Experimental and theoretical studies on some amino acids and their potential activity as inhibitors for the corrosion of mild steel, part 2

Substituent constants and quantum chemical parameters were calculated from PM6, PM3, AM1, RM1 and MNDO. Hamiltonians were used to predict the corrosion inhibition potential of nine amino acids grouped under three skeletons. Skeleton I consisted of cysteine (CYS), serine (SER) and amino butyric acid (ABU). Those in skeleton II included threonine (THR), alanine (ALA) and valine (VAL) while those in skeleton III are aromatic amino acids, which included phenylalanine (PHE), tryptophan (TRP) and tyrosine (TYR). Trends obtained from substituent constants were not entirely useful in predicting the corrosion inhibition potentials of the studied amino acids. However, the results obtained from quantum chemical parameters indicated that the trends for the variation of corrosion inhibition potentials of the studied amino acids in skeletons I, II and III are CYS > SER > ABU, THR > ALA > VAL and TRP > TYR > PHE, respectively. Highest values of inhibition efficiency were obtained for inhibitors in skeleton III and are attributed to the presence of aromatic ring in the molecule while the corrosion inhibition potential of inhibitors in skeletons I and II are attributed to the presence of –SH and –OH functional groups, respectively.

ORIGINAL ARTICLE

Experimental and theoretical studies on some amino

acids and their potential activity as inhibitors for

Nnabuk O Eddy

Department of Chemistry, Ahmadu Bello University, Zaria, Kaduna State, Nigeria

Received 11 January 2010; revised 9 April 2010; accepted 4 July 2010

Available online 25 October 2010

KEYWORDS

Corrosion;

Inhibition;

Amino acids;

Computational chemistry

study

Abstract Substituent constants and quantum chemical parameters were calculated from PM6, PM3, AM1, RM1 and MNDO Hamiltonians were used to predict the corrosion inhibition poten-tial of nine amino acids grouped under three skeletons Skeleton I consisted of cysteine (CYS), ser-ine (SER) and amino butyric acid (ABU) Those in skeleton II included threonser-ine (THR), alanser-ine (ALA) and valine (VAL) while those in skeleton III are aromatic amino acids, which included phen-ylalanine (PHE), tryptophan (TRP) and tyrosine (TYR) Trends obtained from substituent con-stants were not entirely useful in predicting the corrosion inhibition potentials of the studied amino acids However, the results obtained from quantum chemical parameters indicated that the trends for the variation of corrosion inhibition potentials of the studied amino acids in skeletons

I, II and III are CYS > SER > ABU, THR > ALA > VAL and TRP > TYR > PHE, respec-tively Highest values of inhibition efficiency were obtained for inhibitors in skeleton III and are attributed to the presence of aromatic ring in the molecule while the corrosion inhibition potential

of inhibitors in skeletons I and II are attributed to the presence of –SH and –OH functional groups, respectively Analysis of data obtained from relative nucleophilicity/electrophilicity, condensed Fukui and softness functions indicated that the sites for electrophilic attacks for the amino acids

in skeletons I and II are in the amine bonds but for those in skeleton III the sites were in their

q

Eddy NO Part 3 Theoretical study on some amino acids and their

potential activity as corrosion inhibitors for mild steel in HCl Mol

Simul 2010;36(5):354–63.

E-mail addresses: nabukeddy@yahoo.com , nabukeddy@gmail.com

2090-1232 ª 2010 Cairo University Production and hosting by

Elsevier B.V All rights reserved.

Peer review under responsibility of Cairo University.

doi: 10.1016/j.jare.2010.08.005

Production and hosting by Elsevier

Journal of Advanced Research (2011) 2, 35–47

Cairo University Journal of Advanced Research

respective phenyl ring The author proposed that quantum chemical parameters may be used to predict the corrosion inhibition potentials of amino acids

ª 2010 Cairo University Published by Elsevier Ltd All rights reserved.

Introduction

Corrosion is a serious environmental problem in the oil,

fertil-izer, metallurgical and other industries[1–4] Valuable metals,

such as mild steel, aluminium, copper and zinc are prone to

corrosion when they are exposed to aggressive media (such

as acids, bases and salts)[5–7] Therefore, there is a need to

protect these metals against corrosion The use of inhibitors

has been found to be one of the best options available for the protection of metals against corrosion [8] The most effi-cient corrosion inhibitors are organic compounds containing electronegative functional groups and p electrons in their triple

or conjugated double bonds[9] The initial mechanism in any corrosion inhibition process is the adsorption of the inhibitor

on the metal surface[10–13] The adsorption of the inhibitor

on the metal surface can be facilitated by the presence of

NH2 HS

O

OH

NH 2

HO

O

OH H 2 N

O

OH

NH2

NH2

OH

NH2

O

OH

NH2

O

OH

NH2 H

NH2 HO

O

OH

NH2

O OH

Fig 1 Chemical and optimised structures of studied amino acids

hetero atoms (such as N, O, P and S) as well as aromatic ring.

