Accepted Manuscript

Molecular dynamics simulation of inhibition mechanism of 3, 5-dibromo sali-

cylaldehyde schiffs base

Si-Wei Xie, Zheng Liu, Guo-Cheng Han, Wei Li, Jin Liu, ZhenCheng Chen

PII: S2210-271X(15)00146-2

DOI: http://dx.doi.org/10.1016/j.comptc.2015.04.003

Reference: COMPTC 1786

To appear in: Computational & Theoretical Chemistry

Received Date: 24 June 2014

Revised Date: 6 March 2015

Accepted Date: 5 April 2015

Please cite this article as: S-W. Xie, Z. Liu, G-C. Han, W. Li, J. Liu, Z. Chen, Molecular dynamics simulation of

inhibition mechanism of 3, 5-dibromo salicylaldehyde schiffs base, Computational & Theoretical Chemistry

(2015), doi: http://dx.doi.org/10.1016/j.comptc.2015.04.003

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Molecular dynamics simulation of inhibition mechanism of

3, 5-dibromo salicylaldehyde schiff's base

Si-Wei Xiea, Zheng Liu

a*, Guo-Cheng Han

b**, Wei Li

a, Jin Liu

a, ZhenCheng Chen

b

a College of Chemical and Biological Engineering, Guilin University of Technology, Guilin 541004,Guangxi, PR

China

bSchool of Life and Environmental Sciences, Guilin University of Electronic Technology, Guilin 541004, P.R.

China

*Corresponding author: Tel.: +86 773 589 6453; E-mail:lisa4.6@163.com (Z.Liu)

** Corresponding author: Tel.: +86 773 221 7609; E-mail: hangc1981@163.com(G.-C. Han)

Abstract: Molecular dynamics simulation method was adopted to investigate the absorption

behavior, inhibition mechanisms on Fe(1 0 0) surface in aqueous solution and the diffusion

behavior of H3O+, Cl

- and H2O in three of 3, 5-dibromo salicylaldehyde Schiff base inhibitor

films, including 3, 5-dibromo salicylaldehyde-2-pyridinecarboxylic acid hydrazide (L1),

3,5-dibromo salicylaldehyde-2-thiol-phenecarboxylic acid hydrazide(L2), 3,5-dibromo

salicylaldehyde-2-aminobenzothiazole (L3). The effects of the interaction energy, radial

distribution function and the self-diffusion coefficient were studied accompanying with

density functional theory (DFT) study. The results demonstrated that the order of adsorption

energy is E(L2)>E(L1)>E(L3), absorption manner is a multi-center chemical adsorption for

three inhibitor films; for different inhibitor films, the diffusion coefficients followed the order

of D(L3)>D(L1)>D(L2) for the Cl- corrosive particles, the diffusion coefficients followed the

order of D (L3)> D (L1)> D (L2) for the H3O+ corrosive particles. For the three inhibitor films,

the diffusion coefficients of the three corrosive particles all followed the order of

D(H2O)>D(H3O+)>D(Cl

-). The inhibition efficiency order was obtained from the diffusion

coefficient which is agreed well with the experimental results as EI (L2)> EI (L1)> EI (L3).

Three kinds of inhibitor films have good corrosion inhibition efficiency in aqueous solution.

Key words: Molecular dynamics simulation; Schiff base inhibitor; Inhibition mechanism;

Adsorption energy

1. Introduction

Nowadays, computer technology is renewed by the rapid development of science and

technology, accompanying with hot topic of organic inhibitors mechanism study in corrosion

science. It is very useful to develop new inhibitors and study their adsorption behaviors and

inhibition effects. Molecular dynamics simulations (MD) can be carried out to study the

dynamic adsorption process and inhibition mechanism of inhibitor molecules adsorption film

on the metal surface effectively [1-4]. There are some reports focused on corrosion inhibitors

by molecular dynamics simulation [5,6]. Zeng and coworkers studied inhibition effect of

inhibitors on iron, such as PASP, HPMA and PESA in aqueous solution, the results show that

three kinds of corrosion inhibitors can attach on iron surface with good inhibition effects [7].

Khaled and coworkers studied three kinds of corrosion inhibitors adsorption mechanism of

META, PARA and ORTHO on nickel surface [8]. Zhang and coworkers investigated the

adsorption mechanism of imidazoline inhibitor on the Fe surface [9]. Khaled studied the

adsorption behavior and inhibition mechanism of benzimidazole derivatives on the Fe

surface[10]. Formerly, our group studied the adsorption mechanism of dehydroabietylamine

Schiff base derivatives on the carbon steel surface in sea water [11].

