plots (Figure 8).Figure 8
Arrhenius plots for aluminium in 1.0 M HCl at different concentrations of caffeine.The values of change in enthalpy (ΔHa∗) and change in entropy (ΔSa∗) were obtained, respectively, from the slopes and intercepts of the plots of log⁡W/T versus 1/T (Figure 9). All these parameters are collected in Table 3.Table 3
Dissolution parameters of the aluminium in 1.0 M HCl.
C
i
n
h (mM)
E
a (kJ mol−1)
Δ
H
a
∗ (kJ mol−1)
Δ
S
a
∗ (J mol−1K−1)
0
77.9
75.4
-
34.3
0.10
78.8
76.3
-
31.5
0.5
90.6
88.1
2.2
1
97.0
94.5
21.9
5
100.1
97.7
31.2
10
104.9
102.4
45.0Figure 9
Transition state plots of aluminium in 1.0 M HCl at different concentrations of caffeine.From Table3, one can notice that Ea and ΔHa∗ vary in the same way. This result permitted verifying the known thermodynamic relation between the two activation parameters:(24)ΔHa∗=Ea-RT.The activation energies Ea 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 ΔHa∗ 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 entropyΔSa∗ 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.


### 3.2. Quantum Chemical Approach

The quantum chemical parameters of caffeine obtained from the calculations include the energy of the highest occupied molecular orbital (EHOMO), the energy of the lowest unoccupied molecular orbital (ELUMO), the energy gap (ΔE=ELUMO-EHOMO), the dipole moment, μ and the total energy (TE). Based on frontier molecular orbital (FMO), the reactivity parameters such as the ionization energy (I), the electronic affinity (A), the global electronegativity (χ), the global hardness (η), the global softness (S), the fraction of electrons transferred (ΔN), and the electrophilicity index (ω) were also calculated. All these parameters are listed in Table 4.Table 4
Molecular properties of caffeine calculated with B3LYP/6-31G (d).
Parameter
Value
E
H
O
M
O (eV)
-
5.959
E
L
U
M
O (eV)
-
0.819
Δ
E (eV)
5.140
I (eV)
5.959
A (eV)
0.819
χ (eV)
3.389
η (eV)
2.570
S (eV−1)
0.389
μ (Debye)
3.835
Δ
N
0.173
ω (eV)
2.234
TE (a. u)
-
680.4The 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 (EHOMO=-5.959 eV) and a low value of ELUMO (ELUMO=-0.819 eV) when compared with values in the literature [15, 44]. So, the incomplete filled 3p of aluminium (electronic structure: 1s22s22p63s23p1) could bond with the HOMO of caffeine while the filled 3s orbital could interact with its LUMO.The energy gap (ΔE=ELUMO-EHOMO) is an important parameter related to the reactivity of an inhibitor towards the adsorption onto a metallic surface. Lower values of ΔE [45] suggest better adsorption and then better inhibition efficiency. In our case (ΔE=5.140 eV) can be considered [46–48] as a low value when compared with other values in the literature.The dipole momentμ 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 (η) and softness (S) 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 (η=2.570 eV) which can be considered as a low value [54] could explain the inhibiting properties of caffeine.The number(ΔN) of electrons transferred is a parameter which indicates the tendency of a molecule to donate electrons. The higher the value of ΔN, the greater the tendency of the molecule to donate electrons to the metal. In our case (ΔN=0.173), the positive sign shows that the molecule could donate electrons to the metal.The electrophilicity index(ω) 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 ω=2.234 eV shows that caffeine has a good capacity to accept electrons from the metal.TheHOMO and LUMO orbital densities distributions are given in Figure 10.Figure 10
