Solvent and catalyst-free synthesis, corrosion protection, thermodynamic, MDS and DFT calculation of two environmentally friendly inhibitors: Bis-phosphonic acids

Article abstract of DOI:10.1016/j.molstruc.2020.128813

Solvent and catalyst-free synthesis of two bis-phosphonic acids were realized adopting one-pot reaction of 2,6-bis (hydroxymethyl)-4-methoxyphenol or 4-chloro- 2,6-bis (hydroxymethyl) phenol with trialkyl phosphite. Their structures were confirmed by ¹H, ¹³C, ³¹P NMR and IR spectroscopies. The molecular structures have been optimized using DFT, and the related calculated vibrational frequencies and NMR shift confirm the proposed structures. Chemical and electrochemical tests evaluated the inhibitive capabilities on the XC48 steel corrosion against 1 M HCl. The weight loss (chemical one) shows that the adsorption on the XC48 steel surface obeys to Langmuir isotherm, while the polarization measurements suggest that the inhibition process is mixt, the results of Electrochemical Impedance Spectroscopy (EIS) prove an increase of the inhibition efficiency with the increase of inhibitor concentrations. The surface topographies were analyzed by the Atomic Force Microscopy (AFM). The Density Functional Theory (DFT) with B3LYP as correlation function and 6-31G (p, d) as basis sets were used to determine some theoretical physic-chemical parameters affecting the corrosion inhibition and on the other hand the DFT develop the Frontier Molecular Orbitals (FMOs) and the Molecular Electrostatic Potentials (MEP) and through a discussion was made about their relationship with the inhibition of the corrosion. At the end, Molecular Dynamics Simulations (MDS) based on Monte-Carlo calculations were accomplished to simulate the interface configuration between the inhibitor molecules and Fe (110)/H₂O.

Full text of DOI:10.1016/j.molstruc.2020.128813

Journal Pre-proof
Solvent and catalyst-free synthesis, corrosion protection, thermodynamic, MDS and
DFT calculation of two environmentally friendly inhibitors: Bis-phosphonic acids
Louiza Ouksel, Riadh Bourzami, Salah Chafaa, Nadjib Chafai
PII:
S0022-2860(20)31138-8
DOI:
https://doi.org/10.1016/j.molstruc.2020.128813
MOLSTR 128813
Reference:
To appear in: Journal of Molecular Structure
Received Date: 30 May 2020
Revised Date: 30 June 2020
Accepted Date: 1 July 2020
Please cite this article as: L. Ouksel, R. Bourzami, S. Chafaa, N. Chafai, Solvent and catalyst-free
synthesis, corrosion protection, thermodynamic, MDS and DFT calculation of two environmentally
friendly inhibitors: Bis-phosphonic acids, Journal of Molecular Structure (2020), doi: https://
doi.org/10.1016/j.molstruc.2020.128813.
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Credit Author Statement
Author
Credit Statement
Louiza Ouksel
Synthesis and elaboration, Theoretical calculations, Interpretation, Methodology
and Writing.
Riadh Bourzami Molecular structure, Theoretical calculations, Interpretation, Methodology and
Writing.
Salah Chafaa
Nadjib Chafai
Synthesis.
Software.

Solvent and catalyst-free synthesis, corrosion protection, thermodynamic, MDS and
DFT calculation of two environmentally friendly inhibitors: bis-phosphonic acids
Louiza Ouksela,b, Riadh Bourzamic, Salah Chafaaa Nadjib Chafaia
,
a
Laboratoire d’électrochimie des matériaux moléculaires et complexes (LEMMC),
Département de Génie des Procèdes, Faculté de Technologie, Université Ferhat Abbas de
Sétif-1, 19000, Algeria
b
Laboratoire d'Electrochimie et Matériaux (LEM), Département de génie des procédés,
Faculté de Technologie Université Ferhat Abbas, Sétif-1,19000. Algeria
c Research unit of emergent materials, Ferhat Abbas, university, Sétif-1 , Algeria
* Corresponding author: louksel2014@gmail.com
Abstract
Solvent and catalyst-free synthesis of two bis-phosphonic acids were realized adopting one-
pot reaction of 2,6-bis (hydroxymethyl)-4-methoxyphenol or 4-chloro- 2,6-bis
13
(hydroxymethyl) phenol with trialkyl phosphite. Their structures were confirmed by 1H, C,
31P NMR and IR spectroscopies
. The molecular structures have been optimized using DFT,
and the related calculated vibrational frequencies and NMR shift confirm the proposed
structures. Chemical and electrochemical tests evaluated the inhibitive capabilities on the
XC48 steel corrosion against 1 M HCl. The weight loss (chemical one) shows that the
adsorption on the XC48 steel surface obeys to Langmuir isotherm, while the polarization
measurements suggest that the inhibition process is mixt, the results of Electrochemical
Impedance Spectroscopy (EIS) prove an increase of the inhibition efficiency with the increase
of inhibitor concentrations. The surface topographies were analyzed by the Atomic Force
Microscopy (AFM). The Density Functional Theory (DFT) with B3LYP as correlation
function and 6-31G (p, d) as basis sets were used to determine some theoretical physic-
chemical parameters affecting the corrosion inhibition and on the other hand the DFT develop
the Frontier Molecular Orbitals (FMOs) and the Molecular Electrostatic Potentials (MEP) and
through a discussion was made about their relationship with the inhibition of the corrosion. At
the end, Molecular Dynamics Simulations (MDS) based on Monte-Carlo calculations were
accomplished to simulate the interface configuration between the inhibitor molecules and Fe
(110)/H2O.
Keywords: Bis-phosphonic acids; Corrosion inhibition; Adsorption; DFT Calculations; MD
simulations.
1. Introduction

