Abstract
Three sets of ionic liquids such as 1-alkyl-3-methylimidazole chloride [Cnmim]Cl, 1-alkyl-3-methylimidazolium acetate [Cnmim]Ac and 1-octyl-3-methylimidazole salt [Omim]Y (n = 2, 4, 6, 8, and Y = Cl, BF4, HSO4, Ac and TFO) were used as corrosion inhibitor medium for corrosion protection of carbon steel. Electronic structures and reactivity of these ionic liquids, surface energy and electronic structures of the iron surface were systematically analyzed by density functional theory. By increasing the alkyl chain length of the [Cnmim]Cl and [Cnmim]Ac systems, the lowest unoccupied molecular orbital energy (ELUMO), the highest occupied molecular orbital energy (EHOMO), the softness (S) and polarizability (α) increased gradually, whereas electronegativity (χ), energy gap (ΔE), hardness (η), dipole moment (μ)and electrophilic index (ω) gradually decreased. For the [Omim]Y system, the structure parameters of ionic liquids are quite different, and only the polarizability (α) decreases gradually by increasing the length of the alkyl chain. The results show that inhibition is mainly [Cnmim]+ cations of the [Cnmim]Cl system, and the order of inhibition efficiency follows as [C2mim]Cl < [C4mim]Cl < [C6mim]Cl < [C8mim]Cl. Both [Cnmim]+ cations and the Ac− anion have inhibition effect for the [Xmim]Ac system, and the order of inhibition efficiency is [C8mim]Ac > [C6mim]Ac > [C4mim]Ac > [C2mim]Ac. For the [Omim]Y system, [Xmim]+ cations and anions (BF4−, HSO4−, Ac−, TfO−) have inhibition effect, and the order of inhibition efficiency is [Omim]TfO > [Omim]Ac > [Omim]HSO4 > [Omim]BF4 > [Omim]Cl.
Keywords
- inhibitor
- carbon steel
- density functional theory
- active site
- global and local activity parameters
- geometry structure
1. Introduction
Metal corrosion is a ubiquitous phenomenon in the national economy, national defense construction, science and technology and other relative fields [1, 2, 3]. It has tremendous negative effects and need to be treated. Corrosion of metal and its alloys in various fields in each country has caused serious problems in different fields and caused a series of major losses. Therefore, we must adopt effective corrosion protection measures. As latest reported from the National Association of Anticorrosive Engineers, the world’s annual corrosion cost can decrease by 15–35% (US $ 375–75 billion) by adopting appropriate anticorrosion technology [1, 2, 3].
Due to its low cost and excellent physical, chemical and mechanical properties, iron and its alloys are widely used in many fields, and most of them are in contact with corrosive environments, such as tanks, pipelines, boilers, oil and gas production units and refineries [1]. Therefore, it is necessary to take certain protective measures, and the addition of an inhibitor can significantly reduce the corrosion rate of iron and ferroalloy, and the amount of inhibitor is small, which has become an economic and effective method [2]. Traditional inhibitors, such as chromate, nitrite, silicate and molybdate, can react with metals to constitute a passive film or a dense metal salt protective film on the metal surface, preventing the corrosion of metals and alloys [3]. However, this kind of inhibitor is usually used in large amount and high cost. In addition, when the amount of inhibitor is insufficient, it will lead to serious local corrosion of metals and alloys. In addition, these inhibitors have certain toxicity and poor biodegradation ability. When they are accustomed, there are serious environmental and ecological problems and they are increasingly restricted. Therefore, it is very important to develop environment-friendly inhibitors. As an environment-friendly inhibitor, imidazolines, amides and other inhibitors have the characteristics of high efficiency, low toxicity and easy biodegradation, which are commonly used in the corrosion inhibition of iron and alloy [1, 2, 3].
