Room Temperature Columnar Liquid Crystalline Perylene Bisimide as a Novel Corrosion Resistant Surface Film for Mild Steel Surface

The corrosion process can be seen as a widespread phenomenon, which is both pervasive and unstoppable. This is an undesirable phenomenon that reduces the life of materials and takes away their beauty. Potentiodynamic and electrochemical impedance tests are used to explore the corrosion inhibition abilities of a room temperature columnar liquid crystalline perylene bisimide (PBIO10) on mild steel (MS) samples in 1 M HCl. The inhibitor PBIO10 was demonstrated to be an outstanding corrosion inhibitor, with a maximum inhibition efficiency of 76%. In light of potentiometric polarization results, corrosion inhibition was achieved as the inhibitor getting adsorbed on the metal, and they fit into the category of anodic inhibitors. The protective layer was examined from SEM to confirm the protective coating generated on the MS surface. The increase in contact angle confirms the formation of a uniform layer on the MS surface. Analysis of the optical textures observed in POM, the nature of the mesophase under examination to columnar rectangular (Colr) phase. From the TGA, it was found that PBIO10 exhibits higher thermal stability u to 370 ℃. The density functional theory (DFT) and Monte Carlo simulation approach were used to investigate the relationship between molecular structure and inhibitory efficacy. The thermal behavior of PBIO10 was investigated by polarizing optical microscopy (POM), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and X-ray diffraction (XRD) studies. The phase transition from crystal to LC phase was at first examined with the help of POM observation.


Introduction
The corrosion process is a natural one driven by energy considerations. Usually, in pickling baths, sulphuric acid and hydrochloric acids are typically used to remove the metal oxides from the surface. In engineering and commercial processes, inhibitors are used to minimize metal failure and acid utilization [1][2][3]. Almost all environments are corrosive to some extent and contribute to material failure and also lead to huge economic loss to society [4]. Inhibition in a way helps to prevent corrosive attacks on metallic materials [5] extensively studied using corrosion inhibitors that offer simple solutions for protecting metals from corrosion in aqueous environments, which have been extensively studied. By adding a small concentration of an inhibitor, it can reduce the loss of metal, reduce hydrogen embrittlement, effectively prevent pitting of metals, reduce over-pickling and acid fumes, and reduce acid consumption by forming an adsorbed layer or retarding the anode, cathode, or both corrosion [6]. Under the considerations, the mechanism of inhibition by an inhibitor is mainly due to the adsorption that is influenced by the nature of the inhibitor, surface charge, diffusion barrier, electrical double layer, electrode reactions, blocking reaction sites, and chemical structure of inhibitors. Oxygen-containing compounds such as chromates, phosphates, and tungstate inhibit corrosion by anodic inhibition, thereby reversing the corrosion potential's direction to cathodic [7]. Compounds such as sodium, barium, and sulphur are commonly used as cathodic poisons and lower corrosion rates through hydrogen reduction reactions by slowing down the rate of cell reduction to reduce corrosion [8]. By reacting with metal in neutral or alkaline solution, these inhibitors form a protective film on anodes or cathodes, where the main cathodic reaction is an oxygen reduction reaction in which the metal surface is coated with a film oxide or hydroxide or by depositing directly on the metal surface by changing surface charges and dipoles [9]. Some studies have shown that halogen ions to a certain extent can inhibit corrosion in acid solutions that follow the effectiveness order; I − > Br − > Cl − . Generally, the inhibition performance of different inhibitors will follow the opposite order of their electronegativities, for example among S, N, O, and P, the inhibition performance will follow: O < N < S < P. Moreover, heavy metal complexes of Pb 2+ , Ti + , Mn 2+ , and Cd 2+ are capable of inhibiting corrosion in acid media because of their extreme polarizability and low electronegativities [10,11,11]. These inhibitors are effective on metals and non-metals alike since they cover large areas of surface and effortlessly transfer electrons to the vacant orbitals of their atoms [12]. Among lone-pair electron-containing compounds are amines, quinolines, thio compounds, aldehydes, imidazoles, and derivatives of the hydrazide act as excellent corrosion inhibitors [13][14][15][16][17]. The inhibition action of mild steel by benzaldehyde thiosemicarbazone derivatives in 0.5 M H 2 SO 4 is also reported [18]. Nowadays, a key focus of corrosion inhibition has become human health and safety concerns. Therefore, researchers have started focusing on the use of eco-friendly compounds like plant extracts [19][20][21], the nontoxic organic compounds as green alternatives for toxic and harmful compounds [22,23]. Due to the biodegradability, ecofriendliness, low cost, and easy availability of inhibitors they are used as corrosion inhibitors for several metals and metal alloys under different environmental conditions [19,[24][25][26].
There are several literature reports point out that effective organic inhibitors usually possess π-conjugation and polar groups. The physiochemisorption mechanism plays a major role in this inhibition activity of conjugated aromatic organic molecules. The corrosion inhibition process depends on the nucleophilicity of the polar substituents and π-conjugated aromatic cores, which enhances the inhibition response in the corrosion process. Well-ordered applications were also important for anti-corrosion application. Liquid crystals (LCs) are a special state of matter, which have got the prominence due to their unique blend of order and mobility between solids and liquids. They are exceptionally sensitive to external perturbations like temperature, pressure, electric and magnetic fields and are thus successfully employed in the fabrications of display devices and sensors. The application of LCs for the inhibition of corrosion on metal surfaces has been reported. The exceptional inhibition potential of LCs can be accredited to their hydrophobic nature and good protective coat formation on the metal surface due to their self-healing nature [27]. Discotic LCs (DLCs) are an active research area, which is presently studied from the perspective of organic semiconductors capable of 1D charge transport, and is usually composed of a central polyaromatic core and several peripheral flexible chains [28]. DLCs based on perylene bisimides (PBIs) are one such widely studied DLCs due to the presence of an electron-deficient (n-type behavior) central core. PBI-based DLCs usually stabilize columnar (Col) phases that can act as 1D pathways for charge migration, over a wide thermal range and they are also known for high photo, thermal and chemical stability. Thus we envisaged that the Col LCs formed from PBIs may provide a welldefined highly stable hydrophobic supramolecular structure with a large homogeneous corrosion-resistant film on MS substrate over a wide temperature range.

