**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 CdO 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

#### **Figure 4.**

reduced, which indicates that the ability of molecules to attract electrons is weaker and weaker, which is not conducive to accepting the electrons on the iron surface. The global hardness (*η*) of the molecules decreases, while the global softness (*S*) increases by increasing the chain length, which indicates that the interaction between the surface of carbon steel with the molecules becomes stronger and stronger, and the molecules tend to adsorb on the surface. The electrophilic index (*ω*) decreases by increasing the length of the alkyl chain. With the lowest unoccupied molecular orbital energy (*E*LUMO), the ability of the molecule to accept electrons becomes weaker and weaker. Molecular polarizability (*α*) is an important quantum chemical parameter in the field of corrosion protection. From **Table 2**, the order of the magnitude of [C8mim]Cl > [C6mim]Cl > [C4mim]Cl > [C2mim]Cl can be concluded, indicating that [C8mim]Cl is most easily adsorbed on the surface of iron and its corrosion inhibition efficiency is the highest. According to the analysis results in Section 3.1.1, with the increase of the alkyl chain length, the global activity parameters of [Cnmim]Cl system, such as global hardness (*η*), global softness (*S*) and polarizability (*α*), increases; therefore, the corrosion inhibition efficiency

*Global activity parameters for [Cnmim]Cl (n = 2, 4, 6, 8) system with B3LYP/6-311++G(d,p) method.*

**ILs** *μ***/Debye** *χ***/eV** *η***/eV** *S***/eV<sup>1</sup>** *ω***/eV** *α***(a.u.)** [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

**Figure 3** shows the Fukui index *f* distribution of four ionic liquid molecules in [Cnmim]Cl system, with the electrophilic attack index distribution on the left and the nucleophilic attack index *f* <sup>+</sup> distribution on the right. It can be seen from **Figure 3** that the electrophilic attack index of these four ionic liquid molecules is

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

*Fukui indices isosurfaces of [Cnmim]Cl (n = 2, 4, 6, 8) system with B3LYP/6-311++G(d,p) method. The [C2mim]Cl, [C4mim]Cl, [C6mim]Cl and [C8mim]Cl are shown from left to right, respectively.*

increases [29].

**Figure 3.**

**44**

**Table 2.**

*Density Functional Theory Calculations*

*3.1.3 Local activity parameters*

mainly distributed on the Cl atom with the HOMO.

*Equilibrium geometry structures, HOMO and LUMO isosurfaces of [Cnmim]Ac (n = 2, 4, 6, 8) system with B3LYP/6-311++G(d, p) method. From left to right, it is [C2mim]Ac, [C4mim]Ac, [C6mim]Ac and [C8mim] Ac, respectively.*

surface, forming a feedback bond, which makes the imidazole ring easy to be adsorbed on the iron surface.

hardness (*η*) and electrophilic index (*ω*) decrease, while the global activity parameters global hardness (*η*), global softness (*S*) and polarizability (*α*) increase. The order of polarization (*α*) is in agreement with that of corrosion inhibition efficiency.

*Theoretical Study of the Structure and Property of Ionic Liquids as Corrosion Inhibitor*

**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

In this section, we will discuss the structure and property of [Omim]Y (Y = Cl,

**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

*Fukui index isosurfaces of [Cnmim]Ac (n = 2, 4, 6, 8) system with B3LYP/6-311++G(d,p) method. From left*

*to right, it is [C2mim]Ac, [C4mim]Ac, [C6mim]Ac and [C8mim]Ac, respectively.*

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

*3.2.3 Fukui index distribution*

*DOI: http://dx.doi.org/10.5772/intechopen.92768*

**3.3 1-Octyl-3-methylimidazole [Omim]Y system**

*3.3.1 Equilibrium geometry and frontier orbital distribution*

[Cnmim]Cl.

discussed.

