Novel imidazolinium ionic liquids and organic salts

doi:10.1016/j.electacta.2015.01.180

Electrochimica Acta

Volume 159, 20 March 2015, Pages 219-226

https://doi.org/10.1016/j.electacta.2015.01.180 Get rights and content

Highlights

•
A novel imidazolinium family of salts has been synthesised
•
Triflate ionic liquid has higher ionicity than imidazolium equivalent
•
Synergy between imidazolinium and hydroxycinnamate ions for corrosion inhibition

ABSTRACT

The preparation and physical properties of a novel family of ionic liquids and organic salts based on the imidazolinium cation are described, and compared with their imidazolium analogues in some cases. Ionic liquids were obtained with the triflate, formate and salicylate anions, while > 100 °C melting points were observed with acetate and several other benzoate derivatives. The triflate salt was less ion-conductive than the corresponding imidazolium salt, but less so than expected on the basis of its viscosity, suggesting a contribution from proton conductivity. The electrochemical window of the imidazolinium was slightly extended in the reductive direction, due to the lower proton activity produced by the cation in this case. Imidazolinium salts are also known to exhibit anti-corrosion properties and hence a preliminary study of this property is also reported; 2-methylimidazolinium 4-hydroxycinnamate was found to show strong anodic corrosion inhibition on mild steel.

Graphical abstract

Introduction

As a class of materials, ionic liquids (ILs) are generally classified as salts with melting points below 100 °C [1]. Due to the vast number of cations and anions that can be combined to form ILs, they can have greatly varying properties and, as such, ionic liquid research spans a broad range of fields. This includes, but is not limited to, investigation of their use in electrochemical devices [2] and as “green” solvents [3].

Recently this has begun to include development of ILs for use in corrosion protection of metals. Corrosion is an insidious problem that annually causes trillions of dollars' worth of damage worldwide [4]. While a variety of corrosion inhibitors exist, many, such as the extremely effective chromates, are highly toxic [5]. Ionic liquids are being sought that could provide “greener” alternatives for use on a range of metals. One approach is to use ionic liquid pre-treatments to form passivating films on the metal surface. This is particularly useful for protection of reactive metals such as magnesium and aluminium [6]. To date this work has focused predominantly on phosphonium based ILs [7], [8].

A number of publications have also investigated the use of ionic liquids as additive chemical inhibitors, with a particular focus towards protecting steel in acidic environments, a common industrial condition. A range of imidazolium cations have been tested, including traditional alkyl methylimidazoliums [9], [10] and novel substituted cations [11], [12], as well as other cations such as pyridinium [12] and pyridazinium [13].

Most of these examples rely on the use of conventional ILs, despite one of the greatest strengths of the IL platform being the ability in principle to design a compound or a formulation for a given application. In recent years, there has been a shift in the focus of this tuneability. Initial investigations into ILs looked to alter physical properties such as viscosity and thermal stability. Eventually this evolved to tailoring not only the physical properties, but the chemical properties as well, to develop ILs that were chiral [14], [15] or strong coordinating solvents [16]. The ‘third generation” of ILs, as proposed by Rogers and coworkers [17] introduced the idea of manipulating their biological properties. This concept does not, however, have to be limited to pursuing pharmaceutical compounds, but rather can be seen as a shift in the way component ions are chosen. Instead of trying to fit traditional ILs to a given application, one can take an “active” compound and redesign it as an IL with enhanced functionality for a range of applications where bio-activity, or biocompatibility is important. In some of our previous work [18], [19] we have shown how ionic liquids and salts developed this way can tackle the insidious problem of microbially induced corrosion, by combining known antibacterial agent ions with corrosion inhibiting counter ions.

From our continued investigation of such compounds we present here a novel family of task-oriented organic salts and ionic liquids featuring the 2-methylimidazolinium [2-MeHImn]⁺ cation. We describe a range of salts of this cation, some in particular targeted towards use as corrosion inhibitors. Imidazolines and their salts, some of which are liquids, are known and used in a number of industries, notably as corrosion inhibitors, particularly in oil pipelines [20], [21]. The compounds often feature lengthy fatty-acid functional groups and in those cases can be classified as cationic surfactants [20], [21], [22]. As can be seen in Fig. 1, imidazolinium cations are closely related to the more broadly recognised imidazolium cations, with the only difference to the core ring being the saturation of the C4-C5 double bond. Despite this similarity, the imidazolinium cation has not been widely explored in the IL context (for any application) and thus it appears to offer a promising avenue for new IL chemistry. Thus we present here a number of novel ionic liquids and organic salts based on this cation and investigate their properties including an initial investigation of their corrosion inhibition activity.

