**2.4 Study of hydrogen gas evolution over N1 atom of ionic liquid acid**

The hydrogen gas evolution method is one of the ways to determine the corrosion rate of metal in an aggressive medium. Hydrogen gas evolution is a result of a reduction (cathodic) reaction at the metal surface. The volume of hydrogen gas produced in the medium is gradually reduced in the presence of ionic liquid. The increment in alkyl chain length on N1 atoms of the benzimidazolium cation offers maximum surface coverage on the active site. Thus, a lesser amount of hydrogen gas will be produced than the blank medium [24]. As seen, the volume of hydrogen gas is reduced for both methyl and ethyl benzimidazolium ionic liquid in concentration [23]. Furthermore, it can be understood that each concentration of ethyl benzimidazolium ([BMEB]<sup>+</sup> BF4 <sup>−</sup>) possesses a very low volume of H2 gas compared to methyl benzimidazolium ([BMMB]+ Br<sup>−</sup>).

From the above valid point, it is vindicated that the alkyl chain length offers maximum energy or electrons to adhere to the cathode area, which is also the route to sturdy passive film formation between the corrosive medium and the metal surface. The general corrosion mechanism for carbon steel in 1 N HCl acid is described below. As shown in the following equation the ionic liquid is adsorbed onto the anodic and cathodic areas of the carbon steel surface, respectively.

Anodic protection reaction of [BMMB]<sup>+</sup> Br<sup>−</sup>:

$$\text{Fe} + \text{(Cl)}\_{\text{ads}}^{-} \leftrightharpoons \text{(Fe Cl)}\_{\text{ads}}^{-}\tag{1}$$

$$\text{Fe (Cl)}\_{\text{ads}}^{-} \text{ + BMMB}^{+} \leftrightharpoons \left[ \text{Fe (Cl)}^{-} \text{BMMB}^{+} \right]\_{\text{ads}} \tag{2}$$

$$\text{Fe} \left( \text{Br} \right)^{-}\_{\text{ads}} \text{ + BMMB}^{\*} \rightarrow \left| \text{Fe} \left( \text{Br} \right)^{-} \text{BMMB}^{\*} \right|\_{\text{ads}} \tag{3}$$

[BMEB]<sup>+</sup> BF4 −:

*Corrosion Inhibitors*

**acid medium**

**Figure 1.**

**2.3 Significance of carbon chain length on surface protection of metal in** 

*Schematic representation of electrochemical noise analysis (ENA) of ionic liquids.*

On comparing the surface protection effect of any ionic liquid with its alkylated and non-alkylated form, it would help to understand the structure effect of surface protection. Gabler et al. [25] reported that strong binding between metal surface and ionic liquid is key for corrosion reduction. In an electrochemical study of 2-hydroxy- and butylammonium sulfonyl imide, the C▬F bond cleavage was reduced significantly in butyl form compared to the former. On the other hand, Kaczerewska et al. [26] studied the structural effect of inhibition efficiency on metal protection in acid medium. Interconnecting a gemini cation with a bridged oxygen atom showed better surface coverage to avert corrosive ion contact with metal surfaces. The 18-O-18 gemini cationic part of ionic liquid offered improved protection resistance against metal dissolution [27]. Vastag et al. suggested [12] that by increasing the alkyl chain number in N-substituted cations would favor the inhibitor action of organic compounds to isolate metal from further corrosion. Generally, the surface protection of ionic liquid on carbon steel was improved when the N3 atom was alkylated with carbon chain lengths from n-7 to n-9. Infrared spectra also confirmed the shift in wave number [7]. Image examination of metal surfaces exposed to 1-ethyl [12] and 1-allyl [28] 3-butylimidazolium bromide ionic liquid expressed the inhibition effect of ionic liquid. The inductive effect of the allyl group offered increased electrons in its structure in the cation of ionic liquid. Hence, as resonance increased, the adsorption of ionic liquids over the metal surface also increased. Damage to the negative ions in the ionic liquids influenced corrosion protection

<sup>−</sup> has greater tendency to render protection on metal surfaces than

<sup>−</sup>. Sometimes, iminium compounds reduce surface heterogeneity, caused by adsorption of ionic liquid over the metal surface. Apparently, elemental analysis of the inhibited metal showed that the inhibitor's constituents were 15 wt% of carbon and 10 wt% of oxygen, and also corroborated that dodecyl iminium

chloride is more favorably adsorbed than the non-alkylated form [30].