In recent years, ionic liquids (ILs) as a new green solvent, which is formed by organic cations with various inorganic or organic anions, have attracted more and more attention [4, 5, 6]. Due to the heterocyclic structure, heteroatoms (such as N, O and S) and multiple bonds in ionic liquid molecules, they have become effective inhibitors and are commonly used in the field of corrosion protection [6, 7, 8, 9]. The ionic liquid common inhibitors are imidazole ionic liquid, pyridine ionic liquid and quaternary ammonium ionic liquid. Compared with traditional corrosion inhibitors, ionic liquid corrosion inhibitors are always renewable, inexpensive, ecologically acceptable, readily available and environmentally friendly and biodegradable. Various ionic liquids such as imidazoline, pyridine, ammonium, pyridazine and benzimidazole were widely used as corrosion inhibitors for carbon steel. The experimental methods such as surface analysis techniques, electrochemical tests and electrochemical impedance spectroscopy are used extensively to study the performance of ionic liquid inhibitors [9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25].
However, experimental research on ionic liquid inhibitors mainly focuses on the evaluation of their corrosion inhibition performance. The research on the corrosion inhibition mechanism of ionic liquid inhibitors is less, which can not reveal some details of the corrosion inhibition process, such as the quantitative description of the molecular metal atom combination of inhibitors, the determination of the functional groups of inhibitors and the information on the process of ion adsorption and film formation of inhibitors [9, 10, 11]. It is impossible to guide the development of ionic liquid inhibitors by obtaining and diffusing the inhibitors on the metal surface. Therefore, it is necessary to study the mechanism of ionic liquid inhibitors by theoretical calculation, so as to provide theoretical guidance for the development and application of new and efficient ionic liquid inhibitors.
With the rapid improvement and advancement of computer science and technology, and the gradual improvement of related theories, quantum chemical calculation as an important theoretical calculation method has been proved to be very effective in determining the electronic structure of molecules and clarifying the reaction activity of molecules. The inhibition mechanism and performance of inhibitors are often the same as the electronic structure and reaction activity of inhibitors. Therefore, it is very important to study the electronic structure and reaction activity of ionic liquid inhibitors for the design of new ionic liquid inhibitors [26]. Murulana et al. [27] studied 1-octyl-3-methylimidazolium bis(trifluoromethyl-sulfonyl) imide ([Hmim]NTF2), 1-propyl-3-methylimidazolium-bis(trifluoromethyl-sulfonyl) imide ([Pmim]NTF2), 1-propyl-2,3-dimethylimidazolium bis(trifluoromethyl-sulfonyl) imide [PDmim]NTF2 and 1-butyl-3-methylimidazolium bis(trifluoromethyl-sulfonyl) imide ([Bmim]NTF2). The results showed that [PDmim]NTF2 has the strongest capability of electron donor, and [Pmim]NTF2 has capability of electron acceptor. The active centers are located in the ring of imidazole cation and the N and O atoms of anions. Sasikumar et al. [28] studied the behavior of 1-decyl-3-methylimidazolium tetrafluoride ([Dmim]BF4), 1-ethyl-3-methylimidazolium boron tetrafluoride ([Emim]BF4) and 1-butyl-2,3-dimethylimidazolium tetrafluoride ([BDmim]BF4) inhibition on low carbon steel by density functional theory in hydrochloric acid solution. The results showed that the variations of the
As mentioned above, although some progress has been made in the study of the corrosion mechanism and properties of low carbon steel by ionic liquid corrosion inhibitors in the corrosion medium, there are still some problems. Experimental methods (weight-loss method, electrochemical experiment method and surface morphology analysis method) are used to study the ionic liquid, mainly focusing on the evaluation of the inhibition performance and efficiency of ionic liquid, but it is difficult to know the inhibition mechanism of ionic liquid on the iron surface. However, the structure parameters calculated by quantum chemistry are not systematically analyzed. For example, for the imidazole system with different cations, how do the structure parameters and property change with the increase of the cationic alkyl chain length? For the system with the same cations and different anions, how do the structure parameters change? There is no systematic analysis of the properties of different surfaces of iron, but it simply selects a surface as the adsorption surface of ionic liquid. At the same time, different surface of iron may have different electric charge and different electric charge. There are some relations between iron and HOMO and LUMO of ionic liquid molecules, but there are few related researches.