It should be mentioned that corrosion always occurs when the corrosive medium is in

contact with the metal in the process of oil production. Hopefully, inhibitor molecules could be

adsorbed on the metal surface in order to separate water molecules and metal ions in solution to

stop corrosion. When the metal surface is covered by water molecules in solution, the inhibitor

molecules can be adsorbed on the metal surface, they need to expel water molecules on the

metal surface and show stronger adsorption energy than water molecules. The inhibitor

molecules not only obstruct water molecules on the metal surface, but also obstruct water

molecules in aqueous solution. It is foreseeable that the adsorption process will be more

difficult and longer in solution than in vacuum [12-14].

Inhibitor adsorption performance is an important factor to evaluate performance of

corrosion inhibitors. To be a qualified corrosion inhibitor, it not only has good adsorption

performance, but also can form film which can stop the corrosion ions move to metal surface.

Because that inhibitor molecules film can effectively prevent corrosion ions migrate to the

metal surface, avoid corrosion ions contacting with the metal directly, the best corrosion effect

of inhibitors are contributed by the best adsorption performance and the strongest inhibition

[15-18]. Therefore, corrosion inhibitors must have strong ability to inhibit corrosion ions

diffusion in oilfield water [19].

In this paper, molecular dynamics simulation method was adopted to investigate the

absorption behavior, inhibition mechanisms on Fe(1 0 0) surface in aqueous solution and the

diffusion behavior of H3O+, Cl

- and H2O in three of 3, 5-dibromo salicylaldehyde Schiff base

inhibitor films, including 3, 5-dibromo salicylaldehyde- 2- pyridinecarboxylic acid hydrazide

(L1), 3,5-dibromo salicylaldehyde-2-thiol-phenecarboxylic acid hydrazide(L2), 3,5-dibromo

salicylaldehyde-2- aminobenzothiazole (L3). Three key parameters, including interaction

energy, radial distribution function curves and self-diffusion coefficient were selected to

analyze the corrosion inhibition mechanism. The inhibitive properties of these three schiff's

base inhibitors were investigated experimentally, using measurements based on weight loss,

Tafel polarization, electrochemical impedance spectroscopy.

2. Computational methods

The diffusion behaviors of the three corrosive particles in the three corrosion inhibitor

films was carried out using a commercial software package called Materials Studio 5.5

developed by Accelrys Inc.

2.1 Absorption energy calculation details

2.1.1 Establishment of calculation model

Most importantly, it needs to establish the calculation model. First, a Fe lattice structure

was built and a Fe (1 0 0) surface was created and cut into 8 layers, a supercell which contains

580 Fe atoms was built with size of 22.9322.9311.46. Second, a supercell which contains

500 water molecules and a Schiff base molecule was created with size of 22.9322.9328.44.

Third, create a supercell which contains 250 water molecules was created with size of

22.9322.9314.22 . All atoms are fixed in the process of simulation layers, the entire model

size is 22.9322.9361.84 (Fig. 1). Fig. 2 show the side view of the initial adsorption

structure of three Schiff bases on Fe (1 0 0) surface in solution. The relatively dependent

function curves were obtained by analyzing the dynamics simulation trajectory files.

(Figure 1 and 2)

2.1.2 Analysis of dynamic simulation

The temperature, energy and time of simulation system are taken as criterions whether the

system has reached equilibrium, because they will affect the results of calculation. Three kinds

of Schiff base molecules are adsorbed on the Fe (1 0 0) surface in solution, the system reached

equilibrium and are calculated by 500000 step as shown in Fig. 3, it can be seen that potential

energy and nonbond energy have reached a equilibrium from the point of view of

transformation energy for further calculation.

(Figure 3)

2.1.3 Calculation method

The MD simulation of the interaction between corrosion inhibitors molecules L1, L2, L3

on Fe (1 0 0) surface was carried out in a simulation system (Fig.1) with periodic boundary

conditions, a representative part of the interface devoid of any arbitrary boundary effects. The

iron substrate with Fe (1 0 0) plane was first optimized to minimum energy. The behavior of

the inhibitor molecule on the Fe(1 0 0) surface was simulated using the COMPASS force field,

which are compared with the experimental values by using molecular liquid density and

cohesive energy to validate before its using, and to optimize the structures of all components

of the system and represents a technology break-through in forcefield method.

The MD simulation was performed under 298 K, NVT ensemble, with a time step of 1 fs,

per 500 steps output a molecular structure and the total dynamics simulation time is 1000 ps.

The interaction energy Einteraction between the iron surface and the inhibitor molecules was

calculated as following [20]:

Einteraction= Etotal -(Emolecule+E surface (1)

Where Etotal is the total energy of the system containing Schiff base molecules, water

molecules and metal surface; the Emolecule is free Schiff bases molecules energy, the Esurface is

free Schiff base molecules, all water molecules and metal surfaces of energy, respectively.

2.2 Diffusion model calculation details

In order to study the synergistic effects between corrosion inhibitors molecules of L1, L2,

L3 and corrosive particles at the molecular scale, the Amorphous Cell module and Diffusion

module were used[21, 22]. A amorphous system which contain 300 inhibitor molecules, 30

Cl- and 30 H3O

+ was established by using Amorphous Cell module Construction options with

size of 59.7759.77150.46 as shown in Fig.4.