For many industries, corrosion is an expensive, and potentially dangerous problem. It has a
negative impact on system security during a catastrophic failure of machines used in
chemical, petrochemical, construction industries and transport. Corrosion is a spontaneous
and slowly progressing process. The rate is controlled mainly by environmental, metal
composition, biochemical, chemical and electrochemical characteristics [1]. Corrosive factors
are mostly, external media containing oxygen, inorganic, organic acids, or chloride, high
pressure, and high temperature [2], among them, the acidic solutions are the most aggressive
agent on metallic surfaces and the use of organic inhibitors (environmentally friendly) is an
effective choice to protect metals against corrosion.
Among the environmental protection, much importance is linked to corrosion protection.
According to ISO 8044, corrosion is defined as physical interaction within a steel and its
surroundings which induces in changes of the properties of a material and the environment or
the system of which they form a part [3, 4], corrosion inhibition relieves the environment in a
number of ways and becomes a decisive act of environmental protection.
Organophosphorus compounds containing atoms with the electronic lone pair (P, O, N and
S), or π systems, or conjugated bonds, or aromatic rings are considered effective corrosion
inhibitors [5-7]. Especially, compounds with a phosphonic acid are considered the most
effective molecules for inhibiting the corrosion process [8-10]. Really, a donor–acceptor
synergy between the metal and inhibitor forms a thin protective layer on the metal surfaces,
via forms a coordinate covalent bond (chemical adsorption) or the electronic interaction
(physical adsorption) [11]. The produced film isolates the external surface of the metal from
the aggressive environment, which reduces the corrosion extent [12-15]. Because of their low
toxicity, high hydrolysis stability, phosphonic acids has been used extensively in
environmental remediation, they cannot be easily degraded by microorganisms [16, 17]. The
use of phosphonic acids to protect XC48 steel against corrosion in HCl or H2SO4 mediums
has been the subject of various works [18-27].
The objective of this work is to synthesize and compare the corrosion inhibition
efficiencies of 2-hydroxy-5-methoxy-1, 3-phenylene bis (methylphosphonic acid) and 5-
chloro-2-hydroxy-1, 3-phenylene bis (methylphosphonic acid) in 1 M HCl medium on
metallic surfaces (XC48 steel), using polarization curves, Electrochemical Impedance
Spectroscopy (EIS) and weight loss methods. Surface topographies were performed using the
Atomic Force Microscopy (AFM). The Density Functional Theory (DFT) with B3LYP and 6-
31 G (p, d) basis sets was used to optimize the molecular structures and to determine the
energy (EHOMO), the energy (ELUMO), the NBO charges, the Molecular Electrostatic Potential

(MEP), the frontier molecular orbital and other theoretical parameters affecting the effect of
corrosion inhibition. Additionally, to visualize adsorption configurations on the Fe(110),
Molecular Dynamic Simulations (MDS) based on Monte Carlo calculations were also
performed in vacuum slab and aqueous solutions.
2. Experimental
2.1. Methods
2.1.1. Weight loss measurements
Corrosion parameters as corrosion rate and inhibition efficiency were determined
using weight loss measurements. The cleaned specimens with external surface S = 10 cm2,
were immersed by triplicate, for t = 6 h, in 1 M HCl solution with various concentrations c =
0, 1.10‒3, 5.10‒3, 1.10‒2 and 5.10‒2 mM of bis-phosphonic acids at four temperatures T = 298,
303, 313 and 323 K, under aerated condition.
The relations (1, 2, 3 and 4) calculate respectively, the average weight loss ∆W (mg),
the corrosion rates CRW (mg.cm‒2.h‒1), the surface coverage θ and the inhibition efficiency
IEW (%) [28-30]:
∆W = Wꢀ − Wꢁ
(1)
(2)
(3)
ꢂꢃꢄ = ∆W/St = (Wꢀ − Wꢁ)/ꢅꢆ
θ = (Cꢃꢇꢄ − Cꢃꢄ)/Cꢃꢇꢄ
ꢇ
IEꢄ % = (Cꢃꢄ − Cꢃꢄ)/Cꢃꢇꢄ × 100 (4)
( )
ꢈ
ꢉ
Where W1 and W2 are the weight of specimens before and after immersion,
respectively, ꢂꢃꢇꢄ and ꢂꢃꢄ are the corrosion rates in the absence and presence of bis-
phosphonic acids respectively, S is the surface area of the specimen (cm2), t is the immersion
time (h) and θ is surface coverage.
2.1.2. Electrochemical measurements
A three-electrode cell was used for the electrochemical experiments using a PGZ 301
Volta lab 40 and the data were analyzed with Volta Master IV software.
The Counter-Electrode (CE) was a plate of platinum and the Saturated Calomel
Electrode (SCE) was also used as a Reference Electrode (RE). The Working Electrode (WE)
is made by the XC48 steel which has the composition (wt. %) C: 0.520, P: 0.035, Si: 0.35,
Mn: 0.77 and the rest is Fe. Prior to all tests, the working electrode and weight loss specimens
were abraded with a collection of various grades of emery paper (500, 800, 1200, 2400 and
4000 grade). Then, it was rinsed with acetone and bi-distilled water, and dried before their
immersion in the acidic solution.