HOMO (a) and LUMO (b) densities of caffeine.
(a)
(b)In order to ascertain the role of individual atoms in the molecule, its local parameters including Mulliken chargesqN+1, qN, and qN-1, Fukui functions fk+ and fk-, and local softness sk+ and sk- were calculated. All these parameters are listed in Table 5.Table 5
Local parameters of Caffeine.
Atom
q
N
+
1
q
N
q
N
-
1
f
k
+
f
k
-
s
k
+
s
k
-
s
k
+
/
s
k
-
s
k
-
/
s
k
+
1 C
0.4259
0.4753
0.5423
−0.0494
−0.0670
−0.0192
−0.0261
0.7370
1.3568
2 C
0.1933
0.2179
0.3044
−0.0247
−0.0864
−0.0096
−0.0336
0.2855
3.5029
3 C
0.5084
0.6324
0.6942
−0.1240
−0.0618
−0.0482
−0.0240
2.0059
0.4985
4 C
0.7588
0.7852
0.8159
−0.0264
−0.0307
−0.0103
−0.0119
0.8595
1.1634
5 C
0.0637
0.2199
0.2809
−0.1562
−0.0610
−0.0608
−0.0237
2.5623
0.3903
6 H
0.0638
0.1695
0.2487
−0.1058
−0.0792
−0.0411
−0.0308
1.3351
0.7490
7 N
−0.5530
−0.5740
−0.5772
0.0213
0.0029
0.0083
0.0011
7.2401
0.1381
8 N
−0.5700
−0.5900
−0.5595
0.0198
−0.0302
0.0077
−0.0117
−0.6565
−1.5233
9 N
−0.4880
−0.4840
−0.5009
−0.0042
0.0168
−0.0016
0.0065
−0.2478
−4.0357
10 C
−0.2910
−0.3150
−0.3382
0.0239
0.0235
0.0093
0.0091
1.0177
0.9826
11 H
0.1754
0.1982
0.2316
−0.0228
−0.0335
−0.0089
−0.013
0.6820
1.4663
12 H
0.1284
0.1773
0.2196
−0.0489
−0.0423
−0.0190
−0.0165
1.1562
0.8649
13 H
0.1273
0.1772
0.2196
−0.0499
−0.0423
−0.0194
−0.0165
1.1787
0.8484
14 C
−0.3010
−0.3220
−0.3500
0.0208
0.0282
0.0081
0.0110
0.7355
1.3597
15 H
0.1389
0.1977
0.2396
−0.0588
−0.0420
−0.0229
−0.0163
1.4000
0.7143
16 H
0.1410
0.1816
0.2391
−0.0407
−0.0575
−0.0158
−0.0224
0.7072
1.4140
17 H
0.1690
0.1816
0.2391
−0.0126
−0.0575
−0.0049
−0.0224
0.2187
4.5734
18 C
−0.2960
−0.3150
−0.3700
0.0195
0.0546
0.0076
0.0213
0.3568
2.8029
19 H
0.1811
0.1990
0.2571
−0.0179
−0.0581
−0.0070
−0.0226
0.3081
3.2461
20 H
0.1049
0.1991
0.2301
−0.0942
−0.0310
−0.0366
−0.0120
3.0417
0.3288
21 H
0.1069
0.1662
0.2300
−0.0593
−0.0638
−0.0231
−0.0248
0.9297
1.0756
22 O
−0.6400
−0.5400
−0.4402
−0.0994
−0.1002
−0.0387
−0.0390
0.9928
1.0072
23 O
−0.5990
−0.5270
−0.4107
−0.0728
−0.1159
−0.0283
−0.0451
0.6276
1.5933
24 N
−0.5490
−0.5110
−0.4456
−0.0374
−0.0657
−0.0145
−0.0256
0.5691
1.7573The analysis of Table5 shows that carbonC (10) is the probable site for nucleophilic attacks, whereas carbonC (14) is the probable site for electrophilic attacks. However, according to the literature [56], the local parameters fk+, fk-, sk+, and sk- are influenced by the basis sets. So, it is judicious to use relative indexes such as the relative nucleophilicity (sk+/sk-) and the relative electrophilicity (sk-/sk+). So, in our studyN (7) which has the highest value of relative nucleophilicity index sk+/sk-=7.240 could be the probable nucleophilic attack site whereasC (2) with the highest value of relative electrophilicity sk-/sk+=3.503 could be the probable site of electrophilic attack.Figure10 allows verifying the belonging of each atom k 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.Figure 11
Schematic mechanism of aluminium corrosion inhibition in 1.0 M HCl by caffeine.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.