**30**

[29]. HSO4

BF4

$$\text{Fe (Cl)}\text{\textquotedbl{}\_{ads} + BMEB}^{\text{+}} \leftrightharpoons \left[ \text{Fe (Cl)}^{\text{-}} \text{BMEB}^{\text{+}} \right]\_{\text{ads}} \tag{4}$$

$$\text{Fe} \left( \text{Br} \right)^{-}\_{\text{ads}} \text{ + BMEB}^{+} \rightarrow \left| \text{Fe} \left( \text{Br} \right)^{-} \text{BMEB}^{+} \right|\_{\text{ads}} \tag{5}$$

Likewise in the cathodic area, the reduction (hydrogen gas formation) reaction was considerably decreased due to the alkylation impact of ionic liquid on the adsorption effect instead of H+ , and thus hydrogen gas evolution was reduced.

Cathodic protection reaction of [BMMB]<sup>+</sup> Br<sup>−</sup>:

$$\text{Fe} \star \text{BMMB}^{\ast} \leftrightharpoons \text{Fe} \left( \text{BMMB} \right)^{\ast}\_{\text{ads}} \tag{6}$$

$$\text{Fe} \left( \text{BMMB} \right)^{\ast}\_{\text{ads}} \text{+} \text{e}^{\cdot} \to \text{Fe} \left( \text{BMMB} \right)\_{\text{ads}} \tag{7}$$

[BMEB]+ BF4 −:

$$\text{Fe} \star \text{BMEB}^{\ast} \leftrightharpoons \text{Fe} \left(\text{BMEB}\right)^{\ast}\_{\text{ads}}\tag{8}$$

$$\text{Fe} \left( \text{BMEB} \right)^{\ast}\_{\text{ads}} \text{+ e}^{\cdot} \to \text{Fe} \left( \text{BMEB} \right)\_{\text{ads}} \tag{9}$$

As seen in the above equations, the adsorption of the cationic part of ionic liquids is favored in both anodic and cathodic reactive sites. The most favored cationic part among the above three kinds of ionic liquids is [BMEB]+ . The (CH3 − CH2 − ) ethyl group, which effortlessly offered electrons to the metal surface, explains the reason for the occurrence of reduced volume of hydrogen gas in the acid medium. **Figure 2** shows the hydrogen gas evolution reaction in the blank and benzimidazolium ionic liquid.

### **2.5 Influence of alkyl chain length on quantum chemical parameters**

Quantum chemical studies have been used as an efficient method to evaluate the corrosion inhibition performance of any kind of inhibitor. Since corrosion inhibition is adsorption related, dynamic and quantum studies are used to characterize the electronic properties that help to understand the adsorptive properties of the organic compound [31]. Many factors, including heteroatoms, π-electrons, aromatic rings, and carbon chain length, influence the adsorption properties of the inhibitor with metal surfaces [32]. The adsorption properties of ionic liquids will differ by their structural nature, particularly in HOMO and LUMO energy levels. The higher value of HOMO energy level contributes to the electron offering tendency to the acceptor molecule or lower energy state. Gad et al. [33] reported that increasing alkyl chain length increased HOMO energy level in pyridinium bromide ionic liquid. The maximum HOMO level was attained for (C12) 4-mercapto-1-dodecylpyridinium bromide compared to C8 and C10.