Materials
In this study, AR grades were utilized without any purification, solvents were dried following the typical standard procedures. The HCl was procured from Merck, India. MS samples of composition C (0.051 percent), Mn (0.179 percent), S (0.023 percent), P (0.0005 percent), and Fe (99.42 percent) [29,30] were procured from the local market, and were sliced into 1cm 2 area, washed, ultrasonicated in absolute ethyl alcohol and polished by the sandpapers of 200-1600 grids. The samples were then dried and placed in desiccators for future use.

Synthesis
The synthetic methodology utilized for the preparation of trialkoxy phenyl-funtionalized PBI O10 is outlined in Scheme 1. Pyragallol was O-alkylated with n-bromodecane using Williamson ether synthesis using anhydrous K 2 CO 3 as a base to get tri-n-decyloxybenzene (1). This was subjected to nitration to obtain 3,4,5-tridecyloxynitrobenzene (2). The nitro compound was reduced to corresponding aniline (3) by reduction using hydrazine hydrate in the presence of 10% palladium on charcoal. The aniline obtained was condensed with perylene tetracarboxylic bisanhydride in presence of imidazole and zinc acetate in a microwave reactor to give the final desired product with 82% yield. The crude compound was then subjected for repeated column purifications on neutral alumina with chloroform-hexane mixture (50%) as an eluent to obtain the final product as gummy red-colored solid. PBI 10 was completely characterized by 1 H NMR, 13 C NMR, IR spectroscopy and, mass spectrometry (Please see SI for details).