(Figure 4)

Equilibrium of diffusion system of corrosion ions in the inhibitor molecule films was

calculated by 2,000,000 steps. Form the view of energy, potential energy and nonbonding

energies have been equilibrated. It can judge that the system had reached equilibrium. Trace

files was got after the system equilibrium, mean square displacement (MSD) diagrams of

corrosion ions are obtained by opening the Discover module Analysis in the Mean squared

displacement option.

2.3. Density functional theory details

Density functional theory (DFT) can provide a very useful tool for understanding

molecular properties and describing the behaviour of atoms in molecules. DFT methods have

become very popular due to their accuracy from the computational point of view. Molecular

geometry optimisation was performed at the DFT B3LYP level theory using 6-31G* basis set

with Gaussian 03 program. This basis set provided accurate geometry and electronic

properties for a wide range of organic compounds. Besides, molecular electrostatic potential

(MEP) and frontier molecular orbitals were determined at B3LYP/6-31G* level of theory.

3. Experimental procedures

Experiments were carried out using mild steel specimens(0.09% P, 0.38% Si, 0.01% Al,

0.05% Mn, 0.21% C, 0.05% S and remainder iron)(5.01.00.2 cm) as the electrode material.

Steel sheets were mounted in Teflon with surface area of 4.01.0 cm2. The surface area of

1.01.0 cm2 was abraded using emery papers and polished with Al2O3 (0.5 m particle size)

carefully, cleaned in distilled water in an ultrasonic bath, and subsequently rinsed with dry

ethanol and redistilled water for use. The same procedure was also used in case of gravimetric

experiments. Three schiff's base inhibitors were synthesized according to the references

procedure [23] (Scheme 1, 2 and 3). All the original compounds with analytical grade were

obtained from Sigma-Aldrich Chemical Co. The simulative oilfield water was used as

corrosion medium, which compositions are shown in Table 1 with simulated CO2 gas.

(Scheme 1, 2 and 3)

(Table 1)

3.1 Weight loss measurements

Weight loss measurements were carried out in a double glass cell equipped with a

thermostated cooling condenser. The solution volume was 100 mL. The duration of tests was

7 days at 25 C. Duplicate experiments were performed in each case and the mean value of the

weight loss is reported. Weight loss allowed calculation of the mean corrosion rate (g

m-2

h-1

) was evaluated using Eq. (2):

=(w0wt)/(St) (2)

where w0 and wt are the values (g) of weight loss of steel specimens after immersion in

simulative oilfield water without and with inhibitor, respectively. S is the reaction area (m2) of

steel sheet and t is the reaction time (h).

The inhibition efficiency IE (%) was evaluated using Eq. (3):

IE=[(0 t) /0]100% (3)

where 0 and t are the values (g m-2

h-1

) of corrosion rate of steel specimens after immersion

in simulative oilfield water, without and with inhibitors, respectively.

3.2 Electrochemical measurements

A CHI860B electrochemical workstation (Chenhua Instruments, Beijing, China) was used

to perform all electrochemical experiments at 25 C. The electrochemical cell was a classical

three-electrode system with a mild steel specimen electrode as the working electrode, a

saturated calomel electrode as the reference electrode, and a platinum electrode as the counter

electrode. All potentials were referenced to the potential of the saturated calomel reference

electrode (SCE).

For test of Tafel polarization curves, the scanning range is -1.0 ~ 1.0 V with scan rate of

0.001 Vs-1, the sensitivity is set automatically. In this case, the inhibition efficiency IE (%) was

evaluated using Eq.(4):

IE=(i0

corricorr)/i0

corr 100 % (4)

where i0

corr and icorr are the corrosion current density values without and with the addition of

inhibitors in simulative oilfield water, respectively.

For test of electrochemical impedance spectroscopy, measurements were carried out at the

open circuit potential of -1.0 V vs. SCE at a modulation amplitute of 5 mV in the frequency

range of 0.01 Hz to 100 kHz. In this case, the inhibition efficiency IE (%) was evaluated using

Eq. (5):

IE = (RctR0

ct )/Rct 100 % (5)

where R0

ct and Rct are the charge transfer resistance values without and with the addition of

inhibitors in simulative oilfield water, respectively.

4. Results and discussion

4.1. Weight loss study

The corrosion of mild steel specimens in simulative oilfield water, without and with

various concentrations of three tested schiff's base inhibitors was studied by weight loss

measurements. Values of corrosion rate and the inhibition efficiency obtained are given in

Table 2. It can be seen in Table 2, the corrosion rate values of mild steel decrease when the

inhibitor concentration increases. The inhibition efficiency increases with the concentration of

L1, L2 and L3 reaching a maximum value (85.43, 89.76 and 83.71%) at 75 mgL-1, respectively.

