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

#### **Figure 2.**

*<sup>ω</sup>* <sup>¼</sup> *<sup>χ</sup>*<sup>2</sup> 2*η*

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 (*α*) is the average value obtained by calculation, and its relationship is

<sup>3</sup> *<sup>α</sup>*xx <sup>þ</sup> *<sup>α</sup>*yy <sup>þ</sup> *<sup>α</sup>*zz

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 *v*(*r*), the Fukui function is defined as follows [46]:

νð Þ*r*

where, *α*xx, *α*yy and *α*zz are molecular polarizability in x, y and z directions, respectively. The higher the molecular polarizability (*α*) is, the easier adsorption on the metal surface is, and the higher corrosion inhibition performance is [44].

> *f r*ð Þ¼ <sup>∂</sup>ρð Þ*<sup>r</sup> ∂N*

Within the finite difference approximation, the nucleophilic attack can be

<sup>ρ</sup>*<sup>N</sup>*�<sup>1</sup> ð Þ*<sup>r</sup>* [29], where *<sup>ρ</sup><sup>N</sup>* + 1(*r*)、 *<sup>ρ</sup>N*(*r*)、 *<sup>ρ</sup>N*�1(*r*) are the charge density of the atom in the molecule with one unit negative charge, uncharged and one unit positive charge,

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.

**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

þð Þ¼ *r* ρ*<sup>N</sup>*þ<sup>1</sup>ð Þ� *r* ρ*N*ð Þ*r* , and electrophilic attack is *f*

*<sup>α</sup>* <sup>¼</sup> <sup>1</sup>

as follows [44]:

*Density Functional Theory Calculations*

expressed as *f*

respectively.

**42**

**3. Results and discussion**

**3.1 1-Alkyl-3-methylimidazole chloride [Cnmim]Cl**

*3.1.1 Equilibrium geometry structures, HOMO and LUMO*

*:* (8)

*:* (9)

*:* (10)

�ð Þ¼ *r* ρ*N*ð Þ� *r*

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


#### **Table 1.**

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

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 *E*HOMO, the *E*LUMO and the energy gap difference (Δ*E*) of four ionic liquid molecules in [Cnmim]Cl system are shown in **Table 1**. It can be seen from **Table 1** that by increasing the length of the alkyl chain, the *E*HOMO becomes larger and larger, indicating the increased capability of electron donor of the molecule. By increasing the length of the alkyl chain, *E*LUMO also has an increasing trend, which shows that the capability of electron acceptor will weaken. However, the smaller (Δ*E*) is, the better the activity of the molecule is, the easier absorption between the carbon steel surface and molecules, and the higher the inhibition efficiency is. The sequence of inhibition efficiency of these four ionic liquids should be [C2mim]Cl < [C4mim]Cl < [C6mim]Cl < [C8mim]Cl. The [C8mim]Cl has the highest inhibition efficiency and the best inhibition performance, which agrees well with the experimental measurement [29].

#### *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 (*μ*) decreases gradually, indicating that the increase in alkyl chain length will reduce the polarity of the whole molecule. Increasing the length of alkyl chain, the electronegativity (*χ*) is also gradually


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

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

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

**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

*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]*

with iron surface and adsorb on iron surface.

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

(n = 2, 4, 6, 8) system with [Cnmim]Cl system.

**Figure 4.**

**45**

*Ac, respectively.*

*3.2.1 Equilibrium geometry and frontier orbital distribution*

**3.2 1-Alkyl-3-methylimidazolium acetate [Cnmim]Ac system**

**Table 2.**

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

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 increases [29].

#### *3.1.3 Local activity parameters*

**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 mainly distributed on the Cl atom with the HOMO.

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

#### **Figure 3.**

*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.*

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

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.