Before each Polarization (P) and Electrochemical Impedance Study (EIS), an open
circuit potential (OCP) is tested up to 30 mn. The polarization curves were started from the
cathodic to the anodic direction (OCP ± 250 mV) with a scan rate of 0.5 mV/s. EIS
experiments are performed at OCP in the 100 kHz to 30 mHz frequency range, with a signal
amplitude perturbation of 10 mV using alternative current (AC).
The inhibition efficiencies IEP and IEEIS are calculated using the equations (5) and (6)
[31, 32] respectively:
ꢇ
ꢇ
ꢈ
ꢉ
IEꢊ(%) = (iꢋꢌꢍꢍ − iꢋꢌꢍꢍ)/iꢋꢌꢍꢍ × 100
(5)
ꢇ
ꢈ
ꢉ
IEꢎꢏꢐ(%) = (Rꢋꢑ − Rꢋꢑ)/Rꢋꢑ × 100
(6)
Where ꢒꢓꢇꢔꢕꢕ and ꢒꢓꢔꢕꢕ are the corrosion current densities, ꢖꢓꢇꢗ and ꢖꢓꢗ are the resistance
in the absence and presence of inhibitors, respectively.
Double-layer capacitance values were obtained at max frequency (fmax) and
determined using the relation (7):
(
)
Cꢘꢙ = 1/2πfꢚꢛꢜ × ꢝ1/Rꢊꢞ
(7)
2.1.3. Spectroscopy analysis
Infrared spectra (IR) were recorded using a FT-IR 4200 JASCO spectrometer, in the
range 4000-600 cm-1, at room temperature. A Bruker AV III 300 MHz spectrometer was used
to record Nuclear Magnetic Resonance (NMR) spectra, in DMSOd6.
2.1.4. Atomic Force Microscope AFM
Oxford Asylum Research MFP-3D Classic instrument was used to analyze surface
morphology of XC48 steel.
2.2. Computational details
2.2.1. Quantum chemical calculations (DFT)
The use of theoretical methods was in later years a subject of particular interest in the
corrosion inhibition studies field. The detailed calculation of the molecular structures of the
compounds (1) and (2) performed with Gaussian09 program, based on Density Functional
Theory (DFT), with 6-31G basis set and B3LYP correlation functional [33, 34].
For the calculation of the fraction of transferred electrons we use the relation (8)
below:
ꢠ
ꢣ ꢠ
ꢤꢥꢦ
ꢡꢢ
∆ꢟ =
(8)
ꢈꢁ( ꢧ ꢨ ꢧ
)ꢉ
ꢡꢢ
ꢤꢥꢦ
Where χ ꢩꢪ and χ ꢫꢬꢭ are the electronegativity of iron and inhibitor respectively, η ꢩꢪ and
η ꢫꢬꢭ are the hardness of iron and of inhibitor respectively. Theoretical values of η ꢩꢪ = 0
and χ ꢩꢪ = 4.82 ꢮꢯ [35].

2.2.2. Molecular Dynamic Simulations (MDS)
The MDS were performed using the program Materials Studio 7.0 [36]. Fe(110)
surface was chosen for simulations of adsorption modes and was conducted in a simulation
box (17.20 × 22.93 × 22.93 Å). The MDS was realized at 298 K and Normal Volume and
Temperature (NVT) ensemble, using the COMPASS force field, with a time phase of 0.1 fs
and simulation step of 50 ps.
The interaction energy of inhibitors on the Fe(110) surface was determined using the
following relation (9):
Einteraction = Etotal – E(Fe + H2O) – Einh
(9)
Ebonding = – E interaction
(10)
Where Etotal is the total energy of the simulated system, EFe+H2O is the energy of iron
surface together with H2O molecules and Einh is the energy of the free inhibitor molecule. The
binding energy is the negative value of interaction energy.
3. Results and discussion
3.1. Synthesis
The two compounds were synthesized according to the Michaelis-Arbuzov mechanism
[37] (Fig.1) 2,6-bis (hydroxymethyl)-4-methoxyphenol (1) or 4-chloro- 2,6-bis
(hydroxymethyl) phenol (2) (1mmol, purity 99 %, Sigma Aldrich) is added to the P(OCH3)3
(2 mmol, purity 99 %, Sigma Aldrich), a vigorous reaction starts, with the appearance of
methanol (CH3OH) under gentle heating. The reaction was established under a reflux of
nitrogen at room temperature. Yellow color appears after 45 minutes. Thin Layer
Chromatography TLC accompanied the reaction, using a silica gel layer (ethyl acetate/n-
hexane 2:8). The volatile components are removed in vacuum at 50 °C. The ester
intermediates are readily hydrolyzed by H2O/CH3OH. The yields of the intermediate bis-
phosphonic ester as well as its resulting hydrolysis are respectively 40 and 80 %.
3. 2. Spectroscopic analysis
3.2.1. FT-IR spectroscopy
The absorbed frequencies characteristic of the functional groups present in the two
compounds and their assignments are summarized in Table 1. The vibrational frequencies
were computed and then scaled by factor 0.96[38]. The Gauss View software was called for
visual animation and also for examining the normal modes assignment.