Preparation of Sample Medium and Mild Steel
Initially, metal samples with 1 cm 2 exposed surface area were investigated electrochemically, and the epoxy resin was used to cover the unexposed areas. MS specimens were abraded with 200-1600 grade SiC emery sheets before being cleaned in double distilled water, then with tissue paper, the samples were then dipped in ethyl alcohol for five seconds, dried fully, and stored in desiccators [29,30]. In this experiment, we used a commercially available AR grade 37% HCl to prepare 1 M HCl solution. An estimated amount of PBI O10 inhibitor was dissolved in chloroform to prepare concentrations of 100, 200, 300, and 400 parts per million. This inhibitor solution was dropped onto mild steel (MS) surfaces under rapid vibration to ensure the formation of a thin film. In order to ensure proper alignment of the LC film, the MS is heated and cooled several times, then air dried for future use.

Electrochemical Impedence Spectroscopy and Potentiodynamic Polarization Measurements (EIS and PDP)
An electrochemical workstation (CHI660E) was used for the EIS measurements. The measurements were operated using a three-electrode cell containing a platinum electrode, an Ag/AgCl electrode, and a MS plate as a working electrode with a 50 mL solution volume. Various concentrations of the PBI O10 were electrochemically tested at 30 °C after immersing it for 30 min in the test solution [31]. In EIS investigations, the frequency range was from 10.0 kHz to 0.1 kHz, with an amplitude of 0.005 V. The inhibition efficiency (η%) was determined by utilizing the below Eq. (1).
Here, (Rp) a and (Rp) p being polarization resistances in absence and presence of inhibitor, respectively. The potentiodynamic polarization were computed at a scan rate of 0.4 mV/s across a potential range of -200 to + 200 mV [20]. By extending Tafel plots to corrosion potentials, based on anodic and cathodic branches, we can determine the current density (i corr ) and corrosion potential (E corr ). Using the Tafel extrapolation plot and Eq. 2, we were able to determine inhibition efficiency (η%).
Here, i (corr)a and i (corr)p are the corrosion current densities in μAcm −2 in absence and presence of inhibitor, respectively.

Contact Angle Measurements
A Goniometer Insc. was used to measure water contact angles (WCA) on the MS surface, using the sessile drop method [32] and FAMAS (Interface Measurement and Analysis System) software. On MS surfaces, 2µL water droplets were placed. From this analysis, when testing with water, advancing angles will correspond to the hydrophobic domains and receding angles will characterize the hydrophilic domains, on the surface. Thus, contact angle measurements were performed before and after adsorption process of the inhibitor, and the measurement was repeated ten times.

Surface Morphology Studies
The scanning electron microscopy experiment was aimed at observing the morphology of MS metal surfaces before and after the addition of the PBI O10 corrosion inhibitor. Fresh MS samples were air-dried after being immersed in a 1 M HCl solution for 22-24 h. A Tescan Vega 3 scanning electron microscope was used to evaluate inhibitor-treated specimens.
To learn about the effects of the inhibitor at 400 ppm concentrations, the sample was soaked in a 1 M HCl solution and then dried in the air.

Density Functional Theory Calculation (DFT)
DFT experiments utilizing the DMol3 module in Material Studio 2017 (Method: GGA/BLYP; Basis set: DNP 3.5; Solvation model: COSMO) supported the electrochemical findings [33]. These equations were utilized to generate the various DFT indices mentioned in Table 1 [33][34][35][36]based on the energy of frontier molecular orbitals (E HOMO and E LUMO ) Eq. 3 and 4. (3) The energy of the highest-occupied molecular orbital is denoted by E HOMO , whereas the energy of the lowest-unoccupied molecular orbital is denoted by E LUMO . I and A, respectively, stand for the electron affinities and ionization potentials. The symbols X, η, µ, ΔE, and ΔN, respectively, stand for electronegativity, chemical potential, hardness, energy band, and electron transfer percentage. The work function provided by Fe X Fe = 7 eV and Fe ( Fe = 0) was utilized to get the value of ΔN.