Likewise, the LUMO energy level of an inhibitor also depends on the functional group present in the structure. The maximum LUMO value is acquired by (C8) pyridinium bromide ionic liquid compared to the rest. A smaller alkyl chain length will decrease HOMO energy level and increase LUMO energy level. LUMO defines electron-accepting behavior of the molecule from the neighboring environments. Since (C8) ionic liquid has a higher LUMO level, it can withdraw the electron from the metal, and even from feedback bonding, and result in strong adsorption with the metal surface. The change in energy gap is an important factor for measuring reactivity of ionic liquid to adsorb onto the metallic surface. The lower value of ∆*E* is the reason for the higher inhibition efficiency in metal corrosion. This is attributed to the

#### **Figure 2.**

*Pictorial representation of hydrogen gas evolution in the absence and presence of [BMMB]+ Br<sup>−</sup> and [BMEB]<sup>+</sup> BF4<sup>−</sup> in 1 N HCl medium.*

**33**

**Figure 3.**

*Structural Effect in Ionic Liquids Is the Vital Role to Enhance the Corrosion Protection of Metals…*

minimum amount of energy used to transfer the electron from the highest occupied orbital to the vacant "d" orbital of metal [34]. The ∆*E* value of the electrodeposition of polyaniline using tetrabutyl phosphonium bromide is less than that using ethyl tributyl phosphonium diethoxy phosphate. The above facts are the same as above in the phosphonium bromide molecule. The energy difference is much higher when substituting the 4-ethoxybenzyl group in the phosphonium [35] cationic part compared to butyl substitution [36]. Likewise, increasing the carbon chain length on the heterocyclic group also decreases the ∆*E* considerably [37]. The presence of methyl and ethyl groups in the benzimidazolium groups will maintain the minimum energy

In addition, the binding energies of butyl and ethoxy-substituted phosphonium bromide were 2040.9 and 25505.6 kJ, respectively. Ethoxy-substituted phosphonium bromide possessed more negative than butyl-substituted phosphonium bromide. Hence, the ionic liquid is more stable and there is less chance of spitting in the medium because of better passive film on the metal surface. Still, many researchers are studying the structure impact on its corrosion performance. By introducing an alkyl functional group to imidazoline ionic liquid, the relationship between corrosion protection and structure can be discussed. Apparently, partial atomic charges of each atom in the compound describe structural influence using quantum chemical parameters on corrosion protection in the comparison study of [DMIM][BF4] and [BMIM] [BF4] at corrosion inhibition efficiency; the partial atomic charges focus on cationic moiety rather than anionic moiety, because cations possess large molecular size. Finally, the carbon atom of the alkyl chain contains a negative charge. The C5 carbon atom has a higher negative charge in [DMIM][BF4] than in [BMIM][BF4]. This makes DMIM ionic liquid effective in adsorption on the metal surface against corrosion [38]. In addition, Ibrahim et al. noted their point on structural effect in their research [18]. The adsorption of imidazolium ionic liquid took place through the nitrogen atom of the ring. The coordinate bond occurs between nitrogen and iron. On evaluating two ionic liquids (benzyl and ethyl acetate-substituted imidazolium ionic liquids), benzyl-substituted imidazolium ionic liquid has a higher rate of adsorption on the anodic curve area than ethyl acetate-substituted imidazolium ionic liquid, because the former has a higher rate of relaxation of adsorbed ionic

gap between the HOMO and LUMO groups as represented in **Figure 3**.

*Schematic representation of energy gap difference between various benzimidazolium ionic liquids.*

*DOI: http://dx.doi.org/10.5772/intechopen.82422*

liquid from the metal surface.

#### *Structural Effect in Ionic Liquids Is the Vital Role to Enhance the Corrosion Protection of Metals… DOI: http://dx.doi.org/10.5772/intechopen.82422*

minimum amount of energy used to transfer the electron from the highest occupied orbital to the vacant "d" orbital of metal [34]. The ∆*E* value of the electrodeposition of polyaniline using tetrabutyl phosphonium bromide is less than that using ethyl tributyl phosphonium diethoxy phosphate. The above facts are the same as above in the phosphonium bromide molecule. The energy difference is much higher when substituting the 4-ethoxybenzyl group in the phosphonium [35] cationic part compared to butyl substitution [36]. Likewise, increasing the carbon chain length on the heterocyclic group also decreases the ∆*E* considerably [37]. The presence of methyl and ethyl groups in the benzimidazolium groups will maintain the minimum energy gap between the HOMO and LUMO groups as represented in **Figure 3**.