The Fig. 2 (left and right) shows the calculated and experimentally IR spectra of (1)
and (2). Due to the structural resemble of the two molecules, many vibration modes are
common. The spectra show a broad absorption band at (3450, 3409 cm-1) due to the stretching
vibration of the phenolic group of (1) and (2) respectively, the bands at (1644, 1626 cm-1) are
due to ν (C=C) and (1484, 1437 cm-1) are associated to ν(C–C) aromatic of (1) and (2)
respectively. While, the appearance of the bands at (1299, 1219 cm-1) of P=O, the bands at
(2928, 2849 cm-1) of ν(P-OH), in addition, the bands at (951, 916 cm-1) of ν(P–O) and the
bands at (779, 761 cm-1) of ν(P–C) of (1) and (2) respectively confirm all the obtaining of the
bis-phosphonic acid. Finally, the appearance of the band at 1303 cm-1 (Fig.2 left) due to ν
CAromatic-O-C characteristic of methoxy substituted group of (1) and the appearance of the
band at 696 cm-1 (Fig.2 right) corresponds to CAromatic-Cl of the chloro substituted group of
(2).
3.2.2. NMR spectroscopy
1
The calculated and experimental H NMR, 13C NMR and 31P NMR obtained in
DMSOd6 spectra are shown in Fig.3, and the chemical displacements are summarized in Table
2. All values are in very good agreement with the experimental ones, except OH chemical
changes cannot be estimated correctly due to their participation in hydrogen bonding, so they
were not included in the correlation calculations [39].
1
The H NMR spectra of (1) and (2) molecules were shown in Fig.3b (left and right
respectively). The singlet (1H, s) signal at 2.89 and 2.94 ppm corresponds to CAr-CH-P proton
of (1) and (2) respectively. The peak at 4.78 and 4.8 ppm is attributed to the proton of P-OH
acid of (1) and (2) respectively. The aromatic protons CHAromatic of (1) and (2) gave a
multiplet at 6.58 and 7.07 ppm, respectively
6.94 and 7.37 ppm, respectively finally the singlet at 3.81 ppm corresponds to OCH3 proton
of (1).
, the peaks of OH phenolic of (1) and (2) appear at
,
,
13
The C NMR spectra (left and right respectively) show peak at 38.5 and 37.5 ppm
attributed to the carbon of CAr-CH-P of (1) and (2) respectively, the peaks corresponding to
aromatic carbons CHAromatic of (1) and (2) are observed between 111-153 ppm except carbon
linked to chlorine of (2), appears at 126.7 ppm characteristic of para chloro substitution. The
peak at 55.8 ppm is attributed to the carbon of O-CH3 characteristic of para methoxy
substitution of (1).
The 31P-NMR spectra of (1) and (2) exhibit a single peak at 34 and 31 ppm
confirming the presence of phosphorus in (1) and (2) respectively.

3.3. Quantum chemical calculations
In order to compare the difference in electronic and non-linear properties on methoxy
and chloro substitution in para position, the structures of (1) and (2) were optimized. The
geometric optimizations of the two acids were performed by the minimization of energy
utilizing DFT at the BLYP level and 6-31 G (d, p) basis set (Fig.4a).
The NBO charge analysis is applied to determine the charge distribution over the
molecular structure of (1) and (2) (Fig.4b), the Table 3 shows certain selected characteristic
NBO atomic charges, the NBO charge analysis provides an idea of the adsorption cites [40].
In general the distribution of NBO charges is almost the same for the two molecules. All the
hydrogen atoms exhibit a net positive charge and some carbon atoms exhibit a positive
charge, while the other carbon atoms exhibit a negative charge. It is also seen from the Table
3 that the oxygen atoms have a maximum negative charge value between –0.55 and –0.63 in
the OH group for phenolic and bis-phosphonic groups, these oxygen atoms can be adsorbed
on the metal surface through the acceptor–donor type reaction [41], these results promote the
formation of a chelate on XC48 steel surface. While, the maximum positive charge (1.08-
1.12) is principally focused mainly on phosphorus present in the bis-phosphonic groups,
which can be classified as desorbed sites.
The Fig.4c shows the corresponding 3D graph of the Molecular Electrostatic Potential
(MEP) produced around (1) and (2). The colors red and yellow represent the MEP's negative
regions and correspond to the nucleophilic sites, whereas the color blue indicates the positive
regions corresponding to the electrophilic sites. This theoretical finding shows that the
geometrical nucleophilic regions of both (1) and (2) are centered over oxygen atom of a P=O
bond while the electrophilic regions are situated over hydrogen atoms of O-H bonds.
The Fig. 5 illustrates the spatial distribution of the frontier molecular orbital. The
reactive ability of the molecules is related to their frontier molecular orbitals [42], the energy
of the Highest Occupied Molecular Orbital (EHOMO) is directly related to ionization potential,
signifies electron donating ability to attract acceptor molecules with low-energy of empty
molecular orbital, thus can explain the adsorption of (1) and (2) on XC48. While, the energy
of the lowest unoccupied molecular orbital (ELUMO) is entirely related to electron affinity
signifies the electron receiving tendency.

The green and red surfaces reflect the difference in electron density, the red
regions display negative charge (accumulation of electrons) and the green regions display
positive charge (loss of electrons). The distribution of electron density of HOMO and
LUMO of both (1) and (2) is located mainly on the benzene ring, suggesting that the
benzene ring forms active sites for adsorption on XC48.
The energy (EHOMO), the energy (ELUMO), energy gap (∆Egap) and some related physic-
chemical parameters of the optimized molecular structures (1) and (2) were calculated in gas
and water media (Table 4).
The energy of LUMO (-0.58 and -0.67 eV ) of both (1) and (2) is very low, which
entail the high ability to accept electrons from the d-orbital of Fe and a high stability of the
Fe–(1) and Fe–(2) complexes [41]. Otherwise, the dipolar moment (µ) is a measure of the
polarity of the covalent bonds, which is associated to the distribution of electrons in molecules
[43]. It is clearly seen that, the dipolar moment increases in passing from para methoxy to
para chloro substituted bis-phosphonic acid, also, the polarization of the molecule is
influenced by the molecule/solvent interaction [44], moreover, the large values of µ aid the
adsorption. As shown in the Table 4, the low value of the overall hardness (η) and the high
softness value (σ) for both (1) and (2) can produce a high inhibitory efficacy [45]. The global
index electrophilicity (ω) reflects the capacity of inhibitor molecules to accept electrons.
According to Lukovit, if ΔN is little than 3.6, the efficiency of inhibition increases with the
increase in electron transfer capacity to the metal surface[46], the ΔN values for both para-
methoxy and chloro substituted in this study are little than 3.6, showing that the electrons are
transferred from the molecules to the XC48 steel.
About the influence of the solvent on the various quantum chemical parameters, as can
be seen from Table 4, the solvent greatly affected dipolar moment (µ), this change is caused
by the interaction between the molecule and the solvent. Although the presence of solvent
affects moderately the gap energy ∆Egap, electronegativity (χ), fraction of transferred electrons
(ΔN), hardness (η), softness (σ) and NBO charges of both (1) and (2).
3.4. Corrosion results
In this section, chemical and electrochemical measurements were performed in focus
to characterize and compare the (1) and (2) inhibitive efficiencies. The effects of the
concentrations and the temperature were studied.
3.4.1. Weight loss measurements
The weight loss analysis was used to evaluate the corrosion rate CRW (mg.cm‒2.h‒1),
the surface coverage (θ) and the values of inhibition efficiency (IEW (%)) for the XC48 steel