Monte Carlo Calculations
Monte Carlo simulations were utilized to show how the adsorbate and substrate chemicals interacted using Material Studio (2017)'s adsorption locator module. The Adsorption Locator module was created by BIOVIA Company based on the Monte Carlo (MC) simulation methodology [37]. We may get more knowledge using Monte Carlo simulations to examine the interfacial interactions between inhibitor molecules and the steel surface. The pre-geometrized conformers were administered at 298 K using Berendsen's thermostat Displacement, force, and energy convergence criteria, which were established at 0.015 Å, 0.5 kcal.mol −1 . Å, and 10 -3 kcal.mol −1 , respectively. The simulation box was 37.23 × 37.23 × 74.19 Å 3 (8 layers of Fe (110), 60 Å as a vacuum zone, and super cell: 15 × 15), and the system included 500 H 2 O + 5H 3 O + + 5Cl − + 1 inhibitor. We have chosen to replicate the adsorption process on the packed surface of Fe (110) because of its high density and high degree of stability throughout simulation. The bulk atoms on the Fe (110) plane were kept frozen throughout the simulations procedure. The van der Waals potential energy was estimated using the atom-based approach, and the electrostatic potential energy was determined using the Ewald summation method. Every structure in the system under consideration was optimized using the COMPASSII force field [38]. The Forcite module has handled geometry optimization. The studied PBI. O10 's adsorption energy on the Fe substrate was calculated using Eq. 8 and documented in Table 1 [39,40] Here,E total , E Metalsurface , E inhibitor denote to the total energy of a system, energy of a metal surface, and energy of an inhibitor respectively.

Thermal Behavior
The thermal behavior of PBI O10 was investigated by polarizing optical microscopy (POM), differential scanning colorimetry (DSC), thermogravimetric analysis (TGA) and X-ray diffraction (XRD) studies. The phase transition from crystal to LC phase was first examined with the help of POM observation, which showed the appearance of strong birefringence in conjunction with an increase in the fluidity of the compounds on heating. The phase transition temperatures and the corresponding changes in enthalpy were confirmed by DSC thermograms. These values were in agreement with peak temperatures noted in POM studies (Table 2). An analysis of the optical textures observed in POM and the information obtained from XRD studies shown the nature of the mesophase under examination. From the TGA, it was found that PBI O10 exhibits higher thermal stability up to 370 °C (Fig. 1). Under POM observation, PBI O10 exhibits textures corresponding to the columnar rectangular (Col r ) phase. DSC thermograms showed that the mesophase was stable over a broad thermal range, including ambient temperature. The high clearing temperature confirms the strong core-core interactions between the aromatic cores. The assignment of the Col phase was confirmed from the powder XRD studies carried out at different temperatures (Table 3, Fig. 2). The compound PBI O10 , which was a sticky mass, placed between a glass slide and coverslip, on slow heating in a programmable hotstage placed under a POM, showed an increase in fluidity and birefringence. The viscous mass was shearable on the application of mechanical stress and transformed into an isotropic liquid at ≈ 359 °C. Slow cooling of the isotropic melt at a rate of 5 °C/min, showed the mosaic texture emerging from large homeotropic domains, which is a characteristic feature of Col r phases (Fig. 2a,b). The texture remains unaltered even at room temperature, while the DSC shows no signature of crystallization, confirming that the Col phase is stable even at room temperature (Fig. 2b). Powder XRD studies were carried out at different thermal intervals to analyze the mesophase structure. The XRD patterns obtained at 100 °C is comparatively simpler than the ones obtained at room temperature which exhibited many peaks in the low-angle region (Table 3, Fig. 3). The plot of intensity vs 2θ obtained for compound PBI O10 at 28 °C (Fig. 2c) showed a strong peak corresponding to a d spacing of 27.3 Å, followed by weaker peaks centered at smaller d spacings (24.1 Å, 14.1 Å, 12.76, 9.16 Å, 6.94 Å and 5.69 Å) along with two diffused peaks with d-spacings of 4.36 Å and 3.37 Å in the wide angle region (Fig. 2c  inset). These values could be indexed to the (10), (11), (12), (21), (30), (33) and (45) reflections of the Col r phase (Fig. 2c). The first diffuse peak in the wide angle region arises from the packing of aliphatic tails, while the second peak corresponds to the stacking of discs within the columns. The second peak is usually seen when there is superior interaction between the discs in the column, and here it is driven by attractive π-π interactions of the PBI cores. The rectangular lattice parameters calculated were found to be a = 27.30 Å, b = 33.0 Å with c = 3.37 Å shown in a schematic diagram (Fig. 2d).
From the lattice parameters, the lattice area (S) and molecular volume (V) were calculated. From the values of S and V the number of molecules forming the unit rectangular cell, i.e. Z was found to be 1. The XRD pattern obtained at higher temperature (i.e. T = 100 °C, Fig.S5b) also showed a similar pattern but a lesser number of peaks at small angle, confirming the Col r phase. Notably, the core-core peak was not observed here, which is understandable due to the reduced core-core interactions at higher temperatures.