**Figure 5.**

**47**

The *E*HOMO, *E*LUMO and the energy gap Δ*E* of four [Cnmim]Ac (n = 2, 4, 6, 8) ionic liquid are shown in **Table 3**. It is shown in **Table 3** that *E*HOMO and *E*LUMO increase as that of [Cnmim]Cl (n = 2, 4, 6, 8) in **Table 2**, by increasing the length of alkyl chain, indicating that the capability of electron donator enhances and the capability of electron acceptor weakens. However, the smaller Δ*E* is, the better the activity of the molecule is, the easier it is to be adsorbed on the surface of iron and the better the inhibition effect is. The sequence of inhibition efficiency of the four ionic liquids should be [C8mim]Ac > [C6mim]Ac > [C4mim]Ac > [C2mim]Ac, and the inhibition efficiency of [C8mim]Ac is the highest. The order obtained is the same as that of system analysis.

### *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 (*μ*) decreases gradually. Increasing the alkyl chain length will reduce the polarity of the whole molecule. The electronegativity (*χ*) also decreases gradually, which indicates that the capability of attracting electrons is weaker and weaker, and is not conducive to receiving electrons on the iron surface. This results are the same as those of [Cnmim]Cl system. As the alkyl chain length increases, the global hardness (*η*) and the global softness (*S*) of the molecule decrease, which indicates a stronger interaction between the carbon surface with the molecule and the molecule is more stably adsorbed on the iron surface. The electrophilic index (*ω*) decreases by increasing the alkyl chain length. With the increase of *E*LUMO, the capability of electron acceptor of a molecule is weaker and weaker.

It can be seen from **Table 4** that the order of *α* is [C8mim]Ac > [C6mim] Ac > [C4mim]Ac > [C2mim]Ac, indicating that [C8mim]Ac is most easily adsorbed on the iron surface. So, [C8mim]Ac should have the best inhibition effect and the highest inhibition efficiency. Similar to the [Cnmim]Cl system, by increasing the length of alkyl chain, the values of dipole moment (*μ*), electronegativity (*χ*), global


**Table 3.**

E*HOMO,* E*LUMO and Δ*E*(eV)of [Cnmim]Ac (n = 2, 4, 6, 8) with B3LYP/6-311++G(d, p) method.*


**Table 4.**

*Global activity parameters for [Cnmim]Ac (n = 2, 4, 6, 8) system with B3LYP/6-311++G(d,p) method.*

*Theoretical Study of the Structure and Property of Ionic Liquids as Corrosion Inhibitor DOI: http://dx.doi.org/10.5772/intechopen.92768*

hardness (*η*) and electrophilic index (*ω*) decrease, while the global activity parameters global hardness (*η*), global softness (*S*) and polarizability (*α*) increase. The order of polarization (*α*) is in agreement with that of corrosion inhibition efficiency.

## *3.2.3 Fukui index distribution*

surface, forming a feedback bond, which makes the imidazole ring easy to be

The *E*HOMO, *E*LUMO and the energy gap Δ*E* of four [Cnmim]Ac (n = 2, 4, 6, 8) ionic liquid are shown in **Table 3**. It is shown in **Table 3** that *E*HOMO and *E*LUMO increase as that of [Cnmim]Cl (n = 2, 4, 6, 8) in **Table 2**, by increasing the length of alkyl chain, indicating that the capability of electron donator enhances and the capability of electron acceptor weakens. However, the smaller Δ*E* is, the better the activity of the molecule is, the easier it is to be adsorbed on the surface of iron and the better the inhibition effect is. The sequence of inhibition efficiency of the four ionic liquids should be [C8mim]Ac > [C6mim]Ac > [C4mim]Ac > [C2mim]Ac, and the inhibition efficiency of [C8mim]Ac is the highest. The order obtained is the