Molecular simulation is only a simple calculation of adsorption energy between the metal surface and ionic liquid inhibitors, but there are few studies on the quantitative description of the molecular metal atom combination of ionic liquid, the determination of the functional groups of ionic liquid, and the acquisition of the process information of adsorption and film formation, which cannot guide the development of ionic liquid corrosion inhibitors. In this paper, the equilibrium geometry, frontier orbit, global and local reactivity of [Cnmim]Cl system, [Cnmim]Ac system and [Omim]Y (n = 2, 4, 6, 8, Y = Cl, BF4, HSO4, Ac and TFO) are studied by quantum chemical calculation. The active region, inhibition efficiency of possible interaction between ionic liquid molecules and iron surface are preliminaries analyzed.
2. Computational method
The B3LYP method of density functional theory in Gaussian 09 software package [31] is utilized to optimize the geometric configuration and analyze the frequency of ionic liquid. The 6-311++G(d, p) basis set was used in this paper, which is same as the literature [26, 27, 28]. The geometry optimization and frequency analysis of the studied ionic liquid molecules were performed, and the stable geometry was obtained. There are seven interaction sites of 1-alkyl-3-methylimidazolium cations to interact with anions, as shown in Figure 1, where 1–5 represent the five positions of anions in the plane of imidazole ring, and 6 and 7 are respectively located on both sides perpendicular to the plane of imidazole ring. As confirmed by many authors, position 2 is the most active site and the anions are always located in this position, so we just discuss the anions’ contact with cations at position 2. The
According to the frontier molecular orbital theory, the molecules’ reactivity is related to their lowest unoccupied molecular orbits (LUMO) and the highest occupied molecular orbital (HOMO). In general, the level of the
In the concept density functional theory, according to Koopmans theorem [36], ionization potential
Hardness (
Using the finite difference approximation, the global hardness (
According to Koopmans theorem [36], the electronegativity (
Global softness (
Electronegativity (
According to the definition, this parameter is a measure of the capability of electron acceptors. The larger the value is, the stronger the capability of electron acceptor is. As an important parameter of global reaction activity, the molecular polarizability (
where,
The local reactivity of ionic liquid inhibitors was evaluated by their Fukui index. Nucleophilic and electrophilic behavior of the molecule was studied by analyzing its Fukui index distribution. Fukui function is an important concept in density functional theory, which is commonly used to predict the active sites of a molecule [45]. In the case of certain outfield
Within the finite difference approximation, the nucleophilic attack can be expressed as
3. Results and discussion
3.1 1-Alkyl-3-methylimidazole chloride [Cnmim]Cl
In this section, we will analyze the equilibrium geometry, frontier orbital distribution, global and local reactivity, the change rule of the reactive sites and structure parameters of ionic liquids with the increase of the cationic alkyl chain length for the [Cnmim]Cl system. The comparison of the corrosion inhibition performance was achieved for the ionic liquids formed with the same anion and different cations.
3.1.1 Equilibrium geometry structures, HOMO and LUMO
Figure 2 shows the equilibrium geometry, HOMO and LUMO distribution of [Cnmim]Cl (n = 2, 4, 6, 8) system obtained by B3LYP/6-311++G(d, p) method. It can be seen from Figure 2 that the equilibrium geometry HOMO and LUMO profiles of these four ionic liquids are very similar. The anions are all at the C2 atom of the imidazole ring. HOMO is distributed on anions, but not on cations, which indicates that Cl− is easy to form a coordination bond with d-orbital on an iron surface, while imidazole ring is not easy to provide electrons and interact with the iron surface. LUMO is mainly distributed on the imidazole ring, and other C atoms and anions of cation also have little distribution, which indicates that the imidazole ring of cation is easy to accept electrons from the iron surface and form a feedback bond. According to the distribution of HOMO and LUMO, when the [Xmim]Cl ionic liquid adsorbs on the surface of carbon steel, the imidazole ring of the ionic liquid will interact with the surface of carbon steel and lay parallel to the surface.