## 3.1. Mass Loss Technique

Figures2 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.Figure 2
Inhibition efficiency of caffeine against aluminium corrosion in 1.0 M HCl versus concentration for different temperatures.Figure 3
Inhibition efficiency of caffeine against aluminium corrosion in 1.0 M HCl versus temperature for different concentrations.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 theπ-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 acid–metal 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 Orgsol and water molecules at the metal surface H2O(ads):(15)Orgsol+xH2Oads⇄Orgads+xH2Osolwhere x 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:(16)KadsCinh=fθ,xexp⁡-αθ,where fθ,x is the configurational factor, subject to the physical model and assumptions involved in the derivation of the isotherm, α is a molecular interaction parameter, Cinh is the inhibitor concentration, and Kads is the adsorption constant.Attempts were made to fit experimental data (θ and Cinh) to some classical isotherms including Langmuir, El-Awady, Freundlich, and Frumkin. By far the best fits were obtained with the isotherm of Langmuir (R2>0.99) and that of El-Awady (R2>0.98). In Langmuir isotherm, θ and Cinh are related by the following equation:(17)Cinhθ=1Kads+Cinh.Figure 4 depicts the plots of Cinh/θ versus Cinh.Figure 4
Langmuir adsorption plots for aluminium in presence of caffeine.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.Table 1
Parameters of the modified Langmuir adsorption isotherms.
Isotherm
Equation
T (K)
R
2
Slope
Intercept
K
a
d
s (×103 M)
Δ
G
a
d
s
0 (kJmol−1)
Villamil et al.
C
i
n
h
θ
=
n
K
a
d
s
+
n
C
i
n
h
303
0.994
1.3542
0.0007
2.768
-
30.1
308
0.999
1.6943
0.0005
5.648
-
32.4
313
0.998
1.8394
0.0007
4.598
-
32.3
318
0.994
1.9882
0.0012
3.314
-
32.0
323
0.995
2.3771
0.0014
3.962
-
33.0
El-Awady
log
⁡
θ
1
-
θ
=
log
⁡
K
′
+
y
log
⁡
C
i
n
h Kads=K′1/y
303
0.999
0.3421
1.1085
1.738
-
28.9
308
0.996
0.2442
0.6466
0.444
-
25.9
313
0.978
0.2413
0.5539
0.197
-
24.2
318
0.968
0.2435
0.4809
0.094
-
22.6
323
0.992
0.2226
0.2950
0.021
-
19.0The adsorption constantsKads have been calculated using the equations in Table 1. The changes in standard adsorption free enthalpy have been calculated using the following equation:(18)ΔGads0=-RTln⁡55.5×Kads,where R is the gas constant, T is the absolute temperature, and 55.5 is the concentration of water (in mol L−1) in the solution.In Villamil equation “n” is a constant introduced to consider all the factors, not taken into account in the derivation of Langmuir isotherm.The constanty in the isotherm of El-Awady is the number of active sites on the material surface; 1/y less than one implies a multilayer adsorption, while 1/y greater than one suggests that a given inhibitor molecule occupies more than one active site.It is clear from Table1 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 enthalpyΔHads0 and change in standard adsorption entropy ΔSads0, we used the basic equation:(19)ΔGads0=ΔHads0-TΔSads0.Plotting ΔGads0 versus temperature gives the two adsorption parameters (Figure 5). The negative values of ΔGads0 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.Figure 5
Δ
G
a
d
s
0 versus temperature for aluminium in presence of caffeine.Change in standard adsorption enthalpy (ΔHads0=-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 ΔGads0. It may be indicative of both physisorption and chemisorption processes.Change in standard adsorption entropy (ΔSads0=-462 Jmol−1K−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:(20)ln⁡θ=ln⁡θmax-aδ2,where θmax is the maximum surface coverage and δ is the Polanyi potential which is given by(21)δ=RTln⁡1+1Cinh.In this equation, R is the perfect gas constant, T is the absolute temperature, and Cinh is the concentration of the inhibitor expressed in g L−1. Figure 6 gives the plots of ln⁡θ versus δ2. The parameters of this model are in Table 2.Table 2
Parameters of the Dubinin-Radushkevich model.
T (K)
R
2
a (kJ−2 mol2)
θ
m
a
x
E
m (kJ mol−1)
303
0.991
0.0121
0.696
6.4
308
0.995
0.0081
0.582
7.9
313
0.984
0.0063
0.517
8.9
318
0.965
0.0063
0.466
8.9
323
0.974
0.0061
0.401
9.0Figure 6
Dubinin-Radushkevich isotherm in presence of caffeine.The value of the parametera in (20) leads to the mean adsorption energy Em 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. Em is defined as(22)Em=12a.In order to determine the range of temperatures for physisorption and chemisorption, we plot Em versus temperature (Figure 7).Figure 7
Adsorption energy versus temperature for caffeine onto aluminium.According to the literature [39] the magnitude of Em gives information about the adsorption: Em values less than 8 kJ mol−1 indicate physical adsorption, while that higher than 8 kJ mol−1 suggest chemisorption. Using the equation of the straight line in Figure 7, we derived the domains where each type of adsorption is predominant ((T<309.1 K for physisorption) and (T>309.1 K for chemisorption)). The decrease in θmax values confirms that the adsorption decreases with increasing temperatures.