**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 (*μ*) decreases gradually. Increasing the alkyl chain length will reduce the polarity of the whole molecule. The electronegativity (*χ*) also decreases gradually, which indicates that the capability of attracting electrons is weaker and weaker, and is not conducive to receiving electrons on the iron surface. This results are the same as those of [Cnmim]Cl system. As the alkyl chain length increases, the global hardness (*η*) and the global softness (*S*) of the molecule decrease, which indicates a stronger interaction between the carbon surface with the molecule and the molecule is more stably adsorbed on the iron surface. The electrophilic index (*ω*) decreases by increasing the alkyl chain length. With the increase of *E*LUMO, the capability of electron acceptor of a molecule is weaker and

It can be seen from **Table 4** that the order of *α* is [C8mim]Ac > [C6mim] Ac > [C4mim]Ac > [C2mim]Ac, indicating that [C8mim]Ac is most easily adsorbed on the iron surface. So, [C8mim]Ac should have the best inhibition effect and the highest inhibition efficiency. Similar to the [Cnmim]Cl system, by increasing the length of alkyl chain, the values of dipole moment (*μ*), electronegativity (*χ*), global

**ILs [C2mim]Ac [C4mim]Ac [C6mim]Ac [C8mim]Ac** *E*HOMO 5.4007 5.3992 5.3951 5.3943 *E*LUMO 0.9120 0.9009 0.8979 0.8985 Δ*E* 4.4887 4.4982 4.4972 4.4958

E*HOMO,* E*LUMO and Δ*E*(eV)of [Cnmim]Ac (n = 2, 4, 6, 8) with B3LYP/6-311++G(d, p) method.*

**ILs** *μ***/Debye** *χ***/eV** *η***/eV** *S***/eV<sup>1</sup>** *ω α***(a.u.)** [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

*Global activity parameters for [Cnmim]Ac (n = 2, 4, 6, 8) system with B3LYP/6-311++G(d,p) method.*

adsorbed on the iron surface.

*Density Functional Theory Calculations*

same as that of system analysis.

*3.2.2 Global active parameters*

weaker.

**Table 3.**

**Table 4.**

**46**

**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

#### **Figure 5.**

*Fukui index isosurfaces of [Cnmim]Ac (n = 2, 4, 6, 8) system with B3LYP/6-311++G(d,p) method. From left to right, it is [C2mim]Ac, [C4mim]Ac, [C6mim]Ac and [C8mim]Ac, respectively.*

#### **Figure 6.**

*Equilibrium geometry structures, HOMO and LUMO isosurfaces of [Omim]Y system with B3LYP/6-311++G (d, p) method. From left to right, it is [Omim]Cl, [Omim]BF4, [Omim]HSO4, [Omim]Ac and [Omim]TFO.*


#### **Table 5.**

E*HOMO,* E*LUMO and Δ*E *(eV) of [Omim]Y with B3LYP/6-311++G(d, p) method.*

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.

Cl, indicating that [Omim]TFO is the most easily adsorbed on the iron surface, with the best corrosion inhibition effect and the highest corrosion inhibition efficiency.

*Fukui index isosurfaces of [Omim]Y ionic liquids with B3LYP/6-311++G(d,p) method. From left to right, it is*

**ILs** *μ***/Debye** *χ***/eV** *η***/eV** *S***/eV<sup>1</sup>** *ω α***(a.u.)** [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

*Theoretical Study of the Structure and Property of Ionic Liquids as Corrosion Inhibitor*

*DOI: http://dx.doi.org/10.5772/intechopen.92768*

*Global activity parameters for [Omim]Y system with B3LYP/6-311++G(d, p) method.*

*[Omim]Cl, [Omim]BF4, [Omim]HSO4, [Omim]Ac and [Omim]TFO.*

**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

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 *E*HOMO and *E*LUMO, the *E*HOMO of [Omim]Cl is the largest, and the *E*LUMO of [Omim]TFO is the smallest, indicating that [Omim]Cl has the strongest ability to give electrons and [Omim]TFO has the strongest ability to get electrons. The dipole moment (*μ*) of [Omim]HSO4 is the largest, which indicates that the molecular polarity is the largest and it is easier to interact with the iron surface. For electronegativity (*χ*), the maximum is [Omim]BF4 and the second

*3.3.3 Fukui index distribution*

**Table 6.**

**Figure 7.**

**49**

mainly on anion, especially O atom.