The results demonstrate that all studied schiff's base inhibitors exhibit inhibition properties.

The inhibition efficiency of three tested inhibitors follows the order EI (L2)> EI (L1)> EI

(L3).

(Table 2)

4.2. Tafel Polarization curves study

Potentiodynamic polarization was used to investigate the corrosion inhibition effects of

schiff's base inhibitors. Tafel polarization curves recorded for mild steel electrode in

simulative oilfield water, without and with various concentrations of three tested schiff's base

inhibitors are shown in Fig. 5. The associated corrosion parameters, such as corrosion potential

(Ecorr) and corrosion current density (icorr) are listed in Table 3.

(Table 3 and Fig.5)

As shown in Fig.5, with the addition of various concentration inhibitors L1 and L2, the

polarization plots display both cathodic and anodic region of Tafel behaviours, inhibitors L1

and L2 should be regarded as mixed-type inhibitors. However, the polarization plots shift to

anodic region is deeply than to cathodic region both for L1 and L2, from this view of point,

they mainly belong to anodic inhibitors. The polarization plots only display cathodic region of

Tafel behaviour with the addition of inhibitor L3, L3 can be described as cathodic inhibitor.

All the shifts show that the addition of inhibitors to simulative oilfield water can reduces the

anodic dissolution of mild steel and the cathodic hydrogen evolution reaction and to protect

mild steel.

It is clear that the addition of inhibitors all cause sharply change of current densities in

Table 3, to decrease them and demonstrate that corrosion of mild steel electrode are inhibited

by the inhibitors in simulative oilfield water. For example, the addition of 25 mgL-1 inhibitors

in simulative oilfield water can change the value of corrosion current (A cm-2) from 116.8 to

37.08, 37.89 and 41.84, and show IE value as 68.25%, 67.56% and 64.18%, respectively.

When the concentration of inhibitors increase at 75 mgL-1, the values of corrosion current are

30.71, 30.50 and 36.43, and IE values are 86.90%, 90.09% and 85.14%, respectively.

However, higher concentration of inhibitors does not change the value of corrosion current

and IE obviously, so the concentration of 75 mgL-1 inhibitors was chose for following

experiments. The above results demonstrate that all studied schiff's base inhibitors exhibit

inhibition properties. The inhibition efficiency of three tested inhibitors follows the order of EI

(L2)> EI (L1)> EI (L3).

4.3. EIS study

It is evident that the more efficient the inhibitor, the lower the corresponding corrosion

rate and the higher the charge-transfer resistance, Rct. EIS technique was used to evaluate the

efficiency of the three tested inhibitors against the simulative oilfield water corrosion of mild

steel.

The results of the EIS measurements were presented in Fig. 7 as Nyquist plots of mild

steel in different Cl- concentrations of simulative oilfield water, with addition of three tested

inhibitors at 75 mgL-1, respectively. The Nyquist plots are derived from Fig.6, where Rct is

the charge-transfer resistance of electrode and Rs is the solution resistance. Table 4 shows the

electrochemical parameters, together with the inhibition efficiencies IE(%), calculated from

EIS measurements on mild steel electrode in various Cl- concentrations of simulative oilfield

water without and with addition of 75 mgL-1 inhibitors at 25 C.

(Table 4)

(Fig.6 and 7)

It can be seen that the Rct value of mild steel decrease when the concentration of Cl-1

in

simulative oilfield water, not only for the blank simulative oilfield water, but also for the

addition of inhibitors in simulative oilfield water, indicate the increase concentration of Cl-1

will increase the corrosion of mild steel. For the same concentration of Cl-1

, after the addition

of inhibitors, the Rct value of mild steel has a big increase to show the inhibition of mild steel

by the inhibitors. When the the concentration of Cl-1

in simulative oilfield water is 55 mgL-1,

the EI value of three inhibitors all more than 80%, even the concentration of Cl-1

in simulative

oilfield water up to 70 mgL-1, the EI value of three inhibitors all also more than 70% to show

very good inhibition performance of three inhibitors. The results demonstrate that all studied

schiff's base inhibitors exhibit inhibition properties and obtain the order of inhibition

efficiency for three tested inhibitors as EI (L2)> EI (L1)> EI (L3) according to data of Table

4.

4.4 Adsorption energy of study

In the initial of adsorbed configuration, the inhibitor molecules are oriented vertically

attached on the metal surface as shown in Fig.2. After the simulation, the result shows that the

existence of water molecules have a great influence on Schiff base inhibitor molecules,

according to movement of surrounding water molecules, they occur a slight vibrations. This

indicates that there is a interaction between water molecules and inhibitor molecules, which

significantly slow down the speed of the inhibitor molecules. After a period of vibration, the

inhibitor molecules are tilted gradually, slow the trend that water molecules move to the metal

surface. When the inhibitor molecules continue move to the bottom of the solvent layer, they

interact with water molecules, the inhibitor molecules continue tilt slowly until parallel to

metal surface. They expel water molecules on the surface of the metal, and then are adsorbed

parallelly. It can be seen that the three kinds of Schiff base inhibitors are almost parallel to the

metal surface in Fig.8, which is the most stable form of adsorption, because that inhibitor

molecules and the metal surface formed can form a high degree of coverage dense molecular

film[24], which have inhibition effects.