### 3.1.2. Effect of Temperature

To determine the activation parameters of the corrosion process, the Arrhenius and the transition state equations were used:(23)log⁡W=log⁡A-Ea2,303RT,log⁡WT=log⁡Rℵh+ΔSa∗2,303R-ΔHa∗2.303RT,where Ea is the apparent activation energy, R is the perfect gas constant, A is the frequency factor, h is Planck’s constant, ℵ is the Avogadro number, ΔHa∗ is the change in activation enthalpy, and ΔSa∗ is the change in activation entropy.Values of apparent activation energy of corrosion (Ea) for aluminium in HCl in the absence and presence of various concentrations of caffeine were determined from the slope of log⁡W versus 1/T plots (Figure 8).Figure 8
Arrhenius plots for aluminium in 1.0 M HCl at different concentrations of caffeine.The values of change in enthalpy (ΔHa∗) and change in entropy (ΔSa∗) were obtained, respectively, from the slopes and intercepts of the plots of log⁡W/T versus 1/T (Figure 9). All these parameters are collected in Table 3.Table 3
Dissolution parameters of the aluminium in 1.0 M HCl.
C
i
n
h (mM)
E
a (kJ mol−1)
Δ
H
a
∗ (kJ mol−1)
Δ
S
a
∗ (J mol−1K−1)
0
77.9
75.4
-
34.3
0.10
78.8
76.3
-
31.5
0.5
90.6
88.1
2.2
1
97.0
94.5
21.9
5
100.1
97.7
31.2
10
104.9
102.4
45.0Figure 9
Transition state plots of aluminium in 1.0 M HCl at different concentrations of caffeine.From Table3, one can notice that Ea and ΔHa∗ vary in the same way. This result permitted verifying the known thermodynamic relation between the two activation parameters:(24)ΔHa∗=Ea-RT.The activation energies Ea 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 ΔHa∗ 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 entropyΔSa∗ 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.