corrosion against 1 M HCl for different temperatures (T = 298, 303, 313, 323 K), with and
without various concentrations (c = 0, 1.10‒3, 5.10‒3, 1.10‒2 and 5.10‒2 mM) of (1) and (2)
(Table 5). The temperature is one of the variables that can alter the actions simultaneously of
inhibitors and substrates in aggressive environments [47].
3.4.1.1. Effect of temperature and inhibitors concentration
It can be seen from the Fig. 6 that the corrosion rate CRW decreases, in opposite, the
inhibition efficiency IEW (%) raises with concentrations raising and temperature decreasing,
this phenomenon can be attributed to a good adsorption on the metal surface.
It can be seen that the compound (1) containing the methoxy groups in para position
gives higher inhibition efficiency (IE = 93 %) and low corrosion rate (CRW = 0.12 mg.cm‒2.h‒1
)
compared to compound (2) containing in para position the chloro group (IE = 84 %, CRW
0.27 mg.cm‒2.h‒1).
=
3.4.1.2. Thermodynamic calculations and adsorption isotherm
In order to get more information on the corrosion mechanism, the activation
parameters are evaluated using the following Arrhenius (relation 11) [48]:
Cꢃꢄ = Aꢍexp(−Eꢛ/RT)
(11)
Where, Ea is the activation energy, T is the absolute temperature, Ar is the Arrhenius
pre-exponential constant and R is the noble gases constant.
Enthalpy and entropy were calculated using the form of Arrhenius (relation 12):
Cꢃꢄ = RT/Nhexp(∆Sꢛꢇ/R)exp(−∆Hꢛꢇ/RT)
(12)
Where, h is Planck’s constant and N is Avogadro’s constant, R is the gas constant, ∆ꢰꢱ0
is the enthalpy and ∆ꢅꢲꢇ is the entropy.
The activation energy for different concentrations of the (1) and (2) can be estimated
from the plots of logarithm of CRW versus 1/T (Fig. 7). The values of activation energies are
given in Table 6.
The Fig.8 shows the curve of ln(CRW/T) versus 1/T. The straight lines were obtained
with the slope of ‒∆ꢰꢲꢇ/ꢖ and an intercept of ln(R/Nh+∆ꢅꢲꢇ/R) from which the values of ∆ꢰꢲꢇ
and ∆ꢅꢲꢇ were calculated and listed in Table 5. It was found that for both compounds (1) and
(2), with inhibitors ∆ꢰꢲꢇ shows higher values (53, 63 KJ.mol-1) than those obtained from the
uninhibited solution.
It is clear that Ea values (Table 6) within the existence of inhibitors (1) and (2) are
considerably higher than in their absence. A rise in the thickness of the double layer is

suggested in the presence of the inhibitor, resulting in an increase in the activation energy
limit for the corrosion process [49]. While the positive signs of enthalpies are indicative of the
endothermic aspect of the process of steel dissolution [50]. The entropy value in the absence
of the inhibitor is about -127.12 J.mol‒1.K‒1 and in the existence of (1) and (2) is about -34.50
and -41.49 J.mol‒1.K‒1 respectively, which suggests that there is an increase in values entropy,
this change can be explained by a change in the disorder over the duration of the
transformation of the reagents into active complexes. The mechanism of removing the water
(H2O) molecules during the adsorption of (1) and (2) molecules on the steel surface can also
clarify this behavior [51].
The isotherm adsorption can give important information about the mechanism of
interaction between the inhibitor and the steel surface [52]. This was determined by applying
different isothermic adsorption models such as Langmuir, Freundlich, Frumkin and Temkin to
the same set of experimental results, where we found that model Langmuir (Fig. 9) serves
better than all other models (R = 0.9999). According to these isotherms, θ is related to the
inhibitor concentration C via the following relation [53]:
C/θ = 1/Kꢛꢘꢳ + C
(13)
Where, C is the concentration of inhibitor, θ is surface coverage, which have been
determined by weight loss analysis.
The adsorption equilibrium constant (Kads) is attached to the standard free energy of
adsorption (∆ꢴꢲꢇꢵꢶ) of the inhibitor molecules by the relation [54]:
ꢷꢲꢵꢶ = 1/55.5exp(−∆ꢴꢲꢇꢵꢶ/ꢖꢸ)
(14)
Where R is the noble gases constant, T is the temperature in K, and 55.5 value
represents the molar concentration of water in the solution.
The values of Kads and ∆ꢴꢲꢇꢵꢶ are recapitulated in Table 7.
The adsorption enthalpy and entropy (∆ꢰꢲꢇꢵꢶand ∆ꢅꢱ0ꢹꢺ) can be calculated using the
relation:
ln Kꢛꢘꢳ = ln(1/55.5) – (∆Hꢛꢇꢘꢳ/RT) + (∆Sꢛꢇꢘꢳ/R)
(15)
Using the relation (13), the values of ∆ꢰꢲꢇꢵꢶ and ∆ꢅꢲꢇꢵꢶ were obtained from the slope
and the intercept of the plot of lnKads versus 1/T (Fig.S1). The values of ∆ꢰꢲꢇꢵꢶ and ∆ꢅꢲꢇꢵꢶ were
calculated and listed in Table 7. The negative value of ∆ꢰꢲꢇꢵꢶ indicates that the adsorption
process is exothermic.