The *E*HOMO, *E*LUMO and the energy gap Δ*E* of four [Omim]Y ionic liquids are shown in **Table 5**. It is shown in **Table 5** that the order of size of *E*HOMO and *E*LUMO is [Omim]TFO > [Omim]Ac > [Omim]HSO4 > [Omim]Cl > [Omim]BF4, which indicates that [Omim]TFO has the strongest ability to give electrons and [Emim]Cl has the strongest ability to get electrons. The order of Δ*E* is [Omim]BF4 > [Omim] HSO4 > [Omim]Ac > [Omim]TFO > [Omim]Cl, which indicates that [Omim]Cl has the best chemical activity and is the easiest to interact with metal surface.

#### *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 (*μ*), while [Omim]Ac has the smallest dipole moment (*μ*), indicating that [Omim]HSO4 has the largest molecular polarity and easily interacts with the iron surface. For electronegativity (*χ*), the maximum of [Omim] BF4 and the second of [Omim]TFO indicate that the ability of [Omim]BF4 and [Omim]TFO to attract electrons is strong, which makes the molecules stably adsorbed on the iron surface by combining with the electrons on the iron surface. Global hardness (*η*) of [Omim]BF4 is the largest, the global softness (*S*) is the smallest and the electrophilic index (*ω*) is the largest, indicating that [Omim]BF4 is a good ionic liquid inhibitor. It can be seen from **Table 6** that the order of polarizability (*α*) is [Omim]TFO > [Omim]Ac > [Omim]HSO4 > [Omim]BF4 > [Omim]


*Theoretical Study of the Structure and Property of Ionic Liquids as Corrosion Inhibitor DOI: http://dx.doi.org/10.5772/intechopen.92768*

**Table 6.**

*Global activity parameters for [Omim]Y system with B3LYP/6-311++G(d, p) method.*

**Figure 7.**

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 *E*HOMO, *E*LUMO and the energy gap Δ*E* of four [Omim]Y ionic liquids are shown in **Table 5**. It is shown in **Table 5** that the order of size of *E*HOMO and *E*LUMO is [Omim]TFO > [Omim]Ac > [Omim]HSO4 > [Omim]Cl > [Omim]BF4, which indicates that [Omim]TFO has the strongest ability to give electrons and [Emim]Cl has the strongest ability to get electrons. The order of Δ*E* is [Omim]BF4 > [Omim] HSO4 > [Omim]Ac > [Omim]TFO > [Omim]Cl, which indicates that [Omim]Cl has the best chemical activity and is the easiest to interact with metal surface.

E*HOMO,* E*LUMO and Δ*E *(eV) of [Omim]Y with B3LYP/6-311++G(d, p) method.*

*Equilibrium geometry structures, HOMO and LUMO isosurfaces of [Omim]Y system with B3LYP/6-311++G (d, p) method. From left to right, it is [Omim]Cl, [Omim]BF4, [Omim]HSO4, [Omim]Ac and [Omim]TFO.*

**ILs [Omim]Cl [Omim]BF4 [Omim]HSO4 [Omim]Ac [Omim]TfO** *E*HOMO 5.4007 8.2613 5.3992 5.3951 5.3943 *E*LUMO 0.9120 1.4030 0.9009 0.8979 0.8985 Δ*E* 4.4887 6.8583 4.4982 4.4972 4.4958

**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 (*μ*), while [Omim]Ac has the smallest dipole moment (*μ*), indicating that [Omim]HSO4 has the largest molecular polarity and easily interacts with the iron surface. For electronegativity (*χ*), the maximum of [Omim] BF4 and the second of [Omim]TFO indicate that the ability of [Omim]BF4 and [Omim]TFO to attract electrons is strong, which makes the molecules stably adsorbed on the iron surface by combining with the electrons on the iron surface. Global hardness (*η*) of [Omim]BF4 is the largest, the global softness (*S*) is the smallest and the electrophilic index (*ω*) is the largest, indicating that [Omim]BF4 is a good ionic liquid inhibitor. It can be seen from **Table 6** that the order of polarizability (*α*) is [Omim]TFO > [Omim]Ac > [Omim]HSO4 > [Omim]BF4 > [Omim]