(Figure 8)

The adsorption energies of three Schiff base were obtained as listed in Table 2, the

adsorption energy of L1, L2 and L3 are 113.46, 141.43 and 80.47 kcal/mol, respectively, it

follows the order of E(L2)> E(L1)> E(L3) and leads to inhibition efficiency order of EI (L2)>

EI (L1)> EI (L3). Since inhibitor molecules can easily expel the water molecules which are

adsorbed on the metal surface[25-27], the adsorption energy of three kinds of Schiff bases are

much larger than adsorption energy of water molecules (the adsorption energy of water

molecules is 5.89 kcal/mol).

(Table 5)

Schiff base inhibitor molecules have many adsorption points, and that the adsorption

energy of inhibitor molecules is larger than the adsorption energy of water molecules, can

disperse the water molecules which are adsorbed on the metal surface. After reaching stable

adsorption, it still exist interactions between inhibitor molecules and surrounding water

molecules, which inevitably lead to the interactive force will become weaken between inhibitor

molecules and the metal Fe surface [28-30].

4.5 Radial distribution function curves study

Fig.9 shows the radial distribution function curves of C, N, O, S of Schiff base molecules

and Fe atoms. Generally, the peak within 3.5 indicates the formation of chemical bonds

between atoms, the peak outside 3.5 shows interaction of Van der Waals force or Coulomb

force[31, 32]. It is clear known that the three Schiff bases radial distribution curve of the peak

value appear around the 3.5 or less, show that C, N, O, S of Schiff base molecules and Fe

atoms can form chemical bonds, so chemical adsorption occur on the surface of Fe.

(Figure 9)

For L1, the highest peaks of the radial distribution function curve of C, N and O atoms

appear at 3.27 , 3.44 and 3.20 , respectively, the interactive force of O, C and N atom on

the Fe surface is following order of F(O)> F(C)> F(N). For L2, the highest peaks of the radial

distribution function curve of C, N , O and S atoms appear at 3.46 , 3.17 , 3.18 and 3.42 ,

respectively, the interactive force of O, C , N and S atom on the Fe surface is following order

of F(N)> F(O)> F(S)> F(C). For L3, the highest peaks of the radial distribution function curve

of C, N , O and S atoms appear at 3.11, 3.16 , 3.39 and 3.31 , respectively, the

interactive force of C, N , O and S atoms on the Fe surface is following order of F(C)> F(N)>

F(S)> F(O). Overall, the radial distribution function curve of C, N, O, S of three kinds of Schiff

base molecules and the Fe surface show that the highest peak appeared within 3.5 , so

chemical bonds were formed between C, N, O, S of three kinds of Schiff base molecules and Fe

atoms, chemical adsorption had happened at Fe surfaces.

4.6 Diffusion coefficients study

During the simulation, the inhibitor molecules and corrosion ions (Cl-, H3O

+) are moving.

The movement of corrosion ions of Cl- and H3O

+ in the inhibitor molecular films, indicate that

the size of the diffusion intensity in the inhibitor adsorption films, it can reflect the strength of

inhibitor performance. The inhibitor films can inhibit diffusion of corrosion ions, so the

corrosion ions in the inhibitor films movement are weaker and the position is fixed. The motion

of corrosion ions position can be measured by mean square displacement (MSD).

Mean square displacement [23] (MSD) can be expressed as:

MSD={|Ri(t)-Ri(0)|2} (4)

Where Ri(t) in t time for the position of the i ion, Ri(0) for the position of the i ion at the

initial moment.

(Figure 10)

Diffusion coefficient can directly reflect the strength of the corrosion particle diffusion

migration. The diffusion coefficient of corrosion medium Cl- and H3O

+ at three kinds of Schiff

base inhibitor molecules adsorption films have been calculated, and with corrosive media Cl-

and H3O+

diffusion coefficient in the water molecules were compared and analyzed, more

clearly reflect inhibitor performance of the Schiff base. According to Einstein relation can

obtain the diffusion coefficient (D).