The inhibition of the corrosion of metals can also be viewed as

a process that involves the formation of chelate on the metal

surface, which involves the transfer of electrons from the

or-ganic compounds to the surface of the metal and the formation

of a coordinate covalent bond In this case, the metal acts as an

electrophile while the nucleophilic centre is in the inhibitor

Literature reveals that a wide range of compounds have

been successfully investigated as potential inhibitors for the

corrosion of metals[14–17] However, a close examination of

these compounds indicates that some of them are toxic to the

environment while others are expensive These and many other

factors have prompted a continuing search for better inhibitors

Possibilities include plant extracts, some drugs and other

natu-ral occurring products[18–24] It is interesting to note that

ami-no acids are components of living organisms and are precursors

for protein formation Several researchers have investigated the

inhibitory potential of some amino acids and the results

ob-tained from such studies have given some hope for the use of

amino acids as green corrosion inhibitors[25–31]

The present study is aimed at correlating the electronic and

molecular structures of three classes of amino acids (described

as skeletons I, II and III) with their corrosion inhibition

poten-tial Amino acids chosen for skeleton I shall include cysteine

(CYS), serine (SER) and amino butyric acid (ABU) Those

in skeleton II shall include threonine (THR), alanine (ALA)

and valine (VAL), while those in skeleton III shall consist of

the aromatic amino acids, which include, phenylalanine

(PHE), tryptophan (TRP) and tyrosine (TYR) The chemical

and optimised structures of the amino acids chosen for the

study are presented inFig 1

Experimental

Materials

Materials used for the study were mild steel sheet of

composi-tion (wt%); Mn (0.6), P (0.36), C (0.15) and Si (0.03) and the

rest Fe The sheet was mechanically pressed cut into coupons

of dimensions 5· 4 · 0.11 cm Each coupon was degreased

by washing with ethanol, dipped in acetone and allowed to

dry in the air before it was preserved in a desiccator All

reagents used for the study were Analar grade and double

Table 1 Experimental inhibition efficiencies of the studied

amino acids

Skeleton 1

Skeleton II

Skeleton III

Skeleton I

-3.9 -3.8 -3.7 -3.6 -3.5 -3.4 -3.3 -3.2

logC

CYS SER ABU

Skeleton III

-4 -3.9 -3.8 -3.7 -3.6 -3.5 -3.4 -3.3 -3.2

logC

TRP TYR PHE

Skeleton II

-3.7 -3.6 -3.5 -3.4 -3.3 -3.2 -3.1 -3

logC

THR ALA VAL

Fig 2 Langmuir isotherms for the adsorption of the studied inhibitors on mild steel surface

Table 2 Langmuir parameters for the adsorption of the studied amino acids on mild steel surface

Inhibitor Slope log K DG 0

ads (kJ/mol) R 2

distilled water was used for their preparation The test

solu-tions were prepared by dissolving 0.01, 0.02, 0.03 and

0.04 mol of the respective amino acids in 0.1 M H2SO4

Gravimetric method

In the gravimetric experiment, a previously weighed metal

(mild steel) coupon was completely immersed in 250 ml of the

test solution in an open beaker The beaker was covered with

aluminium foil and inserted into a water bath maintained at

303 K Every 24 h the corrosion product was removed by

wash-ing each coupon (withdrawn from the test solution) in a

solu-tion containing 50% NaOH and 100 g l1 of zinc dust The

washed coupon was rinsed in acetone and dried in the air before

re-weighing The difference in weight for a period of 168 h was

taken as the total weight loss From the average weight loss

results (average of three replicate analyses), the inhibition

effi-ciency (Eexp) of the inhibitor and the degree of surface coverage

were calculated using Eqs.(1)and (2), respectively,

where W1and W2are the weight losses (g) for mild steel in the

presence and absence of the inhibitor and h is the degree of

sur-face coverage of the inhibitor

Computational details

Quantum chemical calculations were carried out using PM6,

PM3, AM1, RM1, and MNDO semi-empirical (SCF-MO)