Electrochemical Impedance Spectroscopy Measurements
The inhibitory effect of PBI O10 in 1 M HCl medium was studied over a frequency range of 100 kHz to 0.1 Hz using electrochemical impedance spectroscopy. Figure 3a shows the Nyquist plots for MS in the presence of PBI O10 at concentrations ranging from 100 to 400 ppm in a 1 M HCl acid medium. The semicircular nature of the Nyquist plot suggests that charge transfer is taking place with non-ideal plots with depressed semicircles. The semicircle deviates slightly from a perfect circle due to the inhomogeneity and impurities of the metal surface. Furthermore, the diameter of the graph increases as PBI O10 inhibitor concentration increases, indicating that it is more difficult to transfer charge from MS to the electrolyte (HCl) as inhibitor concentration increases.
The following  Fig. 3b, where it can be seen that the slopes are not equal to -1 due to the nonhomogeneity of the solid surface. Additionally, the observed one-time constant behavior for blank as well as various inhibitor concentrations, displays the charge transfer process as the only relaxation process. An improved corrosion inhibition property is primarily the result of the adsorption of more and more inhibitor molecules to the surface, at higher concentrations on the surface of MS. Using the ZSimpWin software, the EIS spectrum obtained for mild steel was best fitted with the equivalent circuit, as shown in Fig. 3c, where Rs is the solution resistance, Rp is the polarization resistance, and Cdl is the double layer capacitance.

Studies on Potentiodynamic Polarization
The polarization curves for MS in 1 M HCl for a blank and for various concentrations of PBI O10 in 1 M hydrochloric acid are shown in Fig. 4. The corrosion current density i corr (mA cm −2 ) can be calculated from the graph exploration of polarization curves so that the corrosion rate (C R ) can be calculated. Following, the polarization parameters, including corrosion current (i corr ), corrosion potential (E corr ), cathodic Tafel slope (-βc), anodic Tafel slope (βa), and inhibition efficiency (η) have been clearly given in Table 5. The addition of an inhibitor affects both the anodic and cathodic branches, as revealed by the polarization curves. As a result of the presence of some active sites in the molecules, such as aromatic rings, heteroatoms. [26], the inhibitor concentration slows down cathodic reduction and anodic metal dissolution. Therefore, both anodic and cathodic curves shifted towards lower current densities, with deflection values not exceeding ± 85 mV relative to the blank. Anodic corrosion inhibition occurs when the E corr value moves toward positive with regard to a blank, and cathodic corrosion inhibition occurs when the E corr value is negative with respect to a blank. In this case, the values are moving toward the positive, which indicates that predominantly anodic corrosion is occurring. During the adsorption process, inhibitors functional groups and structural characteristics are crucial. Potentiodynamic polarization curves and EIS measurements show good agreement when determining η%.

Contact Angle Measurements
The hydrophobic and hydrophilic characteristics of the MS surface in the presence and absence of an inhibitor were estimated using the liquid sessile drop method and the Goniometer device from Tech Inc. An intermolecular interaction between water and the MS surface was observed using a drop of water placed on the MS surface. Figure 5a illustrates the contact angle of MS in the absence of the inhibitor 75.868°, confirming its partially hydrophilic nature. Figure 5b shows a more hydrophobic nature with 90.948°, in the presence of an inhibitor. The development of a protective layer on MS surfaces helps to improve corrosion resistance.