The
ILs | [C2mim]Cl | [C4mim]Cl | [C6mim]Cl | [C8mim]Cl |
---|---|---|---|---|
−5.2672 | −5.2641 | −5.2623 | −5.2608 | |
−1.1890 | −1.1824 | −1.1820 | −1.1819 | |
Δ | 4.0782 | 4.0817 | 4.0903 | 4.0789 |
3.1.2 Global activity parameters
Table 2 shows the global activity parameters of [Cnmim]Cl system obtained by B3LYP/6-311++G(d, p) method. It can be seen from Table 2 that by increasing the length of the alkyl chain, the dipole moment (
ILs | ||||||
---|---|---|---|---|---|---|
[C2mim]Cl | 12.6974 | 3.2281 | 2.0391 | 0.4904 | 2.5552 | 117.8930 |
[C4mim]Cl | 12.3672 | 3.2232 | 2.0409 | 0.4900 | 2.5453 | 132.8403 |
[C6mim]Cl | 12.2509 | 3.2222 | 2.0402 | 0.4902 | 2.5445 | 158.3847 |
[C8mim]Cl | 12.2083 | 3.2213 | 2.0394 | 0.4903 | 2.5441 | 183.8247 |
3.1.3 Local activity parameters
Figure 3 shows the Fukui index
It is easy to provide electrons when attacked by the dielectric since Cl atom has high electronegativity and high electron density around them. As the same of distribution of LUMO, the nucleophilic attack index of these four kinds of ionic liquid molecules is mainly distributed on the imidazole ring, especially 2C, 4C, 5C and 1N atoms (see Figure 1 for the number of specific atom). The imidazole ring is positively charged, and it is easier to accept electrons when attacked by the nucleophilic medium. Therefore, local reactive sites of [Cnmim]Cl are Cl, 2C, 4C, 5C and 1N atoms (see Figure 1 for the number of specific atom), which are easy to interact with iron surface and adsorb on iron surface.
3.2 1-Alkyl-3-methylimidazolium acetate [Cnmim]Ac system
In this part, we will discuss how the change of chain length affects the change of structure and property of [Cnmim]Ac (n = 2, 4, 6, 8) system with the same anion and different cations through the density functional theory. The equilibrium geometry, frontier orbital distribution, global reactivity and local reactivity, the change rule of reactive sites and structure parameters of ionic liquids by increasing the length of cationic alkyl chain and the comparison of corrosion inhibition performance are obtained and analyzed. In addition, the influence of different anions Cl−, Ac− on the reaction activity of ionic liquids was analyzed by comparing [Cnmim]Ac (n = 2, 4, 6, 8) system with [Cnmim]Cl system.
3.2.1 Equilibrium geometry and frontier orbital distribution
Figure 4 shows the equilibrium geometry, HOMO and LUMO frontier molecular orbital distribution of [Cnmim]Ac (n = 2, 4, 6, 8) system obtained by B3LYP/6-311++G(d,p) method. It can be seen from Figure 4 that the equilibrium geometry HOMO and LUMO distribution for these four ionic liquids is very similar. The anion Ac− is located under the cation, and the optimized C▬O bond is a double bond. HOMO is mainly distributed on the anion Ac−, especially O atom, while C atom on the cation also has a small contribution, which shows that the unpaired O atom on the anion easily forms a coordination bond with the empty d-orbital on the iron surface and adsorbs on the iron surface. LUMO is mainly distributed on the imidazole ring, and there is a small amount of C of other cations and anions, which shows that the imidazole ring of cations easily accepts electrons from the iron surface, forming a feedback bond, which makes the imidazole ring easy to be adsorbed on the iron surface.