From the Table 7, Kads values follow the order: Kads (1) > Kads (2), indicating that the
para methoxy substituted bis-phosphonic acid exhibits a better inhibitive performance than the
para chloro substituted. At last, Kads values decrease with the increase of temperature, these
results explain the values of IEW (%) get from Table 4.
The calculated values of ∆ꢴꢲꢇꢵꢶ (Table 7) were most negative than
‒
40 KJ.mol-1
indicate exchange or transfer of the charge from the inhibitors to the steel surface and forming
a covalent bond between the metal and the inhibitor, indicating thereby, that the adsorption of
the inhibitors (1) and (2) on the steel surface XC48 is a chemical [48].
3.4.2. Potentiodynamic polarization studies
Tafel curves for XC48 steel in 1M HCl solution at various concentration (c = 0, 1.10‒3,
5.10‒3, 1.10‒2 and 5.10‒2 mM) of the (1) and (2) at the temperature T = 298 K are shown in
Fig.S2.
Fig.S2 shows that the addition of the (1) and (2) reduces both anodic and cathodic
current densities, this result indicates the mixt nature of the inhibition process without altering
the release of hydrogen and the anodic deterioration of the metal[55], in addition, a small
change in the corrosion potential towards negative values is observed.
Electrochemical corrosion parameters, as corrosion potential Ecorr (mV/SCE), cathodic
and anodic Tafel slopes ßc and ßa (mV/dec), the corrosion current density icorr (μA.cm‒2) and
inhibition efficiency IEp (%) are given in Table 8.
From the polarization method, the (1) and (2) protect well the XC48 steel surface, this
result is related to the reducing of the icorr and increasing of inhibition efficiencies with rising
concentrations, also the para methoxy substituted (1) has better results than the para chloro
substituted (2), respecting the order: EIp (%) (2) < EIp (%) (1).
3.4.3. Electrochemical impedance spectroscopy
Nyquist plots for mild steel after 1 h immersion in 1M HCl solution in the absence and
the presence of the various concentrations (c = 0, 1.10‒3, 5.10‒3, 1.10‒2 and 5.10‒2 mM) of (1)
and (2) at the temperature T = 298 K are illustrated in Fig. S3.
The curves of impedance present a single capacitive semicircle. These semicircles
Nyquist plots aren't perfect due to the interfacial impedance frequency distribution. This
activity is a product of roughness, surface heterogeneity, and adsorption–desorption action on
XC48 steel surface by inhibitive molecules [56]. Moreover, these diagrams have a

comparable shape for all tested concentrations, revealing no change in the corrosion
mechanism and it is controlled by charge transfer process [57, 58].
The diameters of the imperfect semicircles in the existence of (1) and (2) were greater
than those found in the blank solution (1 M of HCl), as well as increasing concentrations of
inhibitors, which may be correlated with the increase in the surface coverage of inhibitor
molecules on the steel surface.
The Table 9 summarizes the electrochemical values of Rp (Ω cm2), CdI (μF cm-2), fmax
(Hz) and (EEIS %) of the (1) and (2) derived from EIS. The values of CdI decrease while the
values of Rp increase with increased the concentrations. The increase in Rp values is caused
by inhibitor adsorption which reduce the exposed region. The decrease in CdI can also result
from an increase in electric double-layer thickness, suggesting that (1) and (2) inhibitors
operate by adsorption on the metal surface.
The inhibition efficiency values increase with the increase of the concentration of the
inhibitors (Table 9). The electrochemical impedance spectroscopy also shows that (1) and (2)
have a strong inhibition effect, with the same order: EIEIS (%) (2) < EIEIS (%) (1).
3.5. Atomic Force Microscope AFM
The atomic force microscope was used to describe the morphology of microstructures
[59], in our study, it demonstrates the formation of the protective film on XC48 steel. In
Fig.S4, AFM images of XC48 surface before and after 6 h of immersion in acidic solution are
shown with and without the presence of the (1) and (2).
In the Table S1, the measured roughness of XC48 steel in 1 M HCl with and without
inhibitors was summed up. The average roughness value of XC48 steel in HCl (169 nm) was
largely higher than that of the polished one about 15.8 nm but the roughness are reduced to
25.5 and 35.2 nm respectively in the presence of (1) and (2) (Fig.S4). Such findings show that
the HCl acid attacks the steel surface, and the adsorbed molecules of inhibitors reduce the rate
of corrosion and degradation of the metal surface. Finally, we can say that the findings
obtained earlier are verified by the AFM.
3.6. Molecular Dynamic Simulation MDS
The MD Simulations (MDS) were performed on Fe(110) (Fig.S5) in order to study the
adsorption behaviors of (1) and (2) molecules. In absence of solvent, the molecules (1) and (2)
are almost horizontal on the surface of Fe(110), the oxygen atoms will provide the unoccupied
iron d-orbital with electrons to form coordinate bonds (chemical interaction) and the physical