Surface Examination
The SEM images revealed extensive corrosion of MS surfaces in acidic environments. Figure 6a and b illustrate the surface morphology when MS is not exposed to the PBI O10 inhibitor and when it is exposed to the PBI O10 inhibitor, respectively. A SEM image of a fresh MS sample exposed to 1 M HCl at room temperature is shown in Fig. 6a. There were pits, cracks, and cavities on the ing the intensity against 2θ for the Col r phase at 28 °C (c); Schematic diagram of PBI. O10 showing the Col r phase (considered the XRD pattern obtained at 28 °C) (d) exposed surface. Also in Fig. 6b, the PBI O10 modified surface exhibits no pits or cracks, which indicates that PBI O10 molecules have been evenly coated on the MS surface, inhibiting corrosion. Figures 7a and b show the energy dispersive X-ray spectroscopy (EDX) surface composition of the metal in the absence and presence of PBI O10 using EDX images from the composition table (insets), confirming the deposition of. PBI O10 on the surface. The elements present in Fig. 7a are O-24.31%, P-0.04%, S-0.27%, Cl-7.95%, Fe-67.43% clearly shows metal undergoing corrosion dipped in HCl. In contrast, Fig. 7b showing the elemental composition of N-5.11%, C-73.57% clearly shows that inhibitor molecules are deposited on the metal surface.

DFT Calculations
Figure 8a-d displays the PBI O10 optimized structure, molecular electrostatic potential (ESP), HOMO, and LUMO. As shown in Fig. 8c, the HOMO energy is demonstrated to be localized on the heteroatoms (O = C-N group, aromatic group). The LUMO energy is present on the bulk of heteroatoms and the phenyl ring, as seen in Fig. 8d. Additionally, PBI O10 has a HOMO value of −4.806 eV and a LUMO energy of −3.830 eV. They are the ideal places for interacting with the metal surface as a consequence. The ESP example lends further evidence to this. In Fig. 8b (ESP), the yellow region denotes electrophilicity, whereas the blue area denotes nucleophilicity. The yellow regions are dominated   [41,42]. In general, a smaller gap E (Table 1) promotes higher inhibitor efficacy, increased surface adsorption, and easier molecule polarization. In other words, the experimental data strongly confirm the energy gap for PBI. O10 being 0.976 eV [31,32]. Additionally, it displays the proportion of transmitted electrons (ΔN), a crucial statistic for assessing the effectiveness of the inhibition.
The literature indicates that the capacity to transfer electrons to the metal surface is enhanced by raising the inhibition efficacy to a value of ΔN (2.748) < 3.6 [43][44][45]

Monte Carlo Simulations (MCS)
Using Monte Carlo simulations, the inhibitors' adsorption behavior on the surface of Fe (110) was examined while also taking into consideration the testing circumstances (the continuance of the tested compounds in an HCl solution). Figure 9 shows how inhibitors have significantly steadier low-energy adsorption characteristics on the 500 H 2 O + 5 H 3 O + + 5 Cl. − /Fe (110) molecule system. The adsorption energy of the inhibitor evaluated on the Fe (110)/inhibitor system is listed in Table 1. The investigated inhibitor molecules produced an umbrella-like cover on the Fe surface, as illustrated in Fig. 9, with a preference for vertical orientation, optimizing contact and enhancing surface defence. This adsorption paradigm results from the strong interaction between the phenyl rings of inhibitor compounds and the metal surface. The N and O atoms exhibit strong interactions with one another, many electrons, and the metal surface, which may allow vacant orbital electrons of iron coordinate bonds to form, according to these data. Consequently, MCS was able to confirm the inhibitor adsorption and the development of a stable barrier layer on the mild steel surface as a result. This sequence is also supported by the results of surface characterization and electrochemistry [45]      Data Availability Currently, datasets generated and/or analyzed during the current study are not publicly available, but they can be obtained from the corresponding authors upon request.

Conflicts of interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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