## 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 Orgsol and water molecules at the metal surface H2O(ads):(15)Orgsol+xH2Oads⇄Orgads+xH2Osolwhere x 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:(16)KadsCinh=fθ,xexp⁡-αθ,where fθ,x is the configurational factor, subject to the physical model and assumptions involved in the derivation of the isotherm, α is a molecular interaction parameter, Cinh is the inhibitor concentration, and Kads is the adsorption constant.Attempts were made to fit experimental data (θ and Cinh) to some classical isotherms including Langmuir, El-Awady, Freundlich, and Frumkin. By far the best fits were obtained with the isotherm of Langmuir (R2>0.99) and that of El-Awady (R2>0.98). In Langmuir isotherm, θ and Cinh are related by the following equation:(17)Cinhθ=1Kads+Cinh.Figure 4 depicts the plots of Cinh/θ versus Cinh.Figure 4
Langmuir adsorption plots for aluminium in presence of caffeine.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.Table 1
Parameters of the modified Langmuir adsorption isotherms.
Isotherm
Equation
T (K)
R
2
Slope
Intercept
K
a
d
s (×103 M)
Δ
G
a
d
s
0 (kJmol−1)
Villamil et al.
C
i
n
h
θ
=
n
K
a
d
s
+
n
C
i
n
h
303
0.994
1.3542
0.0007
2.768
-
30.1
308
0.999
1.6943
0.0005
5.648
-
32.4
313
0.998
1.8394
0.0007
4.598
-
32.3
318
0.994
1.9882
0.0012
3.314
-
32.0
323
0.995
2.3771
0.0014
3.962
-
33.0
El-Awady
log
⁡
θ
1
-
θ
=
log
⁡
K
′
+
y
log
⁡
C
i
n
h Kads=K′1/y
303
0.999
0.3421
1.1085
1.738
-
28.9
308
0.996
0.2442
0.6466
0.444
-
25.9
313
0.978
0.2413
0.5539
0.197
-
24.2
318
0.968
0.2435
0.4809
0.094
-
22.6
323
0.992
0.2226
0.2950
0.021
-
19.0The adsorption constantsKads have been calculated using the equations in Table 1. The changes in standard adsorption free enthalpy have been calculated using the following equation:(18)ΔGads0=-RTln⁡55.5×Kads,where R is the gas constant, T is the absolute temperature, and 55.5 is the concentration of water (in mol L−1) in the solution.In Villamil equation “n” is a constant introduced to consider all the factors, not taken into account in the derivation of Langmuir isotherm.The constanty in the isotherm of El-Awady is the number of active sites on the material surface; 1/y less than one implies a multilayer adsorption, while 1/y greater than one suggests that a given inhibitor molecule occupies more than one active site.It is clear from Table1 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 enthalpyΔHads0 and change in standard adsorption entropy ΔSads0, we used the basic equation:(19)ΔGads0=ΔHads0-TΔSads0.Plotting ΔGads0 versus temperature gives the two adsorption parameters (Figure 5). The negative values of ΔGads0 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.Figure 5
Δ
G
a
d
s
0 versus temperature for aluminium in presence of caffeine.Change in standard adsorption enthalpy (ΔHads0=-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 ΔGads0. It may be indicative of both physisorption and chemisorption processes.Change in standard adsorption entropy (ΔSads0=-462 Jmol−1K−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:(20)ln⁡θ=ln⁡θmax-aδ2,where θmax is the maximum surface coverage and δ is the Polanyi potential which is given by(21)δ=RTln⁡1+1Cinh.In this equation, R is the perfect gas constant, T is the absolute temperature, and Cinh is the concentration of the inhibitor expressed in g L−1. Figure 6 gives the plots of ln⁡θ versus δ2. The parameters of this model are in Table 2.Table 2
Parameters of the Dubinin-Radushkevich model.
T (K)
R
2
a (kJ−2 mol2)
θ
m
a
x
E
m (kJ mol−1)
303
0.991
0.0121
0.696
6.4
308
0.995
0.0081
0.582
7.9
313
0.984
0.0063
0.517
8.9
318
0.965
0.0063
0.466
8.9
323
0.974
0.0061
0.401
9.0Figure 6
Dubinin-Radushkevich isotherm in presence of caffeine.The value of the parametera in (20) leads to the mean adsorption energy Em 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. Em is defined as(22)Em=12a.In order to determine the range of temperatures for physisorption and chemisorption, we plot Em versus temperature (Figure 7).Figure 7
Adsorption energy versus temperature for caffeine onto aluminium.According to the literature [39] the magnitude of Em gives information about the adsorption: Em values less than 8 kJ mol−1 indicate physical adsorption, while that higher than 8 kJ mol−1 suggest chemisorption. Using the equation of the straight line in Figure 7, we derived the domains where each type of adsorption is predominant ((T<309.1 K for physisorption) and (T>309.1 K for chemisorption)). The decrease in θmax values confirms that the adsorption decreases with increasing temperatures.