interaction can be strong between the benzene ring and the iron surface [60], in the presence
of the solvent, it is clearly seen that the water molecules are positioned mostly at the interface
between the organic molecules and the iron surface resulting a loss of the horizontal
conformation which signifies a lowering of the physical interaction, the space between the
oxygen atoms and the iron atoms is unchangeable, indicating that chemical adsorption is
maintained between both the oxygen and the iron atoms.
Table S2 summarizes the adsorption energies of (1) and (2) on Fe(110), it shows that
the energy of interaction between the (2) molecule and the Fe(110) surface is lower than that
obtained by the (1) molecule. In principle, the high binding energy value is associated with
the most rigid molecules/surface interactions [61]. This indicates that (1) may have better
efficiency as an inhibitor of corrosion than (2) molecule, this result is in accord with the
experimental findings.
4. Conclusion
2-hydroxy-5-methoxy-1, 3-phenylene bis (methylphosphonic acid) and 5-chloro-2-hydroxy-1,
3-phenylene bis (methylphosphonic acid) were synthesized and IR and NMR spectroscopies
were used to establish their molecular structures, and the spectroscopic results were confirmed
by DFT calculation. Also, theoretical DFT calculations show good synthesized molecules
reactivity and predict strong XC48 surface adsorption due to donor-acceptor interactions and
conjugate π bonds. The corrosion inhibition of XC48 steel in 1 M HCl has been studied,
resulting in strong inhibition efficiencies of about 93.5 and 87 % for (1) and (2) respectively.
The adsorption of (1) and (2) on the XC48 steel surface obeys to Langmuir adsorption
isotherm. Also, the surface morphologies of attacked and inhibited surfaces were followed by
AFM, confirming the previous results. The MDS was performed in focus to visualize
adsorption configurations on Fe(110) surfaces, and the determined binding energy, all studies
prove that (1) is more efficient than (2) for the iron protection.

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Figures captions:
Fig. 1: Reaction scheme.
Fig. 2 : FT-IR Spectrum (a) calculated (b) experimental of in the left (1) and in the right (2).
Fig. 3: (a) 13 C NMR (b) 1H NMR and (c) 31P NMR Spectra for (1) in the left and for (2) in the
right.
Fig. 4 : (a) Optimized molecular structures (b) Charge NBO and (c) Total electron density
mapped with electrostatic potential (1) in left and (2) in right.
Fig. 5 : Molecular orbital energy diagram of (1) and (2).
Fig. 6: Corrosion rate and Inhibition efficiency vs. concentration of (1) and (2) in acidic
medium
at various temperatures.
Fig. 7: ln(CRW) versus 1/T for XC48 steel in acid medium in the absence and presence of
various concentrations of (1) and (2).
Fig. 8: ln (CRW/T) versus 1/T for XC48 steel in acidic medium in the absence and presence of
different concentrations of (1) and (2).
Fig. 9: C/θ vs. C of (1) and (2) bis-phosphonic acids for XC48 steel in acidic medium at
various
temperatures.

Table titles:
Table 1. Selected IR frequencies and their assignments.
Table 2. NMR experimental and calculated peaks positions.
Table 3. Selected calculated NBO atomic charges of (1) and (2) gas phase and in aqueous
solution.
Table 4. Calculated quantum chemical parameters of the (1) and (2).
Table 5. Gravimetric results and some deduced thermodynamic parameters.
Table 6. Thermodynamic parameters.
Table 7. Thermodynamic parameters derived from Langmuir adsorption isotherm.
Table 8. Electrochemical parameters derived from TAFEL curves.
Table 9. Electrochemical parameters derived from impedance diagrams.

Table 1.
B3LYP/6-31G
Unscal Scaled
Exp.
Assignements
3450
3250
2928
1644
1484
1303
1299
951
3536
3340
2926
1736
1565
1412
1389
1055
865
3395
3206
2809
1666
1502
1356
1333
1012
830
OH Phenolic
ν (C–H) Aromatic
P–OH
δ (C=C) Aromatic
ν (C–C) Aromatic
ν CAromatic-O-C
ν (P=O)
ν (P–O)
ν (P–C)
779
B3LYP/6-31G
Unscal Scaled
Exp.
Assignements
3409
2932
2849
1626
1437
1219
916
3460
3218
2953
1732
1523
1329
1027
874
3322
3089
2835
1663
1462
1276
986
OH Phenolic
ν (C–H) Aromatic
P–OH
δ (C=C) Aromatic
ν (C–C) Aromatic
ν (P=O)
ν (P–O)
ν (P–C)
761
839
696
742
712
CAromatic-Cl
ν : Elongation vibration, δ : Deformation vibration

Table 2.
1H NMR (ppm)
B3LYP
13C NMR (ppm)
B3LYP
31P NMR (ppm)
B3LYP
Exp.
Assig.
Exp.
Assig.
Exp.
Assig.
/6-31G
2.63
3.76
4.29
6.48
-
/6-31G
40.08
59.96
116.1
128.36
148.0
/6-31G
33.8
2.89
3.81
4.78
6.58
6.94
-
CAr-CH-P
OCH3
P-OH
CH Ar
OH
38.5
55.8
111.5
125.8
146.5
153.0
CAr-CH-P
O-CH3
CH Ar
CAr-C-P
CAr-OH
34
-
-
-
-
2 P
-
-
-
-
-
-
-
-
-
-
-
155.03 CAr-OCH3
-
-
1H NMR (ppm)
B3LYP
13C NMR (ppm)
31P NMR (ppm)
B3LYP
B3LYP
Assig.
Exp.
Assig.
Exp.
Exp.
Assig.
/6-31G
2.82
4.18
6.86
-
/6-31G
/6-31G
2.94
4.80
7.07
7.37
-
CAr-CH-P
P-OH
CH Ar
OH
37.5
126.2
126.7
127.2
152.5
40.06
131.05
138.3
141.0
161.6
CAr-CH-P
CAr-C-P
CAr-Cl
CH Ar
CAr-OH
31
-
-
-
-
30.04
P
-
-
-
-
-
-
-
-
-
-