Section snippets

Materials

All chemicals were purchased from Sigma and were >98% pure. They were used without further purification. Methanol was distilled from sodium metal and stored over 4 Å molecular sieves, acetonitrile and dichloromethane were dried over 4 Å molecular sieves.

Synthesis of 2-methylimidazolinium acetate

Triethylorthoacetate (10.5 mL, 53 mmol) and acetic acid (3.1 mL, 53 mmol) were added to a flask of acetonitrile. Ethylenediamine (3.4 mL, 50 mmol) was added slowly via a dropping funnel, resulting in formation of a white precipitate. The mixture was

Synthesis

[2-MeHImn]⁺ salts were synthesised in a one-pot synthesis from ethylenediamine, triethylorthoacetate and the desired acid, following a method established by Cornia et al. [23] (Fig. 2a). The selection of anions is shown in Fig. 2b, focusing on carboxylates, with corrosion applications in mind. A number of equivalent 2-methylimidazolium ([2-MeHIm]⁺) salts were also synthesised for comparison.

Physical Properties

Differential scanning calorimetry (DSC) was used to determine the thermal behaviour of the materials. As

Conclusions

A series of ILs and organic salts with the [2-MeHImn]⁺ cation were synthesised via a facile one-pot reaction. Depending upon the nature of the anion in the salts, these materials were found to have interesting physical properties such as facile ion transport, as well as demonstrating synergistic corrosion inhibition on mild steel. The imidazolinium cation shows promise as the basis of a new class of ILs. However, more work is required, both to better understand the potential scope of their

Acknowledgements

A. Chong acknowledges her Australian Postgraduate Award and Professors MacFarlane and Forsyth are grateful to the Australian Research Council for support via the Australian Laureate Fellowship program.

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Inhibition of steel corrosion with imidazolium-based compounds – Experimental and theoretical study
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This work aims to investigate the corrosion inhibition of the mild steel in the 1 M HCl solution by 1-octyl-3-methylimidazolium hydrogen sulphate 1-butyl-3-methylimidazolium hydrogen sulphate, and 1-octyl-3-methylimidazolium chloride, using electrochemical, weight loss, and surface analysis methods as well as the full quantum-mechanical treatment. Polarization measurements prove that studied compounds are mixed-type inhibitors with a predominantly anodic reaction. The inhibition efficiency obtained from the polarization curves is about 80–92% for all of the 1-octyl-3-methylimidazolium salts with a concentration higher than 0.005 mol/l, while it is much lower for 1-butyl-3-methylimidazolium hydrogen sulphate. The values measured in the weight loss experiments (after seven days) are to some extent higher (reaching up to 98% efficiency). Furthermore, we have shown that the influence of the alkyl chain length on the inhibition efficiency is much larger than that of the anion type. Furthermore, we obtain a realistic model of a single molecule on iron surface Fe(110) by applying the Density Functional Theory calculations. We use the state-of-the-art computational approach, including the meta-GGA strongly-constrained and appropriately normed semilocal density functional to model the electronic structure properties of both free and bounded-to-surface molecules of 1-butyl-, 1-hexyl-, and 1-octyl-3-methylimizadolium bromide, chloride, and hydrogen sulphate. From the calculations we extract, the HOMO/LUMO gap, hardness, electronegativity, and charge transfer of electrons from/to molecules-in-question. It supports the experimental findings and explains the influence of the alkyl chain length and the functional group on the inhibition process.
An imidazole-based benzilic-dicationic ionic liquid performance in 1.0 M HCl solution to mitigate the mild steel degradation: Electrochemical noise/impedance investigation
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Novel imidazolinium ionic liquids and organic salts

Highlights

ABSTRACT

Graphical abstract

Introduction

Section snippets

Materials

Synthesis of 2-methylimidazolinium acetate

Synthesis

Physical Properties

Conclusions

Acknowledgements

Electrochim. Acta

Electrochim. Acta

Int. J. Electrochem. Sci.

Int. J. Electrochem. Sci.

Tetrahedron

Electrochem. Commun.

Electrochim. Acta

Corros. Sci.

Ionic Liquids in Synthesis

Electrodeposition from Ionic liquids

Ionic Liquids: Industrial Applications for Green Chemistry ACS Symposium Series 818

ACS Appl. Mater. Interfaces

Adv. Mater. Res.

Port

Electrochim. Acta

Int. J. Electrochem. Sci.

Chirality

Green Chem.