## 3.1.2. Effect of Temperature

To determine the activation parameters of the corrosion process, the Arrhenius and the transition state equations were used:(23)log⁡W=log⁡A-Ea2,303RT,log⁡WT=log⁡Rℵh+ΔSa∗2,303R-ΔHa∗2.303RT,where Ea is the apparent activation energy, R is the perfect gas constant, A is the frequency factor, h is Planck’s constant, ℵ is the Avogadro number, ΔHa∗ is the change in activation enthalpy, and ΔSa∗ is the change in activation entropy.Values of apparent activation energy of corrosion (Ea) for aluminium in HCl in the absence and presence of various concentrations of caffeine were determined from the slope of log⁡W versus 1/T plots (Figure 8).Figure 8
Arrhenius plots for aluminium in 1.0 M HCl at different concentrations of caffeine.The values of change in enthalpy (ΔHa∗) and change in entropy (ΔSa∗) were obtained, respectively, from the slopes and intercepts of the plots of log⁡W/T versus 1/T (Figure 9). All these parameters are collected in Table 3.Table 3
Dissolution parameters of the aluminium in 1.0 M HCl.
C
i
n
h (mM)
E
a (kJ mol−1)
Δ
H
a
∗ (kJ mol−1)
Δ
S
a
∗ (J mol−1K−1)
0
77.9
75.4
-
34.3
0.10
78.8
76.3
-
31.5
0.5
90.6
88.1
2.2
1
97.0
94.5
21.9
5
100.1
97.7
31.2
10
104.9
102.4
45.0Figure 9
Transition state plots of aluminium in 1.0 M HCl at different concentrations of caffeine.From Table3, one can notice that Ea and ΔHa∗ vary in the same way. This result permitted verifying the known thermodynamic relation between the two activation parameters:(24)ΔHa∗=Ea-RT.The activation energies Ea 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 ΔHa∗ 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 entropyΔSa∗ 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.