methods in the MOPAC 2008 program Calculations were

per-formed for both gas and aqueous phases using an HP

compat-ible Pentium V (2.0 GHz and 4 GB RAM) computer The

following quantum chemical indices were calculated: the

en-ergy of the highest occupied molecular orbital (EHOMO), the

energy of the lowest unoccupied molecular orbital (ELUMO),

the energy gap (EL–H), the dipole moment (l), the total energy

(TE) and dielectric energy (Edielec) Ab initio parameters

(Muliken and Lowdin charges on the atoms) were computed

using the MP2 correlation type/method and B3LYP-6-31G**

Basis in the GAMESS program Statistical analyses were

per-formed using SPSS program version 15.0 of Windows while all

structures were drawn and optimised using the Chem3D

pack-age in the Ultra Chem 2008 version

Results and discussion

Experimental results

Table 1presents values of inhibition efficiencies for the studied

amino acids From the results, it can be seen that the inhibition

efficiency of the studied amino acids increases with the

increas-ing concentration, which suggests that the studied amino acids

are adsorption inhibitors.Table 1also reveals that for skeleton I,

the trend for the variation of inhibition efficiency is

CY-S > CY-SER > ABU The corresponding trends for skeletons II

and III are THR > ALA > VAL and TRP > TYR > PHE,

respectively

The adsorption characteristics of the studied inhibitors

were investigated by the fitting data obtained for the degree

of surface coverage into different adsorption isotherms

including Langmuir, Temkin, Freundlich, Florry Huggins, Bockris-Swinkel and Frumkin adsorption isotherms The tests indicated that the adsorption of the studied amino acids on a mild steel surface is best described by the Langmuir adsorption model, which can be expressed as follows:

where C is the concentration of the inhibitor in the bulk elec-trolyte and K is the equilibrium constant of adsorption.Fig 2 presents the Langmuir isotherms for the adsorption of the studied amino acids Values of adsorption parameters deduced from the isotherms are presented inTable 2 From the results obtained, the slopes and R2values for the plots are closer to unity, indicating that the adsorption of the studied amino acids

is consistent with the Langmuir adsorption model

The equilibrium constant of adsorption deduced from the Langmuir adsorption isotherm is related to the free energy

of adsorption of the inhibitor as follows:

DG0

where K is the equilibrium constant of adsorption, 55.5 is the molar concentration of water, DG0

ads is the free energy of adsorption of the inhibitor, R is the gas constant and T is the temperature Calculated values of the free energy are recorded

inTable 2 From the results obtained, the free energies are neg-atively less than the range of value (20 to 40 kJ/mol) ex-pected for the mechanism of chemical adsorption Therefore, the adsorption of the studied amino acids on a mild steel surface

is spontaneous and is consistent with the mechanism of electro-static transfer of charge from the charged inhibitor’s molecule

to the charged metal surface, which supports physiosorption Theoretical study

Substituent constants Values of substituent constants calculated for the studied

ami-no acids are presented inTable 3 According to Lukovitis et al [32], substituent constants are empirical quantities which ac-count for variations of the structure once the parent structures are identical This implies that the substituent constants do not depend on the parent structure but vary with the substituent Together with other substituent constants (i.e., C log P,

MR, tPSA and CMR), log P accounts for the hydrophobicity

of a molecule The higher the value of log P, the more hydro-phobic is the molecule; hence, water solubility is expected to

Table 3 Substituent constants for some amino acids

Inhibitor log P C log P tPSA MR (cm 3 /mol) CMR

Table 4 Quantum chemical parameters for the studied amino acids in gas phase.

Skeleton I

CYS

SER

ABU

Skeleton II

THR

ALA

VAL

Skeleton III

TRP

TYR

PHE

decrease with increasing values of log P From the point of

view of the corrosion inhibition process, the processes of

inhi-bition that are affected by hydrophobicity are not well

estab-lished However, Lukovitis et al [32] stated that it is most

probable that hydrophobicity can be used to predict the

mech-anism of formation of the oxide/hydroxide layer on the metal

surface (which reduces the corrosion process drastically)