The
ILs | [C2mim]Ac | [C4mim]Ac | [C6mim]Ac | [C8mim]Ac |
---|---|---|---|---|
−5.4007 | −5.3992 | −5.3951 | −5.3943 | |
−0.9120 | −0.9009 | −0.8979 | −0.8985 | |
Δ | 4.4887 | 4.4982 | 4.4972 | 4.4958 |
3.2.2 Global active parameters
Table 4 shows the global active parameters of [Cnmim]Ac system obtained by B3LYP/6-311++G(d,p) method. It can be seen from Table 4 that by increasing the length of the alkyl chain, the dipole moment (
ILs | ||||||
---|---|---|---|---|---|---|
[C2mim]Ac | 9.9354 | 3.1563 | 2.2444 | 0.4456 | 2.2194 | 120.9223 |
[C4mim]Ac | 9.6985 | 3.1500 | 2.2491 | 0.4446 | 2.2059 | 146.3850 |
[C6mim]Ac | 9.6243 | 3.1465 | 2.2486 | 0.4447 | 2.2015 | 171.8383 |
[C8mim]Ac | 9.5918 | 3.1464 | 2.2479 | 0.4449 | 2.2020 | 197.2063 |
It can be seen from Table 4 that the order of
3.2.3 Fukui index distribution
Figure 5 shows the Fukui index distribution of four ionic liquid [Cnmim]Ac systems. It can be seen from Figure 5 that the electrophilic attack index of these four ionic liquid molecules is mainly distributed on the anion Ac−, especially O atom, and the cation also has a small amount of distribution, indicating that the anion containing oxygen group is easy to provide the d-orbital combination of electron and iron surface space, and can stably be adsorbed on the iron surface. The nucleophilic attack index is mainly distributed in 2C, 4C and 5C atoms in imidazole ring (the number of atoms is shown in Figure 1) and O atom of anion, indicating that both cations and anions of [Cnmim]Ac system will interact with the surface of iron, making the adsorption of molecules on the surface of iron more stable. So the local reactive sites of [Cnmim]Ac are placed in O, 2C, 4C, 5C and 1N atoms. Local reactive sites are almost the same for [Cnmim]Ac and [Cnmim]Cl, except for Cl in [Cnmim]Cl.
3.3 1-Octyl-3-methylimidazole [Omim]Y system
In this section, we will discuss the structure and property of [Omim]Y (Y = Cl, BF4, HSO4, Ac, TFO) ionic liquids formed with the same cation and different anions. The equilibrium geometry, frontier orbital distribution, global reactivity and local reactivity, the change of reactive sites, structure parameters and corrosion inhibition performance of ionic liquids with different anions are obtained and discussed.
3.3.1 Equilibrium geometry and frontier orbital distribution
Figure 6 shows the equilibrium geometry, HOMO and LUMO distribution of [Omim]Y system obtained by B3LYP/6-311++G(d, p) method. It can be seen from Figure 6 that the equilibrium geometry of the four ionic liquids is very similar, and the anions are all under the cations. LUMO is mainly distributed on the imidazole ring in the same way as [Cnmim]Cl system and [Cnmim]Ac system, which indicates that the imidazole ring of cation can easily accept electrons from the iron surface and form a feedback bond, which will make the imidazole ring stably adsorb on the iron surface. However, HOMO distribution of [Omim]Cl is very different. The HOMO of [Omim]BF4 is mainly distributed on the imidazole ring and the alkyl group, and only a small amount of anions is distributed. The HOMO of [Omim]HSO4, [Omim]Ac and [Omim]TFO are all allocated on anions, especially O atom. It is further stated that the oxygen-containing group easily forms a coordination bond with the d-orbit of the iron surface and is stably adsorbed on the iron surface.