1

lim 0 26

n

it

dD Ri t Ri

dx (5)

Obtained by equation 1-2

1lim

6 t

dMSDD

dx (6)

Diffusion coefficients can be made of the following equation to solve:

6

mD (7)

M for MSD curve slope

(Table 6)

Table 6 lists the diffusion coefficients of corrosive ions of Cl- and H3O

+ in three kinds of

Schiff bases inhibitor molecules films and water. It can be seen from the table, diffusion

coefficients in corrosive ions of Cl- and H3O

+ is much smaller than in the water, indicating that

the inhibitors have very strong inhibition ability for corrosive ions of Cl-

and H3O+, can

effectively control the diffusion migration of the corrosive ions of Cl- and H3O

+, avoid metal

surface contacting with corrosive ions of Cl- and H3O

+, thus it can reduce corrosion, and show

very good corrosion inhibition performance [33].

The corrosive ions displacement curves is flat to show that the smaller move of corrosion

ions in the inhibitor molecule adsorption films, the weaker of the diffusion ability is, the

stronger the inhibition ability of the inhibitors and the better the inhibition effect will be; the

larger the slope of corrosion ion mean square displacement (MSD) curve is , the greater move

of the corrosion ions in the inhibitor molecule adsorption films; the stronger the diffusion

ability, the weaker the control ability of the inhibitor will be. As can be seen in Table 3 and

Fig.7, for the different inhibitor films, the diffusion coefficients followed the order of D (L3)>

D (L1)> D (L2) for the Cl- corrosive particles, the diffusion coefficients followed the order of

D (L3)> D (L1)> D (L2) for the H3O+ corrosive particles. For the three inhibitor films, the

diffusion coefficients of the three corrosive particles all followed the order of D(H2O)>

D(H3O+)>D(Cl

-), three kinds of inhibitor films have corrosion inhibition efficiency in aqueous

solution, which can be applied for corrosion protection in oil field.

4.7 DFT study

Mulliken charge is related to the electronic density and is a very useful descriptor in

understanding sites for electrophilic attack and nucleophilic reactions as well as hydrogen

bonding interactions [34].

In order to predict reactive sites for electrophilic attack of inhibitors, Mulliken charge was

calculated as shown in Fig. 11. As easily can be seen in Fig. 11, those molecules have two

possible sites for electrophilic attack. The negative regions are mainly over the N and O atoms,

which are the most susceptible sites for electrophilic attacks and could offer electrons to the

mild steel surface to form a coordinate-type of bond.

(Fig.11)

It is well known that the reactivity of molecules depends entirely on the electronic

distribution of their molecular orbitals. Frontier molecular orbitals are the controlling unit to

investigate the molecular reactivity [35]. HOMO is associated with the electron donating

capability of the molecule, whereas LUMO is related to its capability of accepting electrons.

The frontier molecular orbital energies of three test inhibitors obtained from quantum

chemical calculations as shown in Table 7 and Fig.12. HOMO orbital of geometry optimized

inhibitor molecules (L1, L2 and L3) have electron density over the ring of 3, 5-dibromo

salicylaldehyde and C=N bond. L1, L2 and L3 can donate their electrons to vacant d-orbitals

of acceptor Fe for the formation of co-ordinate type bonds as above discussion. The LUMO

electron density locates on the ring of pyridine segment for L1, and on ring of benzimidazole

for L3, indicating the preferred active sites for accepting electrons. For L2, LUMO electron

density is not restricted in a particular region, and distributed on the entire molecule. It is

therefore reasonable to assume that only quinoline ring of L1, L3 is responsible for accepting

electrons, whereas all the parts of molecule L2 are susceptible in accepting electrons from

d-orbitals of metal by back bonding.

Quantum chemical parameters, such as EHOMO, ELUMO, the energy gap between HOMO

and LUMO orbitals (E), dipolemoment (), electronegativity (), global hardness () and

fraction of electron transferred (N) are tabulated. Generally, it is assumed that EHOMO is

related to the capability of a molecule to donate electrons. Higher the EHOMO value, stronger

will be the ability of the molecules to donate electrons. From Table 7, it can be concluded that

EHOMO values of the three inhibitor molecules follow the order L2>L1>L3, which is in good

agreement with the experimentally determined inhibition efciency. The energy of LUMO

(ELUMO) represents the ability of molecules to accept electrons from the metal surface. The

lower of the value of ELUMO, the greater ability of the molecule to accept electrons will be.

The values of ELUMO follow the order L3>L1>L2. This results again support the relative

inhibition efciency of three inhibitors following the order EI (L2)> EI (L1)> EI (L3).

The energy gap (E) between HOMO and LUMO exhibits the reactivity of the

molecules towards the metal surface [36]. The reactivity of the molecules increases as E

decreases, because the energy need to remove an electron from the last occupied molecular

orbital. Thus, the smaller the energy gap (E) is, the more it will be polarizable and the better

of electrons transport performance. It is found L2 has the smallest energy gap (E) comparing

with L1 and L3. Dipolemoment () of molecule is related to the polarity of the polar covalent

bond. It is dened as the charge of product and the distance between the two concerned atoms.

It can be seen that dipolemoment decreases in the trend similar with results of EHOMO, ELUMO

and E. Therefore, results of theoretical studies support that L2 has the greatest potentiality to

get be adsorbed on the mild steel surface, when compare with L1 and L3.