## 3.2. Quantum Chemical Approach

The quantum chemical parameters of caffeine obtained from the calculations include the energy of the highest occupied molecular orbital (EHOMO), the energy of the lowest unoccupied molecular orbital (ELUMO), the energy gap (ΔE=ELUMO-EHOMO), the dipole moment, μ and the total energy (TE). Based on frontier molecular orbital (FMO), the reactivity parameters such as the ionization energy (I), the electronic affinity (A), the global electronegativity (χ), the global hardness (η), the global softness (S), the fraction of electrons transferred (ΔN), and the electrophilicity index (ω) were also calculated. All these parameters are listed in Table 4.Table 4
Molecular properties of caffeine calculated with B3LYP/6-31G (d).
Parameter
Value
E
H
O
M
O (eV)
-
5.959
E
L
U
M
O (eV)
-
0.819
Δ
E (eV)
5.140
I (eV)
5.959
A (eV)
0.819
χ (eV)
3.389
η (eV)
2.570
S (eV−1)
0.389
μ (Debye)
3.835
Δ
N
0.173
ω (eV)
2.234
TE (a. u)
-
680.4The 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 (EHOMO=-5.959 eV) and a low value of ELUMO (ELUMO=-0.819 eV) when compared with values in the literature [15, 44]. So, the incomplete filled 3p of aluminium (electronic structure: 1s22s22p63s23p1) could bond with the HOMO of caffeine while the filled 3s orbital could interact with its LUMO.The energy gap (ΔE=ELUMO-EHOMO) is an important parameter related to the reactivity of an inhibitor towards the adsorption onto a metallic surface. Lower values of ΔE [45] suggest better adsorption and then better inhibition efficiency. In our case (ΔE=5.140 eV) can be considered [46–48] as a low value when compared with other values in the literature.The dipole momentμ 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 (η) and softness (S) 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 (η=2.570 eV) which can be considered as a low value [54] could explain the inhibiting properties of caffeine.The number(ΔN) of electrons transferred is a parameter which indicates the tendency of a molecule to donate electrons. The higher the value of ΔN, the greater the tendency of the molecule to donate electrons to the metal. In our case (ΔN=0.173), the positive sign shows that the molecule could donate electrons to the metal.The electrophilicity index(ω) 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 ω=2.234 eV shows that caffeine has a good capacity to accept electrons from the metal.TheHOMO and LUMO orbital densities distributions are given in Figure 10.Figure 10
HOMO (a) and LUMO (b) densities of caffeine.
(a)
(b)In order to ascertain the role of individual atoms in the molecule, its local parameters including Mulliken chargesqN+1, qN, and qN-1, Fukui functions fk+ and fk-, and local softness sk+ and sk- were calculated. All these parameters are listed in Table 5.Table 5
Local parameters of Caffeine.
Atom
q
N
+
1
q
N
q
N
-
1
f
k
+
f
k
-
s
k
+
s
k
-
s
k
+
/
s
k
-
s
k
-
/
s
k
+
1 C
0.4259
0.4753
0.5423
−0.0494
−0.0670
−0.0192
−0.0261
0.7370
1.3568
2 C
0.1933
0.2179
0.3044
−0.0247
−0.0864
−0.0096
−0.0336
0.2855
3.5029
3 C
0.5084
0.6324
0.6942
−0.1240
−0.0618
−0.0482
−0.0240
2.0059
0.4985
4 C
0.7588
0.7852
0.8159
−0.0264
−0.0307
−0.0103
−0.0119
0.8595
1.1634
5 C
0.0637
0.2199
0.2809
−0.1562
−0.0610
−0.0608
−0.0237
2.5623
0.3903
6 H
0.0638
0.1695
0.2487
−0.1058
−0.0792
−0.0411
−0.0308
1.3351
0.7490
7 N
−0.5530
−0.5740
−0.5772
0.0213
0.0029
0.0083
0.0011
7.2401
0.1381
8 N
−0.5700
−0.5900
−0.5595
0.0198
−0.0302
0.0077
−0.0117
−0.6565
−1.5233
9 N
−0.4880
−0.4840
−0.5009
−0.0042
0.0168
−0.0016
0.0065
−0.2478
−4.0357
10 C
−0.2910
−0.3150
−0.3382
0.0239
0.0235
0.0093
0.0091
1.0177
0.9826
11 H
0.1754
0.1982
0.2316
−0.0228
−0.0335
−0.0089
−0.013
0.6820
1.4663
12 H
0.1284
0.1773
0.2196
−0.0489
−0.0423
−0.0190
−0.0165
1.1562
0.8649
13 H
0.1273
0.1772
0.2196
−0.0499
−0.0423
−0.0194
−0.0165
1.1787
0.8484
14 C
−0.3010
−0.3220
−0.3500
0.0208
0.0282
0.0081
0.0110
0.7355
1.3597
15 H
0.1389
0.1977
0.2396
−0.0588
−0.0420
−0.0229
−0.0163
1.4000
0.7143
16 H
0.1410
0.1816
0.2391
−0.0407
−0.0575
−0.0158
−0.0224
0.7072
1.4140
17 H
0.1690
0.1816
0.2391
−0.0126
−0.0575
−0.0049
−0.0224
0.2187
4.5734
18 C
−0.2960
−0.3150
−0.3700
0.0195
0.0546
0.0076
0.0213
0.3568
2.8029
19 H
0.1811
0.1990
0.2571
−0.0179
−0.0581
−0.0070
−0.0226
0.3081
3.2461
20 H
0.1049
0.1991
0.2301
−0.0942
−0.0310
−0.0366
−0.0120
3.0417
0.3288
21 H
0.1069
0.1662
0.2300
−0.0593
−0.0638
−0.0231
−0.0248
0.9297
1.0756
22 O
−0.6400
−0.5400
−0.4402
−0.0994
−0.1002
−0.0387
−0.0390
0.9928
1.0072
23 O
−0.5990
−0.5270
−0.4107
−0.0728
−0.1159
−0.0283
−0.0451
0.6276
1.5933
24 N
−0.5490
−0.5110
−0.4456
−0.0374
−0.0657
−0.0145
−0.0256
0.5691
1.7573The analysis of Table5 shows that carbonC (10) is the probable site for nucleophilic attacks, whereas carbonC (14) is the probable site for electrophilic attacks. However, according to the literature [56], the local parameters fk+, fk-, sk+, and sk- are influenced by the basis sets. So, it is judicious to use relative indexes such as the relative nucleophilicity (sk+/sk-) and the relative electrophilicity (sk-/sk+). So, in our studyN (7) which has the highest value of relative nucleophilicity index sk+/sk-=7.240 could be the probable nucleophilic attack site whereasC (2) with the highest value of relative electrophilicity sk-/sk+=3.503 could be the probable site of electrophilic attack.Figure10 allows verifying the belonging of each atom k 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.Figure 11
Schematic mechanism of aluminium corrosion inhibition in 1.0 M HCl by caffeine.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.


## 4. 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 ofΔGads0 suggests a spontaneous adsorption process. The values of change in adsorption free enthalpy ΔGads0 and that of the activation energy Ea indicate the presence of both physisorption and chemisorption. The quantum chemical calculations are in good agreement with experimental results.



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