The
ILs | [Omim]Cl | [Omim]BF4 | [Omim]HSO4 | [Omim]Ac | [Omim]TfO |
---|---|---|---|---|---|
−5.4007 | −8.2613 | −5.3992 | −5.3951 | −5.3943 | |
−0.9120 | −1.4030 | −0.9009 | −0.8979 | −0.8985 | |
Δ | 4.4887 | 6.8583 | 4.4982 | 4.4972 | 4.4958 |
3.3.2 Global active parameters
Table 6 shows the global activity parameters of [Omim]Y ionic liquids obtained by B3LYP/6-311++G(d, p) method. It is shown in Table 6 that [Omim]HSO4 has the largest dipole moment (
ILs | ||||||
---|---|---|---|---|---|---|
[Omim]Cl | 12.2083 | 3.2213 | 2.0394 | 0.4903 | 2.5441 | 183.8247 |
[Omim]BF4 | 12.2330 | 4.8321 | 3.4292 | 0.2916 | 3.4045 | 175.4047 |
[Omim]HSO4 | 12.9717 | 3.8610 | 2.5009 | 0.3999 | 2.9804 | 195.8180 |
[Omim]Ac | 9.5918 | 3.1464 | 2.2479 | 0.4449 | 2.2020 | 197.2063 |
[Omim]TfO | 12.2888 | 4.1213 | 2.6048 | 0.3839 | 2.2604 | 204.3150 |
3.3.3 Fukui index distribution
Figure 7 shows the Fukui index distribution of four ionic liquid [Omim]Y systems. From Figure 7, it can be seen that the electrophilic attack index of [Omim]Cl is mainly distributed on the Cl atom, and 2C (the number of atoms is shown in Figure 1) also has a small amount of the distribution. The nucleophilic attack index is allocated on 1N, 2C, 4C and 5C atoms. The electrophilic attack index of [Omim]BF4 is 2C, 4C and F, and the electrophilic attack index is mainly distributed on 1N, 2C and 3N. Electric attack indexes of [Omim]HSO4, [Omim]Ac and [Omim]TFO are mainly distributed on the O atom of anion, while the nucleophilic attack index is mainly distributed on the imidazole ring. In general, we found that the nucleophilic attack site of [Omim]Y is on the imidazole ring, and the electrophilic attack site is mainly on anion, especially O atom.
Compared with the HOMO and LUMO distribution of these three set ionic liquids systems of [Cnmim]Cl, [Cnmim]Ac and [Omim]Y in Figures 2, 4 and 6, we found that the HOMO distribution is mainly located on the anion containing Cl and O atoms, while the LUMO distribution is mainly located on the imidazole ring of the cation. It shows that the anion containing Cl and O atom, and imidazole ring are the active regions of the ionic liquid. For
4. Conclusion
The reaction activities such as
In [Cnmim]Cl and [Cnmim]Ac system, increasing the length of the alkyl chain, the HOMO energy
The active region of [Cnmim]Cl system is the imidazole ring and anion Cl−, while the [Cnmim]Ac system is located in the imidazole ring and the O atoms of anion. The active region of [Omim]Y system is located in the imidazole ring and an oxygen-containing group. The reaction is the active region of ionic liquid molecules will help interact with the iron surface and stably adsorb on the iron surface.
For [Cnmim]Cl system, the order of inhibition efficiency predicted is [C8mim]Cl > [C6mim]Cl > [C4mim]Cl > [C2mim]Cl, and [Omim]Cl may have the best inhibition effect. For [Cnmim]Ac system, the order of inhibition efficiency is [C8mim]Ac > [C6mim]Ac > [C4mim]Ac > [C2mim]Ac, and [Omim]Ac may have the best inhibition effect. Inhibition efficiency increases by increasing the alkyl chain length. For [Omim]Y system, [Omim]TFO may have better inhibition efficiency, and the sequence of inhibition efficiency needs further study.
Acknowledgments
This work was supported in part by the National Natural Science Foundation of China (51774158, 5126402) and Back-up Personnel Foundation of Academic and Technology Leaders of Yunnan Province (2011HR013).
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