(Fig.12)

(Table 7)

5. Conclusion

In this study, measurements include weight loss, Tafel polarization, electrochemical

impedance spectroscopy were adopted to investigate inhibitive properties of three schiff's base

inhibitors on mild steel. Three key parameters, including interaction energy, radial distribution

function curves and self-diffusion coefficient accompanying with DFT study were selected to

study adsorption behaviors and corrosion inhibition mechanism of three inhibitors on Fe (1 0 0)

surface in aqueous solution and corrosive ions of Cl- and H3O

+.

(1) The three kinds of Schiff base inhibitors are almost parallel to the adsorption on the

metal surface, which is the most stable form of adsorption, show that inhibitor molecules form

a high coverage and compact structure molecular film on the metal surface, have inhibition

effects. The order of adsorption energy is E(L2)> E(L1)> E(L3), and the order of adsorption

abilities is L2> L1> L3.

(2) The radial distribution function curves of the C, N, O, S atoms and Fe atom show that

those peak values are less than 3.5 , indicate between the C, N, O, S atoms of the Schiff base

inhibitor molecules structure with Fe atoms form a chemical bond, chemical adsorption occur

on the surface of Fe.

(3)The mean square displacement curves of corrosive media ions of H3O+, Cl

- and H2O in

three kinds of inhibitor adsorption film show that for the different inhibitor films, the diffusion

coefficients followed the order of D (L3)> D (L1)> D (L2) for the Cl- corrosive particles, the

diffusion coefficients followed the order of D (L3)> D (L1)> D (L2) for the H3O+ corrosive

particles. For the three inhibitor films, the diffusion coefficients of the three corrosive

particles followed the order of D(H2O)>D(H3O+)>D(Cl

-). The results of DFT study confirm

the inhibition efciency of three inhibitors following the order EI (L2)> EI (L1)> EI (L3).

(4) Three kinds of inhibitor films can be applied for corrosion protection in oil field, due

to their corrosion inhibition efficiency in aqueous solution.

Acknowledgments

This work was supported by the National Nature Science Foundation of China

(Nos.21266006, 61301038, 61271119) and the Nature Science Foundation of Guangxi

Province (No.2012GXNSFAA053034).

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Figure Captions

Fig. 1 The adsorption of three Schiff bases in solution

Fig. 2 Side view of initial adsorption of three Schiff bases on Fe(1 0 0) surface in solution

Fig. 3 Energy equilibrium curve of three Schiff bases are adsorbed on Fe(1 0 0) surface in

solution

Fig. 4 Inhibitor monolayer with corrosion particles

Fig.5 Polarization curves of mild steel in simulative oilfield water with various concentrations

of inhibitors

Fig.6 Equivalent circuit used to fit experimental EIS data.

Fig. 7 Nyquist plots of mild steel in different Cl- concentrations of simulative oilfield water

with 75 mgL-1 inhibitors

Fig. 8 Side view of the final adsorption of three Schiff bases on Fe(1 0 0) surface in solution

Fig. 9 The pair correlation function of C, S, O and N atoms from three Schiff bases with Fe

atoms from Fe(1 0 0) surface in solution

Fig. 10 Mean square displacement curve of corrosion particles in inhibitor monolayers

Fig. 11The Mulliken charge population of inhibitors L1, L2 and L3.

Fig.12 The frontier molecule orbital density distributions of inhibitors L1, L2 and L3.

Table 1 Compositions of simulative oilfield water

Table 2 Corrosion rate and inhibition efficiency IE (%) values obtained from weight loss

measurements for mild steel in simulative oilfield water without and with various

concentrations of inhibitors at 25 C.

Table 3 Electrochemical parameters, together with the inhibition efficiencies IE (%)

calculated from polarization measurements on the steel electrode in simulative oilfield water

without and with various concentrations of inhibitors at 25 C.

Table 4 Electrochemical parameters, together with the inhibition efficiencies IE(%),

calculated from EIS measurements on mild steel electrode in various Cl

- concentrations of

simulative oilfield water with addition of 75 mg/L inhibitors at 25 C.

Table 5 The adsorption energy of three Schiff bases on Fe surface in solution

Table 6 The diffusion coefficient of Cl-, H3O

+ at 298K

Table 7 Calculated quantum chemical parameters of the studied inhibitors.