*3.3.2 Global active parameters*

**Figure 6.**

*Density Functional Theory Calculations*

**Table 5.**

**48**

*Fukui index isosurfaces of [Omim]Y ionic liquids with B3LYP/6-311++G(d,p) method. From left to right, it is [Omim]Cl, [Omim]BF4, [Omim]HSO4, [Omim]Ac and [Omim]TFO.*

Cl, indicating that [Omim]TFO is the most easily adsorbed on the iron surface, with the best corrosion inhibition effect and the highest corrosion inhibition efficiency.

#### *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 *E*HOMO and *E*LUMO, the *E*HOMO of [Omim]Cl is the largest, and the *E*LUMO of [Omim]TFO is the smallest, indicating that [Omim]Cl has the strongest ability to give electrons and [Omim]TFO has the strongest ability to get electrons. The dipole moment (*μ*) of [Omim]HSO4 is the largest, which indicates that the molecular polarity is the largest and it is easier to interact with the iron surface. For electronegativity (*χ*), the maximum is [Omim]BF4 and the second is [Omim]TFO, which indicates that [Omim]BF4 and [Omim]TFO have strong ability to attract electrons. The global hardness (*η*) of [Omim]BF4 and [Omim]TFO is larger, the global softness (*S*) is smaller and the electrophilic index (*ω*) is larger, which indicates that [Omim]BF4 and [Omim]TFO are ionic liquids with better corrosion inhibition performance. For the polarizability (*α*), [Omim]TFO is the largest, which indicates that [Omim]TFO is the most easily adsorbed on the iron surface, and its corrosion inhibition effect is the best. In conclusion, [Omim]TFO has the best corrosion inhibition performance and the highest efficiency among the 11 ionic liquids studied.

### **4. Conclusion**

The reaction activities such as *E*LUMO, *E*HOMO, the softness (*S*) and polarizability (*α*) increase gradually, whereas electronegativity (*χ*), energy gap (Δ*E*), hardness (*η*), dipole moment (*μ*) and electrophilic index (*ω*) of three sets ionic liquids 1 alkyl-3-methylimidazole chloride [Cnmim]Cl, 1-alkyl-3-methylimidazolium acetate [Cnmim]Ac and 1-octyl-3-methylimidazole salt [Omim]Y were studied by density functional theory. The results are as follows:

In [Cnmim]Cl and [Cnmim]Ac system, increasing the length of the alkyl chain, the HOMO energy *E*HOMO, LUMO energy ELUMO, softness (*S*) increase, while the dipole moment (*μ*), energy gap Δ*E*, electronegativity (*χ*), electrophilic index (*ω*), hardness (*η*) and polarizability (*α*) gradually decrease. In [Omim]Y (Y = Cl, BF4, HSO4, Ac, TFO) ionic liquids, the structure parameters of ionic liquids vary greatly with different anions.

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.

#### **Acknowledgements**

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).

**Author details**

**51**

Guocai Tian\* and Weizhong Zhou

and Technology, Yunnan, Kunming, China

provided the original work is properly cited.

\*Address all correspondence to: tiangc@kust.edu.cn

State Key Laboratory of Complex Nonferrous Metal Resource Clean Utilization, Faculty of Metallurgical and Energy Engineering, Kunming University of Science

*Theoretical Study of the Structure and Property of Ionic Liquids as Corrosion Inhibitor*

*DOI: http://dx.doi.org/10.5772/intechopen.92768*

© 2020 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

#### **Conflict of interest**

The authors declare no conflict of interest.

*Theoretical Study of the Structure and Property of Ionic Liquids as Corrosion Inhibitor DOI: http://dx.doi.org/10.5772/intechopen.92768*