From the results obtained, the inhibition efficiency of the

stud-ied amino acids is better predicted by the variation in the

val-ues of C log P (for skeleton I), CMR (for skeleton III) and MR

(for skeleton II) This suggests that the substituent constants

are not unique parameters for predicting the direction of the

corrosion inhibition potential of the studied amino acids

Global reactivity

Table 4present values of some quantum chemical parameters

calculated for the studied amino acids in gas and aqueous

phases, using various Hamiltonians (PM6, PM3, AM1, RM1

and MNDO) The frontier molecular orbital energies (energy

of the highest occupied molecular orbital, EHOMO, and that

of the lowest unoccupied molecular orbital, ELUOMO) are

important parameters for defining the reactivity of a chemical

species A good correlation has been found between corrosion

inhibition efficiency and some quantum chemical parameters

including EHOMOand ELUMO EHOMOis associated with the

disposition of the inhibitor’s molecule to donate electrons to

an appropriate acceptor with an empty molecular orbital

Therefore, an increase in the value of EHOMO can facilitate

the adsorption and, therefore, better inhibition efficiency On

the other hand, ELUMOindicates the ability of the inhibitor’s

molecule to accept electrons, which implies that the inhibition

efficiencies of the studied amino acids are expected to increase

with decreasing values of ELUMO[33–35] From the results

ob-tained for EHOMOand ELUMO, it can be stated that the

inhibi-tion efficiencies of the studied amino acids are consistent with

the trend obtained from experimental results

If it is assumed that after physical adsorption,

chemisorp-tion of organic molecules occurs due to chelachemisorp-tion on metal

sur-face by donation of electrons to unoccupied d-orbital of the

metal and the subsequent acceptance of the electrons from

the d-orbital, using antibonding molecular orbital, then the

formation of a feedback bond would be characterised by the

increasing values of EHOMOand the decreasing values of E

LU-MO,which is proposed for the observed trend The energy gap

(DE = EHOMO ELUMO) of an inhibitor is another parameter

that can be used to predict the extent of corrosion inhibition

Larger values of the energy gap imply low reactivity to a

chem-ical species From the results of the study, the inhibition

effi-ciencies of the studied amino acids were found to increase

with the decreasing values of the energy gap and the trend is

consistent with experimental results[36]

Tables 4also presents the calculated values of dipole moment

(l) for various semi-empirical models Based on the decrease in

dipole moment of the amino acid, the expected trend for the

variation of inhibition efficiency is also consistent with the trend

deduced from frontier molecular orbital energies[37]

In Fig 3, representative plots showing the variation of

quantum chemical parameters with experimental inhibition

efficiency are presented From the plots, it is evident that there

is a strong correlation (R2 1) between the experimental

inhibition efficiencies and EHOMO, ELUMO, EL–H, dielectric en-ergy (Edielect) and dipole moment (l) These findings are also applicable to data obtained for gas and aqueous phases (Table 5) From the values of the ground state energy of the systems, the ionization energy (IE) and the electron affinity (EA) of the amino acids were calculated using Eqs.(5) and (6), respectively [38,39],

IE¼ EðN1Þ EðNÞ ð5Þ

EA¼ EðNÞ EðNþ!Þ ð6Þ

where E(N1), E(N)and E(N+1)are the ground state energies of the system with N 1, N and N + 1 electrons, respectively Calculated values of IE and EA (for gas and aqueous phases) are presented inTable 6 Values of IE calculated from Eq.(5) compare favourably with those obtained from semi-empirical calculations for both gas and aqueous phases Moreover, the expected trend for the variation of inhibition efficiencies is also consistent with the experimental results The close similarity between the values of IE and EHOMOand also between the val-ues of EA and ELUMO can be explained as follows Semi-empirical calculations estimate ionization energy and electron

Skeleton III

R 2 = 0.9999

50 60 70 80 90 100

LUMO energy (eV)

IEexp

Skeleton II

R 2 = 0.953

0 10 20 30 40 50 60 70 80

LUMO energy (eV)

IEexp

Skeleton I

R 2 = 0.9839

50 60 70 80 90 100

Lumo energy (eV)

IEexp

Fig 3 Variation of experimental inhibition efficiency with the energy of the LUMO for skeletons I, II and III

affinity through the value of EHOMOand ELUMO, respectively.