In addition, the binding energies of butyl and ethoxy-substituted phosphonium bromide were 2040.9 and 25505.6 kJ, respectively. Ethoxy-substituted phosphonium bromide possessed more negative than butyl-substituted phosphonium bromide. Hence, the ionic liquid is more stable and there is less chance of spitting in the medium because of better passive film on the metal surface. Still, many researchers are studying the structure impact on its corrosion performance. By introducing an alkyl functional group to imidazoline ionic liquid, the relationship between corrosion protection and structure can be discussed. Apparently, partial atomic charges of each atom in the compound describe structural influence using quantum chemical parameters on corrosion protection in the comparison study of [DMIM][BF4] and [BMIM] [BF4] at corrosion inhibition efficiency; the partial atomic charges focus on cationic moiety rather than anionic moiety, because cations possess large molecular size. Finally, the carbon atom of the alkyl chain contains a negative charge. The C5 carbon atom has a higher negative charge in [DMIM][BF4] than in [BMIM][BF4]. This makes DMIM ionic liquid effective in adsorption on the metal surface against corrosion [38].

In addition, Ibrahim et al. noted their point on structural effect in their research [18]. The adsorption of imidazolium ionic liquid took place through the nitrogen atom of the ring. The coordinate bond occurs between nitrogen and iron. On evaluating two ionic liquids (benzyl and ethyl acetate-substituted imidazolium ionic liquids), benzyl-substituted imidazolium ionic liquid has a higher rate of adsorption on the anodic curve area than ethyl acetate-substituted imidazolium ionic liquid, because the former has a higher rate of relaxation of adsorbed ionic liquid from the metal surface.

**Figure 3.**

*Schematic representation of energy gap difference between various benzimidazolium ionic liquids.*

*Corrosion Inhibitors*

Fe(BMEB)ads

pyridinium bromide compared to C8 and C10.

among the above three kinds of ionic liquids is [BMEB]+

<sup>+</sup> + e

**2.5 Influence of alkyl chain length on quantum chemical parameters**

As seen in the above equations, the adsorption of the cationic part of ionic liquids is favored in both anodic and cathodic reactive sites. The most favored cationic part

group, which effortlessly offered electrons to the metal surface, explains the reason for the occurrence of reduced volume of hydrogen gas in the acid medium. **Figure 2** shows the hydrogen gas evolution reaction in the blank and benzimidazolium ionic liquid.

Quantum chemical studies have been used as an efficient method to evaluate the corrosion inhibition performance of any kind of inhibitor. Since corrosion inhibition is adsorption related, dynamic and quantum studies are used to characterize the electronic properties that help to understand the adsorptive properties of the organic compound [31]. Many factors, including heteroatoms, π-electrons, aromatic rings, and carbon chain length, influence the adsorption properties of the inhibitor with metal surfaces [32]. The adsorption properties of ionic liquids will differ by their structural nature, particularly in HOMO and LUMO energy levels. The higher value of HOMO energy level contributes to the electron offering tendency to the acceptor molecule or lower energy state. Gad et al. [33] reported that increasing alkyl chain length increased HOMO energy level in pyridinium bromide ionic liquid. The maximum HOMO level was attained for (C12) 4-mercapto-1-dodecyl-

Likewise, the LUMO energy level of an inhibitor also depends on the functional

group present in the structure. The maximum LUMO value is acquired by (C8) pyridinium bromide ionic liquid compared to the rest. A smaller alkyl chain length will decrease HOMO energy level and increase LUMO energy level. LUMO defines electron-accepting behavior of the molecule from the neighboring environments. Since (C8) ionic liquid has a higher LUMO level, it can withdraw the electron from the metal, and even from feedback bonding, and result in strong adsorption with the metal surface. The change in energy gap is an important factor for measuring reactivity of ionic liquid to adsorb onto the metallic surface. The lower value of ∆*E* is the reason for the higher inhibition efficiency in metal corrosion. This is attributed to the

*Pictorial representation of hydrogen gas evolution in the absence and presence of [BMMB]+*

<sup>−</sup> → Fe(BMEB)ads (9)

− CH2 − ) ethyl

*Br<sup>−</sup> and* 

. The (CH3

**32**

**Figure 2.**

*[BMEB]<sup>+</sup>*

*BF4<sup>−</sup> in 1 N HCl medium.*
