Elsevier

Corrosion Science

Volume 52, Issue 6, June 2010, Pages 2059-2065
Corrosion Science

Molecular modeling of the inhibition mechanism of 1-(2-aminoethyl)-2-alkyl-imidazoline

https://doi.org/10.1016/j.corsci.2010.02.018Get rights and content

Abstract

Inhibition mechanism of five 1-(2-aminoethyl)-2-alkyl-imidazoline derivatives for carbon steel against CO2 corrosion was studied by molecular modeling. Molecular reactivity derived from quantum chemical calculation is insensitive to alkyl length. Inhibitor molecules can be adsorbed preferentially on metal surface with imidazoline ring attached on the surface. And with increase of alkyl length, interaction between inhibitor molecule and metal surface is enhanced to enable more stable adsorption of inhibitor molecules, which will form more compact self-assembly membrane with higher inhibition efficiency. The efficiency order of the inhibitors obtained by theoretical analysis was verified by experimental results.

Introduction

Corrosion prevention is of great importance in industrial application of materials. Among numerous corrosion prevention measures, corrosion inhibitor, bearing advantages of economy, high-efficiency, and wide-applicability, has been applied in various fields, such as petroleum extraction and refining, iron and steel, electric power, and construction, etc. In recent years, with the recognition of environmental protection, development of more effective and environment-friendly corrosion inhibitors has drawn more attention [1], [2], [3]. Traditional research and developing routes are based on large scale trial-and-error experiments, and bring along high cost and long cycle [4], [5]. Therefore, theoretical guidance is critical in design and application of new corrosion inhibitors.

With the advances in computer hardware and development of related theory, molecular modeling has grown to be an effective technique to explore complex systems at molecular level. Molecular structure, electron distribution and detailed adsorption process can be obtained via this approach, which is helpful for investigation of inhibition mechanism. Vosta first studied the inhibition mechanism using quantum chemistry method in 1971 [6], and established quantum corrosion electrochemistry. Thereafter, researches in this field are focused on the quantitative structure–activity relationship and abundant valuable results have been obtained [7], [8], [9]. In the 1990s, research on self-assembled mechanism of inhibitor molecules on metal surfaces became another hot spot by using molecular mechanics [10], [11], plentiful research results illustrated the effect of the adsorption process on inhibition performance, which is of important significance to explore the inhibition mechanism. At the end of the 20th century, much research based on molecular dynamics simulation was conducted to investigate the inhibitor mechanism on micro to mesoscopic scale [12], [13], [14]. The effect of corrosive environment factors, such as solvent, temperature and pressure, etc., on absorption of inhibitor molecule on metal surfaces was also investigated. These research results have provided theoretical guidance for molecular design. In general, though the achievement obtained via molecular modeling, previous studies were mainly conducted on single scale, viz. quantum chemistry, molecular mechanics and molecular dynamics simulation.

Quantum chemistry was principally used to study structural characteristics and chemical reactivity of isolated inhibitor molecules, such as the energy gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital, chemical hardness and Fukui index, etc. The disadvantage of this method is that the workload for calculation of electronic interactions was huge and time-consuming. Therefore the system dealt by this method is relatively small, and it could not be used to investigate the adsorption process on metal surface. On the other hand, molecular mechanics can be adopted to investigate the geometrical properties of inhibitor self-assembled monolayer on metal surface like chain length, title angle and membrane thickness. As for molecular dynamics simulation, this method could be used to obtain information about interaction between inhibitor molecules and metal surfaces. In the above-mentioned methods, by molecular mechanics and molecular dynamics simulation, the atom–atom interaction was calculated without consideration of interaction of electrons and global or local reactivity indices of inhibitor molecules. So researches via different methods on different scales only provided fractional information about inhibition mechanism. Obviously, it would be facile and effective to obtain full scale and detailed information by combination of these three methods, which provide a new research route in investigation of inhibition mechanism. Based on this idea, the inhibition mechanism was systemically investigated via combination of above three methods in this article.

Imidazoline and its derivatives are important sort of inhibitors which could effectively inhibit corrosion of carbon steel against CO2 and H2S [15], [16]. Among them, 1-(2-aminoethyl)-2-alkyl-imidazoline compounds with various alkyl chain lengths was selected as research objects, which inhibition efficiency have been evaluated through weight loss measurement to provide reference for theoretical evaluation [10]. In order to control workload and facilitate the calculation process, five 1-(2-aminoethyl)-2-alkyl-imidazoline compounds with odd number carbon atoms in alkyl chain (R), where R = –(CH2)n–CH3 (n = 6, 8, 10, 12, 14), were investigated by quantum chemistry, molecular dynamics simulation and molecular mechanics. The calculated results provided detailed information about molecular reactivity of inhibitors, interaction mechanism between the inhibitors and metal surfaces, and the structural characteristics of self-assembled monolayers. After all these calculation, the results of predicted inhibition performance were verified with the reported experimental results [10] to conform the effectiveness of the proposed theoretical method for evaluation of inhibition performance of new inhibitor (Fig. 1).

Section snippets

Quantum chemistry calculation

Density functional theory (DFT) was adopted to precisely calculate the information about molecular geometric and electron distribution. It has been widely used for analysis of inhibitor efficiency and inhibitor-surface interaction. In this paper, the geometric configuration of molecules involved was optimized with DFT/B3LYP method with basis set of 6-31G∗, and the frequency analysis was performed to ensure that the researched molecules reached their respective ground state.

Selection of adsorption surface

In CO2 corrosion

Molecular reactivity

The bond length and bond angle of the optimized configurations of molecule A–E were shown in Tables 1 and 2, respectively. It could be seen that the structures of all the molecules had similar characteristics: the bond length of N(4)–C(8) was about 0.140 nm, which was a featured bond length of a single bond. As for the bond length of N(7)–C(8), which was about 0.129 nm, close to the bond length of a double bond. The bond length and bond angle within each imidazoline ring were in the range of

Conclusions

Inhibition performance of five 1-(2-aminoethyl)-2-alkyl-imidazoline corrosion inhibitors with various alkyl chain length was investigated by molecular modeling methods including quantum chemistry, molecular dynamic simulation and molecular mechanics. First, the research by quantum chemistry revealed that the reactive sites of imidazoline molecules are located at imidazoline ring, and these molecules can form coordinate and back-donating bonds with atoms on metal surface. Further comparison

Acknowledgments

This work was financially supported by Natural Science Foundation of Shandong Province (Y2006B35) and CNPC Innovation Foundation (07E1021, 2008D-5006-02).

References (29)

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