On the other hand, Eqs.(5) and (6)are based on the finite

dif-ference methods Ionization energy measures the tendency

to-ward loss of electrons while electron affinity measures the

tendency toward the acceptance of electrons Therefore, IE is

closely related with EHOMOwhile EA is related to ELUMO In

this case, two systems, Fe (in mild steel) and inhibitor are

brought together, hence, electrons will flow from the lower

sys-tem with lower electronegativity (inhibitor) to the syssys-tem with

higher electronegativity until the chemical potential becomes

equal Based on the decreasing value of IE and the increasing

value of EA, the trend for the variation of inhibition potentials

of the studied amino acids agrees with experimental findings

Global softness (S) of the inhibitors was estimated using the

finite difference approximation, which can be expressed as

fol-lows[40],

S¼ 1=½ðEðN1Þ EðNÞÞ  ðEðNÞ EðNþ!ÞÞ ð7Þ

On the other hand, global hardness, g is the inverse of global

softness and is given as g = 1/S.Table 6also presents the

cal-culated values of IP, EA, S and g for the studied amino acids in

gas and aqueous phases Global hardness and softness are

re-lated to the energy gap (DE) of a molecule because a hard

mol-ecule has a large energy gap while a soft molmol-ecule has a small

energy gap implying that a soft molecule is more reactive than

a hard molecule From the results presented inTable 6, g

val-ues are relatively lower for CYS (in skeleton I), THR (in

skel-eton II) and TRP (in skelskel-eton III) indicating that the best

inhibitors are characterised by lower values of global hardness

but higher values of global softness These findings support the

results obtained from the experiment

The fraction of electron transferred, d, can be expressed as

follows[41],

where vFeand vinhare the electronegativity of the inhibitor and

Fe, respectively v = (IP + EA)/2 g and g are the global

hardness of Fe and the inhibitor, respectively In order to val-idate Eq.(8)for this study, the theoretical values of vFe= 7 eV and gFe= 0 were used for the computation of d values re-corded inTable 6 Calculated values of d obtained for the stud-ied amino acids appear to be relatively higher for the inhibitors that have better inhibition potential

Local selectivity The local selectivity of an inhibitor can be analysed using densed Fukui and condensed softness functions The con-densed Fukui function and the concon-densed softness functions are indices that allow for the distinction of each part of a mol-ecule on the basis of its chemical behaviour due to different substituent functional groups The Fukui function is stimu-lated by the fact that if an electron d is transferred to an N elec-tron molecule, it will tend to distribute so as to minimize the energy of the resulting N + d electron system The resulting change in electron density is the nucleophilic and electrophilic Fukui functions, which can be calculated using the finite differ-ence approximation as follows[42],

where q, is the density of electron q(N+1),q(N)and q(N1)are the Milliken or Lowdin charges of the atom with N + 1, N and N1 electrons, respectively Calculated values of f+

and

ffor the carbon, nitrogen and oxygen atoms in cysteine, ser-ine and phenylalanser-ine molecules are presented inTable 7 It is expected that the site for nucleophilic attack is the place where the value of f+ is maximum while the site for electrophilic attack is controlled by the value of f Table 8presents the Huckel charges on carbon and other electronegative atoms

in the studied amino acids Considering that the protonated forms of the inhibitors have a net positive charge, the site for electrophilic attacks can be analysed as follows

Table 5 R2values between calculated quantum chemical parameters in gas phase (aqueous phase) and the experimental inhibition efficiencies

E L–H (eV) E LUMO (eV) E HOMO (eV) l (eV) E L–H (eV) E LUMO (eV) E HOMO (eV) l (eV) E Hyd (eV) Skeleton I

Skeleton II

Skeleton III

Table 6 Calculated quantum descriptors for the studied amino acids in gas and aqueous phase.

IE (eV) EA (eV) v (eV) S (eV) g (eV) d IE (eV) EA (eV) v (eV) S (eV) g (eV) d Skeleton I

CYS

SER

ABU

Skeleton II

THR

ALA

VAL

Skeleton III

TRP

TYR

PHE

Table 7 Global and local selectivity parameters for N, O and C atoms in some amino acids (calculated from MP2-6-31G).

CYS

CYS

SER

ABU

THR

VAL

TYR

TRP

In CYS, the site for electrophilic attack is in the amine bond

(i.e., N2–C3) whose bond length is 1.435 A˚, while the site for

nucleophilic attack is in the thiol bond (i.e., C4–S5, bond

length = 1.815 A˚) It is an established fact that heteroatoms

(such as S, N, O and P) in an inhibitor provide the centre

for the adsorption of an inhibitor on the metal surface From

the Huckel charges of the atoms in CYS (Table 8), it can be

seen that the charges on the amine bond are more positive than

the charges on the thiol bond Therefore, the inhibitor is

pref-erentially adsorbed through the amine bond On the other

hand, the charges on the thiol bond are more negative than

the charges on the amine bond; therefore, the thiol bond is

the centre for nucleophilic attack It can also be stated that

the bond lengths in the amine and thiol bonds are shorter than the expected bond length indicating that there is conjugation For reasons explained for CYS, the sites for the electro-philic and nucleoelectro-philic attacks in SER and ABU are similar

in the amine and thiol bonds However, in ABU, the site for the nucleophilic attack is in the amine bond This shift may

be attributed to the influence of the two carbonyl oxygen atoms in ABU For compounds in skeleton III, the sites for electrophilic and nucleophilic attacks are also in the respective amine bonds except in valine where the nucleophilic centre is in C5

In skeleton III, the sites for electrophilic attacks in TYR and PHE are in their respective phenyl carbon atoms (i.e.,

Table 7 (continued)

PHE

Table 8 Huckel charges on carbon and electronegative elements in the studied amino acids