Table 3.
(1)
Gas
(2)
Gas
1.12
Atom
P1
P2
O1
O7
O4
C8
Water Atom
Water
1.10
1.08
-0.63
-0.61
-0.61
-0.50
-0.10
-0.04
1.08
1.09
-0.55
-0.63
-0.60
-0.49
-0.08
0.24
1.07
1.11
P1
P2
O7
O3
O6
C8
C4
Cl
1.08
-0.60
-0.65
-0.63
-0.50
-0.09
0.24
-0.61
-0.63
-0.57
-0.48
-0.09
-0.02
C9
C1

Table 4.
(1)
(2)
Theoretical parameters
Gas
Water
-
-5.82 -6.22 -6.47
-0.53 -0.67 -0.81
5.82
0.53
5.29
11.68
2.64
0.38
3.17
1.90
0.31
Gas
-
Water
-
Etot (KeV)
EHOMO (eV)
ELUMO (eV)
-44.52
-5.86
-0.58
5.86
0.58
5.27
8.23
2.63
0.34
3.22
1.97
0.30
Ionization potential (I, eV)
Electron affinity (A, eV)
∆E gap (eV)
Dipolar moment (μ,Debye)
Global hardness (η, eV)
Global softness (σ, eV-1)
Electronegativity ( χ , eV)
Global electrophilicity(ω)
∆N
6.22
0.67
5.55
4.89
2.77
0.38
3.44
2.13
0.25
6.47
0.81
5.66
10.94
2.83
0.35
3.64
2.34
0.21
I=-EHOMO, A=-ELUMO, χ =-[0.5(ELUMO+ EHOMO)], η=0.5(ELUMO- EHOMO), ω= χ2/2η. σ=1/ η.

Table 5.
(1)
θ
-
(2)
θ
-
0.51 52
0.79 79
0.80 80
0.84 84
T
C
IE
-
IE
-
CRW
1.72
CRW
1.72
0.83
0.35
0.34
0.27
2.69
1.34
0.74
0.59
0.51
3.71
1.93
1.05
0.89
0.74
5.80
3.53
2.37
1.85
1.74
Blank
1.10‒3 0.48 0.72 72
5.10‒3 0.34 0.80 80
1.10‒2 0.15 0.91 91
5.10‒2 0.12 0.93 93
Blank
2.69
-
-
-
-
1.10‒3 0.81 0.70 70
0.50 50
0.72 72
0.78 78
0.81 81
510‒3
0.69 0.74 74
1.10‒2 0.40 0.85 85
5.10‒2 0.35 0.87 87
Blank
3.71
-
-
-
-
1.10‒3 1.15 0.69 69
5.10‒3 1.04 0.72 72
1.10‒2 0.63 0.83 83
5.10‒2 0.55 0.85 85
0.48 48
0.71 72
0.76 76
0 .8
80
-
0.39 39
0.59 59
0.68 68
0.70 70
Blank
5.80
-
-
-
1.10‒3 2.03 0.65 65
5.10‒3 1.74 0.70 70
1.10‒2 1.45 0.75 75
5.10‒2 1.23 0.79 79
System unit: C(mM), T(K), CRW(mg.cm‒2.h‒1), IEW(%).

Table 6.
Ea-∆ꢀꢁꢂ
3.0
2.7
2.7
2.7
2.7
2 .5
2 .5
2 .5
2 .6
∆ꢀꢁꢂ
33.50 ‒ 127.12
∆ꢃꢁꢂ
C
Ea
Blank 36.50
1.10‒3 43.56
5.10‒3 50.69
1.10‒2 54.73
5.10‒2 56.02
1.10‒3 42.98
5.10‒3 47.93
1.10‒2 65.68
5.10‒2 67.76
40.90
47.97
51.97
53.22
‒108.4
‒92.45
‒80.55
‒ 34.50
40.49 ‒114.31
45.31 ‒100.10
63.17
65.18
‒ 46.68
‒ 41.49
System unit: C(mM), Eads (KJ.mol-1, ∆ꢀꢁꢂꢄꢅ (KJ.mol-1), ∆ꢃꢁꢂꢄꢅ(J.mol‒1.K‒1).

Table 7.
∆ꢇꢁꢂꢄꢅ
‒45.72
‒ 46.26
‒ 47.73
‒49.01
‒ 45.10 ‒18.62
‒ 45.62
‒ 46.60
∆ꢀꢁꢂꢄꢅ
‒ 05.82 133.6
∆ꢃꢁꢂꢄꢅ
T(K)
298
303
313
325
298
303
313
325
ꢆꢁꢄꢅ
1862
1723
1665
1523
1452
1319
1078
0800
(1)
(2)
088.7
‒ 47.29
System unit: ∆ꢇꢁꢂꢄꢅ (KJ.mol-1), Kads (104.L.mol-1, ∆ꢀꢁꢂꢄꢅ (KJ.mol-1), ∆ꢃꢁꢂꢄꢅ(J.mol‒1.K‒1).