Table 1

Composition NaCl MgCl2 CaCl2 Na2S NaHCO3 Na2SO4

Concentration(g L-1) 70.0 4.0 6.0 1.50 0.48 0.58

Table 2

Inhibitor concentration

(mgL-1)

corrosion rate

(gm-2h-1) IE%

Blank 0 3.351

L1

25 0.922 72.48

50 0.542 83.83

75 0.488 85.43

100 0.492 85.31

125 0.499 85.10

L2

25 0.936 72.07

50 0.491 75.34

75 0.343 89.76

100 0.372 88.89

125 0.369 88.99

L3

25 1.156 65.50

50 0.718 78.57

75 0.546 83.71

100 0.576 82.81

125 0.606 81.92

Table 3

Inhibitor concentration(mgL-1) Ecorr(mV) Icorr(A cm-2

) IE(%)

Blank 836.8 116.8

L1

25 770.7 37.08 68.25

50 796.4 30.71 73.71

75 798.9 15.30 86.90

100 867.4 14.49 87.59

125 794.6 14.36 87.71

L2

25 739.5 37.89 67.56

50 735.2 30.50 73.89

75 845.4 11.58 90.09

100 775.4 11.66 90.02

125 853.1 11.95 89.77

L3

25 860.6 41.84 64.18

50 872.4 36.43 68.81

75 885.2 17.35 85.14

100 845.4 18.04 84.55

125 855.2 18.39 84.26

Table 4

Inhibitor concentration

of Cl-(gL-1)

Rs( .cm2)

Rct( .cm2) CPE -T

(Fcm-2

)

CPE -P

(Fcm-2

)

IE (%)

Blank

55 2.560 123.0 0.0446 0.8543

60 3.012 115.6 0.0538 0.7625

65 2.513 67.26 0.0379 0.8187

70 2.596 43.68 0.0391 0.8459

L1

55 2.410 778.8 0.0789 0.7573 84.21

60 2.555 633.3 0.0405 0.8566 81.75

65 2.047 317.9 0.0801 0.7041 78.84

70 2.715 190.4 0.0722 0.6959 77.06

L2

55 2.337 855.7 0.0770 0.7463 85.62

60 2.901 660.9 0.0546 0.7498 82.51

65 2.187 333.2 0.0814 0.6384 79.81

70 4.626 202.3 0.0580 0.6513 78. 40

L3

55 2.850 716.1 0.0575 0.7167 82.82

60 2.563 548.0 0.0395 0.8519 78.90

65 2.684 271.3 0.0787 0.6908 75.21

70 2.425 170.9 0.0884 0.7658 74.44

Table 5

Molecular L1 L2 L3 H2O

Energy/(kcal/mol) 113.46 141.43 80.47 5.89

Table 6

Molecular D H3O+(10-9

m2/s) D Cl- (10

-9m

2/s)

H2O 0.8740 0.3490

L1 0.0026 0.0024

L2 0.0018 0.0015

L3 0.0045 0.0037

Table 7

Inhibitor EHOMO

(eV)

ELUMO

(eV)

E

(eV)

(Debye)

I=-E

HOMO

A=-E

LUMO

S N

L1 -6.50846 -2.63909 3.86937 2.9527 6.50846 2.63909 4.5737 1.93468 0.5538 0.7324

L2 -5.95294 -2.95924 2.9937 2.4674 5.95294 2.95924 4.4561 1.49685 0.6274 0.8345

L3 -6.51171 -2.62789 3.88382 3.2854 6.51171 2.62789 4.5698 1.94191 0.5219 0.7125

Fig. 1

L1 L2 L3

Fig. 2

L1 L2 L3

Fig. 3

Fig. 4

Fig.5

Fig.6

Fig. 7

Fig. 8

L1 L2 L3

Fig. 9

Fig. 10

Fig. 11

L1

L2

L3

Fig.12

(a)HOMO (b)LUMO

L1

(a)HOMO (b)LUMO

L2

(a)HOMO (b)LUMO

L3

Graphical abstract

L1 L2 L3

Fig.2

L1 L2 L3

Fig.8

In the initial of adsorbed configuration, the inhibitor molecules are oriented vertically

attached on the metal surface as shown in Fig.2. After the simulation, the result shows that the

existence of water molecules have a great influence on Schiff base inhibitor molecules,

according to movement of surrounding water molecules, they occur a slight vibrations. When

the inhibitor molecules continue move to the bottom of the solvent layer, they interact with

water molecules, the inhibitor molecules continue tilt slowly until parallel to metal surface

(Fig.8).

Highlights

1. The adsorption energy of three Schiff base inhibitors followed the order of E(L2)>

E(L1)> E(L3).

2. The study of radial distribution function curves show that C, N, O, S atoms of Schiff base

and Fe atoms form a chemical bond, chemical adsorption occur on the surface of Fe.

3. The diffusion coefficients of three inhibitor films followed the order of D (L3)> D (L1)>

D (L2) for the Cl- corrosive particles, and followed the order of D (L3)> D (L1)> D

(L2) for the H3O+ corrosive particles.

4. For the three inhibitor films, the diffusion coefficients of the three corrosive particles

all followed the order of D(H2O)>D(H3O+) > D(Cl

-).

5. The results of DFT study confirm the inhibition efciency of three inhibitors following

the order EI (L2)> EI (L1)> EI (L3).